Physiology_Levy

Physiology_Levy. We are pleased that the following section authors have continued as members of the seventh edition team: Drs. Kalman Rubinson and Eric Lang (nervous system), Dr. James Watras (muscle), Dr. Achilles Pappano (cardiovascular system), Drs. Michelle Cloutier and Roger Thrall (respiratory system), Drs. Kim Barrett and Helen Raybould (gastrointestinal system), and Dr. Bruce White (endocrine and reproductive systems). We also welcome the following authors: Dr. Withrow Gil Wier (cardiovascular system), and Dr. John Harrison (endocrine and reproduction systems).


Physiology_Levy

Physiology_Levy. As in the previous editions of this textbook, we have attempted to emphasize broad concepts and to minimize the compilation of isolated facts. Each chapter has been written to make the text as lucid, accurate, and current as possible. We have included both clinical and molecular information in each section, as feedback on these features has indicated that this information serves to provide clinical context and new insights into physiologic phenomena at the cellular and molecular levels. New to this edition is a list of sources that the reader can consult for further information on the topics covered in each chapter. We hope that you find this a valuable addition to the book.


Physiology_Levy

Physiology_Levy. The human body consists of billions of cells that are organized into tissues (e.g., muscle, epithelia, and nervous tissue) and organ systems (e.g., nervous, cardiovascular, respiratory, renal, gastrointestinal, endocrine, and reproductive). For these tissues and organ systems to function properly and thus allow humans to live and carry out daily activities, several general conditions must be met. First and foremost, the cells within the body must survive. Survival requires adequate cellular energy supplies, maintenance of an appropriate intracellular milieu, and defense against a hostile external environment. Once cell survival is ensured, the cell can then perform its designated or specialized function (e.g., contraction by skeletal muscle cells). Ultimately, the function of cells, tissues, and organs must be coordinated and regulated. All of these functions are the essence of the discipline of physiology and are presented throughout this book. What follows is a brief introduction to


Physiology_Levy

Physiology_Levy. and organs must be coordinated and regulated. All of these functions are the essence of the discipline of physiology and are presented throughout this book. What follows is a brief introduction to these general concepts.


Physiology_Levy

Physiology_Levy. Cells need a constant supply of energy. This energy is derived from the hydrolysis of adenosine triphosphate (ATP). If not replenished, the cellular ATP supply would be depleted in most cells in less than 1 minute. Thus, ATP must be continuously synthesized. This in turn requires a steady supply of cellular fuels. However, the cellular fuels (e.g., glucose, fatty acids, and ketoacids) are present in the blood at levels that can support cellular metabolism only for a few minutes. The blood levels of these cellular fuels are maintained through the ingestion of precursors (i.e., carbohydrates, proteins, and fats). In addition, these fuels can be stored and then mobilized when ingestion of the precursors is not possible. The storage forms of these fuels are triglycerides (stored in adipose tissue), glycogen (stored in the liver and skeletal muscle), and protein. The maintenance of adequate levels of cellular fuels in the blood is a complex process involving the following tissues, organs,


Physiology_Levy

Physiology_Levy. glycogen (stored in the liver and skeletal muscle), and protein. The maintenance of adequate levels of cellular fuels in the blood is a complex process involving the following tissues, organs, and organ systems:


Physiology_Levy

Physiology_Levy. Liver: Converts precursors into fuel storage forms (e.g., glucose → glycogen) when food is ingested, and converts storage forms to cellular fuels during fasting (e.g., glycogen → glucose and amino acids → glucose). Skeletal muscle: Like the liver, stores fuel (glycogen and protein) and converts glycogen and protein to fuels (e.g., glucose) or fuel intermediates (e.g., protein → amino acids) during fasting. Gastrointestinal tract: Digests and absorbs fuel precursors. Adipose tissue: Stores fuel during feeding (e.g., fatty acids → triglycerides) and releases the fuels during fasting. Cardiovascular system: Delivers the fuels to the cells and to and from their storage sites. Endocrine system: Maintains the blood levels of the cellular fuels by controlling and regulating their storage and their release from storage (e.g., insulin and glucagons).


Physiology_Levy

Physiology_Levy. Endocrine system: Maintains the blood levels of the cellular fuels by controlling and regulating their storage and their release from storage (e.g., insulin and glucagons). Nervous system: Monitors oxygen levels and nutrient content in the blood and, in response, modulates the cardiovascular, pulmonary, and endocrine systems and induces feeding and drinking behaviors. In addition to energy metabolism, the cells of the body must maintain a relatively constant intracellular environment to survive. This includes the uptake of fuels needed to produce ATP, the export from the cell of cellular wastes, the maintenance of an appropriate intracellular ionic environment, the establishment of a resting membrane potential, and the maintenance of a constant cellular volume. All of these functions are carried out by specific membrane transport proteins.


Physiology_Levy

Physiology_Levy. The composition of the extracellular fluid (ECF) that bathes the cells must also be maintained relatively constant. In addition, the volume and temperature of the ECF must be regulated. Epithelial cells in the lungs, gastrointestinal tract, and kidneys are responsible for maintaining the volume and composition of the ECF, while the skin plays a major role in temperature regulation. On a daily basis, H2O and food are ingested, and essential components are absorbed across the epithelial cells of the gastrointestinal tract. This daily intake of solutes and water must be matched by excretion from the body, thus maintaining steady-state balance. The kidneys are critically involved in the maintenance of steady-state balance for water and many components of the ECF (e.g., Na+ , K+ , HCO3 − , pH, Ca++ , organic solutes). The lungs ensure an adequate supply of O2 to “burn” the cellular fuels for the production of ATP and excrete the major waste product of this process (i.e., CO2). Because CO2


Physiology_Levy

Physiology_Levy. Ca++ , organic solutes). The lungs ensure an adequate supply of O2 to “burn” the cellular fuels for the production of ATP and excrete the major waste product of this process (i.e., CO2). Because CO2 can affect the pH of the ECF, the lungs work with the kidneys to maintain ECF pH.


Physiology_Levy

Physiology_Levy. Because humans inhabit many different environments and often move between environments, the body must be able to rapidly adapt to the challenges imposed by changes in ambient temperature and availability of food and water. Such adaptation requires coordination of the function of cells in different tissues and organs as well as their regulation. The nervous and endocrine systems coordinate and regulate cell, tissue, and organ function. The regulation of function can occur rapidly (seconds to minutes), as is the case for levels of cellular fuels in the blood, or over much longer periods of time (days to weeks), as is the case for acclimatization when an individual moves from a cool to a hot environment or changes from a high-salt to a low-salt diet.


Physiology_Levy

Physiology_Levy. The function of the human body represents complex processes at multiple levels. This book explains what is currently known about these processes. Although the emphasis is on the normal function of the human body, discussion of disease and abnormal function is also appropriate, as these often illustrate physiologic processes and principles at the extremes. The authors for each section have presented what they believe to be the most likely mechanisms responsible for the phenomena under consideration. We have adopted this compromise to achieve brevity, clarity, and simplicity. Bruce M. Koeppen, MD, PhD Bruce A. Stanton, PhD 29TheGastricPhaseoftheIntegratedResponsetoaMeal,529 30TheSmallIntestinalPhaseoftheIntegratedResponsetoaMeal,541 31TheColonicPhaseoftheIntegratedResponsetoaMeal,559 32TransportandMetabolicFunctionsoftheLiver,568 Section 7: The Renal System, 580 37RoleoftheKidneysintheRegulationofAcid-BaseBalance,670 Section 8: The Endocrine and Reproductive Systems, 685


Physiology_Levy

Physiology_Levy. Section 7: The Renal System, 580 37RoleoftheKidneysintheRegulationofAcid-BaseBalance,670 Section 8: The Endocrine and Reproductive Systems, 685 SECTION 1Cellular Physiology BRUCE M. KOEPPEN AND BRUCE A. STANTON Upon completion of this chapter, the student should be able to answer the following questions: 1. What organelles are found in a typical eukaryotic cell, and what is their function? 2. What is the composition of the plasma membrane? 3. What are the major classes of membrane transport proteins, and how do they transport biologically important molecules and ions across the plasma membrane? 4. What is the electrochemical gradient, and how it is used to determine whether the transport of a molecule or ion across the plasma membrane is active or passive? 5. What are the driving forces for movement of water across cell membrane and the capillary wall?


Physiology_Levy

Physiology_Levy. 5. What are the driving forces for movement of water across cell membrane and the capillary wall? In addition, the student should be able to define and understand the following properties of physiologically important solutions and fluids: he human body is composed of billions of cells. Although cells can perform different functions, they share certain common elements. This chapter provides an overview of these common elements and focuses on the important function of the transport of molecules and water into and out of the cell across its plasma membrane. Overview of Eukaryotic Cells


Physiology_Levy

Physiology_Levy. Overview of Eukaryotic Cells Eukaryotic cells are distinguished from prokaryotic cells by the presence of a membrane-delimited nucleus. With the exception of mature human red blood cells and cells within the lens of the eye, all cells within the human body contain a nucleus. The cell is therefore effectively divided into two compartments: the nucleus and the cytoplasm. The cytoplasm is an aqueous solution containing numerous organic molecules, ions, cytoskeletal elements, and a number of organelles. Many of the organelles are membrane-enclosed compartments that carry out specific cellular function. An idealized eukaryotic cell is depicted in Fig. 1.1 , and the primary function of some components and compartments of the cell are summarized in . Readers who desire a more in-depth presentation of this material are encouraged to consult one of the many textbooks on cell and molecular biology that are currently available. The Plasma Membrane


Physiology_Levy

Physiology_Levy. The Plasma Membrane The cells within the body are surrounded by a plasma membrane that separates the intracellular contents from the extracellular environment. Because of the properties of this membrane and, in particular, the presence of specific membrane proteins, the plasma membrane is involved in a number of important cellular functions, including the following: Selective transport of molecules into and out of the cell. A function carried out by membrane transport proteins. Cell recognition through the use of cell surface antigens. Cell communication through neurotransmitter and hormone receptors and through signal transduction pathways. Tissue organization, such as temporary and permanent cell junctions, and interaction with the extracellular matrix, with the use of a variety of cell adhesion molecules. Membrane-dependent enzymatic activity.


Physiology_Levy

Physiology_Levy. Membrane-dependent enzymatic activity. Determination of cell shape by linkage of the cytoskeleton to the plasma membrane. In this chapter, the structure and function of the plasma membrane of eukaryotic cells are considered. More specifically, the chapter focuses on the transport of molecules and water across the plasma membrane. Only the principles of membrane transport are presented here. Additional details that relate to specific cells are presented in the various sections and chapters of this book. The plasma membrane of eukaryotic cells consists of a 5-nm-thick lipid bilayer with associated proteins ( Fig. 1.2 ). Some of the membrane-associated proteins are integrated into the lipid bilayer; others are more loosely attached to the CHAPTER 1 Principles of Cell and Membrane Function 3 • Fig. 1.1 Schematic drawing of a eukaryotic cell. The top portion of the cell is omitted to illustrate the nucleus and various intracellular organelles. See text for details.


Physiology_Levy

Physiology_Levy. Primary Functions of Some Eukaryotic Cellular Components and Compartments 1.1 CytosolMetabolism, protein synthesis (free ribosomes)CytoskeletonCell shape and movement, intracellular transportNucleusGenome (22 autosomes and 2 sex chromosomes), DNA and RNA synthesisMitochondriaATP synthesis by oxidative phosphorylation, Ca2+ storageSmooth endoplasmic reticulumSynthesis of lipids, Ca2+ storageFree ribosomesTranslation of mRNA into cytosolic proteinsRough endoplasmic reticulumTranslation of mRNA into membrane associated proteins or for secretion out of the cellLysosomeIntracellular degradationEndosomeCellular uptake of cholesterol, removal of receptors from the plasma membrane, uptake of small molecules and water into the cell, internalization of large particles (e.g., bacteria, cell debris)Golgi apparatusModification, sorting, and packaging of proteins and lipids for delivery to other organelles within ATP, adenosine triphosphate; mRNA, messenger RNA.


Physiology_Levy

Physiology_Levy. ATP, adenosine triphosphate; mRNA, messenger RNA. inner or outer surfaces of the membrane, often by binding molecules that contain a charged (or polar) hydrophilic to the integral membrane proteins. head and two (nonpolar) hydrophobic fatty acyl chains ( Fig. 1.3 ). The amphipathic nature of the phospholipid molecule Membrane Lipids is critical for the formation of the bilayer: The hydrophobic The major lipids of the plasma membrane are phospholipids fatty acyl chains form the core of the bilayer, and the polar and phosphoglycerides. Phospholipids are amphipathic head groups are exposed on the surface. • Fig. 1.2 Schematic diagram of the cell plasma membrane. Not shown are lipid rafts. See text for details. (Modified from Cooper GM. The Cell—A Molecular Approach. 2nd ed. Washington, DC: Sinauer; 2000, Fig. 12.3.) Phospholipid Glycolipid (e.g., phosphatidylcholine) (e.g., galactosylceramide) Cholesterol


Physiology_Levy

Physiology_Levy. Phospholipid Glycolipid (e.g., phosphatidylcholine) (e.g., galactosylceramide) Cholesterol Sugar (e.g., galactose) • Fig. 1.3 Models of the major classes of plasma membrane lipids, depicting the hydrophilic and hydrophobic regions of the molecules. The molecules are arranged as they exist in one leaflet of the bilayer. The opposing leaflet is not shown. One of the fatty acyl chains in the phospholipid molecule is unsaturated. The presence of this double bond produces a “kink” in the fatty acyl chain, which prevents tight packing of membrane lipids and increases membrane fluidity. (Modified from Hansen JT, Koeppen BM: Netter’s Atlas of Human Physiology. Teterboro, NJ: Icon Learning Systems; 2002.)


Physiology_Levy

Physiology_Levy. The majority of membrane phospholipids have a glycerol carbons in length and may be saturated or unsaturated (i.e., “backbone” to which are attached the fatty acyl chains, contain one or more double bonds). and an alcohol is linked to glycerol via a phosphate group. The phospholipid composition of the membrane The common alcohols are choline, ethanolamine, serine, varies among different cell types and even between the inositol, and glycerol. Another important phospholipid, bilayer leaflets. For example, in the erythrocyte plasma sphingomyelin, has the amino alcohol sphingosine as its membrane, phosphatidylcholine and sphingomyelin are “backbone” instead of glycerol. lists these common found predominantly in the outer leaflet of the membrane, phospholipids. The fatty acyl chains are usually 14 to 20 whereas phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol are found in the inner leaflet.


Physiology_Levy

Physiology_Levy. , phosphatidylinositol plays an important role in signal transduction, and its location in the inner leaflet of the membrane facilitates this signaling role. The sterol molecule cholesterol is also a critical component of the bilayer (see Fig. 1.3 ). It is found in both leaflets and serves to stabilize the membrane at normal body temperature (37°C). As much as 50% of the lipids found in the membrane can be cholesterol. A minor lipid component of the plasma membrane is glycolipids. These lipids, as their name indicates, consist of two fatty acyl chains linked to polar head groups that consist of carbohydrates (see Fig.


Physiology_Levy

Physiology_Levy. Fig. 1.3 ). As discussed in the section on , one glycolipid, glycosylphosphatylinositol (GPI), plays an important role in anchoring proteins to the outer leaflet of the membrane. Both cholesterol and glycolipids, like the phospholipids, are amphipathic, and they are oriented with their polar groups on the outer surface of the leaflet in which they are located. Their hydrophobic portion is thus located within the interior of the bilayer. The lipid bilayer is not a static structure. The lipids and associated proteins can diffuse within the plane of the membrane. The fluidity of the membrane is determined by temperature and by its lipid composition. As temperature increases, the fluidity of the membrane increases. The presence of unsaturated fatty acyl chains in the phospholipids and glycolipids also increases membrane fluidity. If a fatty acyl chain is unsaturated, the presence of a double bond introduces a “kink” in the molecule (see Fig. 1.3).


Physiology_Levy

Physiology_Levy. Fig. 1.3). This kink prevents the molecule from associating closely with surrounding lipids, and, as a result, membrane fluidity is increased. Although the lipid bilayer is “fluid,” movement of proteins in the membrane can be constrained or limited. For example, membrane proteins can be anchored to components of the intracellular cytoskeleton, which limits their movement. Membrane domains can also be isolated from one another. An important example of this can be found in epithelial tissues. Junctional complexes (e.g., tight junctions) separate the plasma membrane of epithelial cells into two domains: apical and basolateral (see ). The targeted localization of membrane proteins into one or other of these domains allows epithelial cells to carry


Physiology_Levy

Physiology_Levy. CHAPTER 1 Principles of Cell and Membrane Function out vectorial transport of substances from one side of the epithelium to the opposite side. The ability to carry out vectorial transport is crucial for the functioning of several organ systems (e.g., the gastrointestinal tract and kidneys). In addition, some regions of the membrane contain lipids (e.g., sphingomyelin and cholesterol) that aggregate into what are called lipid rafts. These lipid rafts often have an association with specific proteins, which diffuse in the plane of the membrane as a discrete unit. Lipid rafts appear to serve a number of functions. One important function of these rafts is to segregate signaling molecules. As much as 50% of the plasma membrane is composed of proteins. These membrane proteins are classified as integral, lipid-anchored, or peripheral.


Physiology_Levy

Physiology_Levy. As much as 50% of the plasma membrane is composed of proteins. These membrane proteins are classified as integral, lipid-anchored, or peripheral. Integral membrane proteins are imbedded in the lipid bilayer, where hydrophobic amino acid residues are associated with the hydrophobic fatty acyl chains of the membrane lipids. Many integral membrane proteins span the bilayer; such proteins are termed transmembrane proteins. Transmembrane proteins have both hydrophobic and hydrophilic regions. The hydrophobic region, often in the form of an α helix, spans the membrane. Hydrophilic amino acid residues are then exposed to the aqueous environment on either side of the membrane. Transmembrane proteins may pass through the membrane multiple times.


Physiology_Levy


Physiology_Levy

Physiology_Levy. A protein can also be attached to the membrane via lipid anchors. The protein is covalently attached to a lipid molecule, which is then embedded in one leaflet of the bilayer. Glycosylphosphatidylinositol (GPI) anchors proteins to the outer leaflet of the membrane. Proteins can be attached to the inner leaflet via their amino-terminus by fatty acids (e.g., myristate or palmitate) or via their carboxyl-terminus by prenyl anchors (e.g., farnesyl or geranylgeranyl). Peripheral proteins may be associated with the polar head groups of the membrane lipids, but they more commonly bind to integral or lipid-anchored proteins.


Physiology_Levy

Physiology_Levy. Peripheral proteins may be associated with the polar head groups of the membrane lipids, but they more commonly bind to integral or lipid-anchored proteins. In many cells, some of the outer leaflet lipids, as well as many of the proteins exposed on the outer surface of the membrane, are glycosylated (i.e., have short chains of sugars, called oligosaccharides, attached to them). Collectively, these glycolipids and glycoproteins form what is called the glycocalyx. Depending on the cell these glycolipids and glycoproteins may be involved in cell recognition (e.g., cell surface antigens) and formation of cell-cell interactions (e.g., attachment of neutrophils to vascular endothelial cells).


Physiology_Levy

Physiology_Levy. Although plasma membrane proteins perform many important cellular functions, as noted previously, the remainder of this chapter focuses on one group of plasma membrane proteins: the membrane transport proteins, or transporters. It has been estimated that approximately 10% of human genes (≈2000) code for transporters. They are also targets for numerous drugs. The normal function of cells requires the continuous movement of water and solutes into and out of the cell. The intracellular and extracellular fluids are composed primarily of H2O, in which solutes (e.g., ions, glucose, amino acids) are dissolved. The plasma membrane, with its hydrophobic core, is an effective barrier to the movement of virtually all of these biologically important solutes. It also restricts the movement of water across the membrane. The presence of specific membrane transporters in the membrane is responsible for the movement of these solutes and water across the membrane.


Physiology_Levy

Physiology_Levy. Membrane transporters have been classified in several different ways. In this chapter, the transporters are divided into four general groups: water channels, ion channels, solute carriers, and adenosine triphosphate (ATP)–dependent transporters. lists these groups of membrane transporters, their modes of transport, and estimates of the rates at which they transport molecules or ions across the membrane.


Physiology_Levy

Physiology_Levy. lists these groups of membrane transporters, their modes of transport, and estimates of the rates at which they transport molecules or ions across the membrane. Water channels, or aquaporins (AQPs), are the main routes for water movement into and out of the cell. They are widely distributed throughout the body (e.g., the brain, lungs, kidneys, salivary glands, gastrointestinal tract, and liver). Cells express different AQP isoforms, and some cells even express multiple isoforms. For example, cells in the collecting ducts of the kidneys express AQP3 and AQP4 in their basolateral membrane and AQP2 in their apical membrane. Moreover, the abundance of AQP2 in the apical membrane is regulated by antidiuretic hormone (also called arginine vasopressin), which is crucial for the ability of the kidneys to concentrate the urine (see


Physiology_Levy

Physiology_Levy. Although all AQP isoforms allow the passive movement of H2O across the membrane, some isoforms also provide a pathway for other molecules such as glycerol, urea, mannitol, purines, pyrimidines, CO2, and NH3 to cross the membrane. Because glycerol was one of the first molecules identified as crossing the membrane via some AQPs, this group of AQPs is collectively called aquaglyceroporins (see also ). Regulation of the amount of H2O that can enter or leave the cell via AQPs occurs primarily by altering the number of AQPs in the membrane. EachAQPmoleculeconsistsofsixmembrane-spanningdomainsandacentralwater-transportingpore.FourAQPmonomersassembletoformahomotetramerintheplasmamembrane,witheachmonomerfunctioningasawaterchannel.


Physiology_Levy

Physiology_Levy. Ion channels are found in all cells, and are especially important for the function of excitable cells (e.g., neurons and muscle cells). Ion channels are classified by their selectivity, conductance and mechanism of channel gating (i.e., opening and closing). Selectivity is defined as the nature of the ions that pass through the channel. At one extreme, ion channels can be highly selective, in that they allow only a specific ion through. At the other extreme, they may be nonselective, allowing all or a group of cations or anions through. Channel conductance refers to the number of ions that pass through the channel and is typically expressed in picosiemens (pS). The range of conductance is considerable: Some channels have a conductance of only 1 to 2 pS, whereas others have a conductance of more than 100 pS. For some channels, the conductance varies, depending on the direction in which the ion is moving. For example, if the channel has a larger conductance when ions are moving into the


Physiology_Levy

Physiology_Levy. more than 100 pS. For some channels, the conductance varies, depending on the direction in which the ion is moving. For example, if the channel has a larger conductance when ions are moving into the cell than when they are moving out of the cell, the channel is said to be an inward rectifier. Moreover, ion channels fluctuate between an open state or a closed state, a process called gating (


Physiology_Levy

Physiology_Levy. Fig. 1.4 ). Factors that can control gating include membrane voltage, extracellular agonists or antagonists (e.g., acetylcholine is an extracellular agonist mitochondria,andwaterchannels(i.e.,aquaporins)thatfunctionas that controls the gating of a cation-selective channel in the apore. ATP,adenosinetriphosphate.motor end plate of skeletal muscle cells; see Chapter 6), intracellular messengers (e.g., Ca++ , ATP, cyclic guanosine CHAPTER 1 Principles of Cell and Membrane Function •Fig. 1.4 Recordingofcurrentflowthroughasingleionchannel.Thechannelspontaneouslyfluctuatesbetweenanopenstateandaclosedstate.Theamplitudeofthecurrentisapproximately2pA(2× 10−12amps);thatis,12.5millionions/secondcrossthemembrane. monophosphate), and mechanical stretch of the plasma membrane. Ion channels can be regulated by a change in the number of channels in the membrane or by gating of the channels.


Physiology_Levy

Physiology_Levy. Solute carriers (denoted SLCs by the HUGO Gene Nomenclature Committee) represent a large group of membrane transporters categorized into more than 50 families; almost 400 specific transporters have been identified to date. These carriers can be divided into three groups according to their mode of transport. One group, uniporters (or facilitated transporters), transports a single molecule across the membrane. The transporter that brings glucose into the cell (glucose transporter 1 [GLUT-1], or SLC2A1) is an important member of this group. The second group, symporters (or cotransporters), couples the movement of two or more molecules/ions across the membrane. As the name implies, the molecules/ions are transported in the same direction. The Na+,K+,2Cl− (NKCC) symporter found in the kidney (NKCC2, or SLC12A1), which is crucial for diluting and concentrating the urine (see ), is a member of this group. The third group, antiporters (or exchange transporters), also couples the movement of


Physiology_Levy

Physiology_Levy. (NKCC2, or SLC12A1), which is crucial for diluting and concentrating the urine (see ), is a member of this group. The third group, antiporters (or exchange transporters), also couples the movement of two or more molecules/ions across the membrane; in this case, however, the molecules/ions are transported in opposite directions. The Na+-H+ antiporter is a member of this group of solute carriers. One isoform of this antiporter (NHE-1, or SLC9A1) is found in all cells and plays an important role in regulating intracellular pH.


Physiology_Levy

Physiology_Levy. The ATP-dependent transporters, as their name implies, use the energy in ATP to drive the movement of molecules/ ions across the membrane. There are two groups of ATP-dependent transporters: the ATPase ion transporters and the ATP-binding cassette (ABC) transporters. The ATPase ion transporters are subdivided into P-type ATPases and V-type ATPases. The P-type ATPases are phosphorylated during the transport cycle. Na+,K+-ATPase is an important example of a P-type ATPase. With the hydrolysis of each ATP molecule, it transports three Na+ ions out of the cell and two K+ ions into the cell. Na+,K+-ATPase is present in all cells and plays a critical role in establishing cellular ion and electrical gradients, as well as maintaining cell volume (see


Physiology_Levy

Physiology_Levy. V-type H+-ATPases are found in the membranes of several intracellular organelles (e.g., endosomes, lysosomes); as a result, they are also referred to as vacuolar H+-ATPases. The aAnother type of ATPases, F-type ATPases, is found in the mitochondria, and they are responsible for ATP synthesis. They are not considered in this chapter.


Physiology_Levy

Physiology_Levy. Na+,K+-ATPase(alsocalledtheNa+,K+-pump orjusttheNa+pump)isfoundinallcellsandisresponsibleforestablishingthegradientsofNa+ andK+ acrosstheplasmamembrane.Thesegradientsinturnprovideenergyforseveralessentialcellfunctions(see ).Na+,K+-ATPaseiscomposedofthreesubunits(α,β,andγ),andtheproteinexistsinthemembranewithastoichiometriccompositionof1α,1β,1γ.Theα subunitcontainsbindingsitesforNa+,K+ andATP.Itisalsothesubunitthatbindscardiacglycosides(e.g.,ouabain),whichspecificallyinhibittheenzyme.Ithasatransmembranedomainandthreeintracellulardomains:phosphorylation(P-domain),nucleotidebinding(N-domain),andactuator(A-domain).Althoughtheα subunitisthefunctionalsubunitoftheenzyme(i.e.,ithydrolyzesATP,bindsNa+ andK+ ,andtranslocatesthemacrossthemembrane),itcannotfunctionwithouttheβ subunit.Theβ subunitisresponsiblefortargetingtheα subunittothemembraneandalsoappearstomodulatetheaffinityoftheNa+,K+-ATPaseforNa+ andK+ .Theα andβ subunitscancarryoutNa+ andK+ transportintheabsenceoftheγ subunit.However,theγ


Physiology_Levy

Physiology_Levy. subunittothemembraneandalsoappearstomodulatetheaffinityoftheNa+,K+-ATPaseforNa+ andK+ .Theα andβ subunitscancarryoutNa+ andK+ transportintheabsenceoftheγ subunit.However,theγ subunitappearstoplayaregulatoryrole.Theγ subunitisamemberofafamilyofproteinscalledFXYD proteins (sonamedfortheFXYDaminoacidsequencefoundintheprotein).


Physiology_Levy

Physiology_Levy. H+-ATPase in the plasma membrane plays an important role in urinary acidification (see ABC transporters represent a large group of membrane transporters. They are found in both prokaryotic and eukaryotic cells, and they have amino acid domains that bind ATP (i.e., ABC domains). Seven subgroups of ABC transporters in humans and more than 40 specific transporters have been identified to date. They transport a diverse group of molecules/ions, including Cl−, cholesterol, bile acids, drugs, iron, and organic anions.


Physiology_Levy

Physiology_Levy. Because biologically important molecules enter and leave cells through membrane transporters, membrane transport is specific and regulated. Although some membrane transporters are ubiquitously expressed in all cells (e.g., Na+,K+-ATPase), the expression of many other transporters is limited to specific cell types. This specificity of expression tailors the function of the cell to the organ system in which it is located (e.g., the sodium-glucose–linked transporters SGLT-1 and SGLT-2 in the epithelial cells of the intestines and renal proximal tubules). In addition, the amount of a molecule being transported across the membrane can be regulated. Such regulation can take place through altering the number of transporters in the membrane or altering the rate or kinetics of individual transporters (e.g., the time an ion channel stays in the open versus closed state), or both.


Physiology_Levy

Physiology_Levy. Solute and water can be brought into the cell through a process of endocytosis and released from the cell through the process of exocytosis. Endocytosis is the process whereby a piece of the plasma membrane pinches off and


Physiology_Levy


Physiology_Levy

Physiology_Levy. U.S.FoodandDrugAdministrationapprovedlumacaftor/ivacaftor(Orkambi),adrugthatincreasestheamountofF508delCFTRintheplasmamembraneoflungepithelialcells. is internalized into the cell interior, and exocytosis is the process whereby vesicles inside the cell fuse with the plasma membrane. In both of these processes, the integrity of the plasma membrane is maintained, and the vesicles allow for the transfer of the contents among cellular compartments. In some cells (e.g., the epithelial cells lining the gastrointestinal tract), endocytosis across one membrane of the cell is followed by exocytosis across the opposite membrane. This allows the transport of substances inside the vesicles across the epithelium, a process termed transcytosis.


Physiology_Levy

Physiology_Levy. Endocytosis occurs in three mechanisms. The first is pinocytosis, which consists of the nonspecific uptake of small molecules and water into the cell. Pinocytosis is a prominent feature of the endothelial cells that line capillaries and is responsible for a portion of the fluid exchange that occurs across these vessels. The second form of endocytosis, phagocytosis, allows for the cellular internalization of large particles (e.g., bacteria, cell debris). This process is an important characteristic of cells in the immune system (e.g., neutrophils and macrophages). Often, but not always, phagocytosis is a receptor-mediated process. For example, CHAPTER 1 Principles of Cell and Membrane Function 9


Physiology_Levy

Physiology_Levy. Proteins within the plasma membrane of cells are constantly being removed and replaced with newly synthesized proteins. As a result, membrane proteins are constantly being replaced. One mechanism by which membrane proteins are “tagged” for replacement is by the attachment of ubiquitin to the cytoplasmic portion of the protein. Ubiquitin is a 76–amino acid protein that is covalently attached to the membrane protein (usually to lysine) by a class of enzymes called ubiquitin protein


Physiology_Levy

Physiology_Levy. protein ligases. One important group of these ligases is the developmentally downregulated protein 4 (Nedd4)/Nedd4-like family. Once a membrane protein is ubiquitinated, it undergoes endocytosis and is degraded either by lysosomes or by the proteosome. Cells also contain deubiquitinating enzymes (DUBs). Thus the amount of time a protein stays in the plasma membrane depends on the rate that ubiquitin groups are added by the ligases versus the rate that they are removed by the DUBs. For example, Na+ reabsorption by the collecting ducts of the kidneys is stimulated by the adrenal hormone aldosterone (see ). One of the actions of aldosterone is to inhibit Nedd4-2. This prevents ubiquitination of ENaC in the apical membrane of epithelial cells. Thus the channels are retained for a longer period of time in the membrane, and as a result, more Na+ enters the cell and is thereby reabsorbed.


Physiology_Levy

Physiology_Levy. macrophages have receptors on their surface that bind the Fc portion of immunoglobulins. When bacteria invade the body they are often coated with antibody, a process called opsonization. These bacteria then attach to the membrane of macrophages via the fragment crystallizable (Fc) portion of the immunoglobulin, undergo phagocytosis, and are destroyed inside the cell. The third mechanism of endocytosis is receptor-mediated endocytosis, which allows the uptake of specific molecules into the cell. In this form of endocytosis, molecules bind to receptors on the surface of the cell. Endocytosis involves a number of accessory proteins, including adaptin, clathrin, and the GTPase dynamin ( Fig. 1.5


Physiology_Levy

Physiology_Levy. Exocytosis can be either constitutive or regulated. Constitutive secretion occurs, for example, in plasma cells that are secreting immunoglobulin or in fibroblasts secreting collagen. Regulated secretion occurs in endocrine cells, neurons, and exocrine glandular cells (e.g., pancreatic acinar cells). In these cells, the secretory product (e.g., hormone, neurotransmitter, or digestive enzyme), after synthesis and processing in the rough endoplasmic reticulum and Golgi apparatus, is stored in the cytoplasm in secretory granules until an appropriate signal for secretion is received. These signals may be hormonal or neural. Once the cell receives the appropriate stimulus, the secretory vesicle fuses with the plasma membrane and releases its contents into the extracellular fluid. Fusion of the vesicle with the membrane is mediated by a number of accessory proteins. One important group is the SNARE (soluble N-ethylmaleimide sensitive fusion protein [NSF] attachment protein receptors)


Physiology_Levy

Physiology_Levy. of the vesicle with the membrane is mediated by a number of accessory proteins. One important group is the SNARE (soluble N-ethylmaleimide sensitive fusion protein [NSF] attachment protein receptors) proteins. These membrane proteins help target the secretory


Physiology_Levy

Physiology_Levy. Receptor Formation of Formation of coated pit coated vesicle Dynamin Uncoated vesicle ready to fuse Vesicle (e.g., lysosome) uncoating • Fig. 1.5 Receptor-mediated endocytosis. Receptors on the surface of the cell bind the ligand. A clathrin-coated pit is formed with adaptin linking the receptor molecules to clathrin. Dynamin, a guanosine triphosphatase (GTPase), assists in separation of the endocytic vesicle from the membrane. Once inside the cell, the clathrin and adaptin molecules dissociate and are recycled. The uncoated vesicle is then ready to fuse with other organelles in the cell (e.g., lysosomes). (Adapted from Ross MH, Pawlina W: Histology. 5th ed. Baltimore: Lippincott Williams & Wilkins; 2006.)


Physiology_Levy

Physiology_Levy. Cholesterol is an important component of cells (e.g., it is a key component of membranes). However, most cells are unable to synthesize cholesterol and therefore must obtain it from the blood. Normally, cholesterol is ingested in the diet, and it is transported through the blood in association with lipoproteins. Low-density lipoproteins (LDLs) in the blood carry cholesterol to cells, where they bind to LDL receptors in the plasma membrane. After the receptors bind LDL, they collect into “coated pits” and undergo endocytosis as clathrin-coated vesicles. Once inside the cell, the endosomes release LDL and then recycle the LDL receptors back to the cell surface. Inside the cell, LDL is then degraded in lysosomes, and the cholesterol is made available to the cell. Defects in the LDL receptor prevent cellular uptake of LDL. Individuals with this defect have elevated levels of blood LDL, often called “bad cholesterol,” because it is associated with the development of cholesterol-containing


Physiology_Levy

Physiology_Levy. prevent cellular uptake of LDL. Individuals with this defect have elevated levels of blood LDL, often called “bad cholesterol,” because it is associated with the development of cholesterol-containing plaques in the smooth muscle layer of arteries. This process, atherosclerosis, is associated with an increased risk for heart attacks as a result of occlusion of the coronary arteries.


Physiology_Levy

Physiology_Levy. vesicle to the plasma membrane. The process of secretion is usually triggered by an increase in the concentration of intracellular Ca++ ([Ca++]). However, two notable exceptions to this general rule exist: (1) Renin secretion by the juxtaglomerular cells of the kidney occurs with a decrease in intracellular Ca++ (see ), as does (2) the secretion of parathyroid hormone by the parathyroid gland (see Basic Principles of Solute and Water Transport


Physiology_Levy

Physiology_Levy. Basic Principles of Solute and Water Transport As already noted, the plasma membrane, with its hydrophobic core, is an effective barrier to the movement of virtually all biologically important molecules into or out of the cell. Thus membrane transport proteins provide the pathway that allows transport to occur into and out of cells. However, the presence of a pathway is not sufficient for transport to occur; an appropriate driving force is also required. In this section, the basic principles of diffusion, active and passive transport, and osmosis are presented. These topics are discussed in greater depth, as appropriate, in the other sections of the book.


Physiology_Levy

Physiology_Levy. Diffusion is the process by which molecules move spontaneously from an area of high concentration to one of low concentration. Thus wherever a concentration gradient exists, diffusion of molecules from the region of high concentration to the region of low concentration dissipates the gradient (as discussed later, the establishment of concentration gradients for molecules requires the expenditure of energy). Diffusion is a random process driven by the thermal motion of the molecules. Fick’s first law of diffusion quantifies the rate at which a molecule diffuses from point A to point B: Equation 1.1 where J = the flux or rate of diffusion per unit time D = the diffusion coefficient A = area across which the diffusion is occurring ΔC = the concentration difference between point A and B ΔX = the distance along which diffusion is occurring


Physiology_Levy

Physiology_Levy. The diffusion coefficient takes into account the thermal energy of the molecule, its size, and the viscosity of the medium through which diffusion is taking place. For spherical molecules, D is approximated by the Stokes-Einstein equation: Equation 1.2 where k = Boltzmann’s constant T = temperature in degrees Kelvin r = radius of the molecule η= viscosity of the medium According to eqs. 1.1 and 1.2 , the rate of diffusion will be faster for small molecules than for large molecules. In addition, diffusion rates are high at elevated temperatures, in the presence of large concentration gradients, and when diffusion occurs in a low-viscosity medium. With all other variables held constant, the rate of diffusion is linearly related to the concentration gradient.


Physiology_Levy

Physiology_Levy. Fick’s equation can also be applied to the diffusion of molecules across a barrier, such as a lipid bilayer. When applied to the diffusion of a molecule across a bilayer, the diffusion coefficient (D) incorporates the properties of the bilayer and especially the ability of the molecule to diffuse through the bilayer. To quantify the interaction of the molecule with the bilayer, the term partition coefficient (β) is used. For a molecule that “dissolves” equally in the fluid bathing the lipid bilayer (e.g., water) and in the lipid bilayer, β= 1. If the molecule dissolves more easily in the lipid bilayer, β> 1; and if it dissolves less easily in the lipid bilayer, β< 1. For a simple lipid bilayer, the more lipid soluble the molecule is, the larger the partition coefficient is, and thus the diffusion coefficient—therefore the rate of diffusion of the molecule across the bilayer—is greater. In this situation, ΔC represents the concentration difference across the membrane, A is the membrane


Physiology_Levy

Physiology_Levy. coefficient—therefore the rate of diffusion of the molecule across the bilayer—is greater. In this situation, ΔC represents the concentration difference across the membrane, A is the membrane area, and ΔX is the thickness of the membrane.


Physiology_Levy

Physiology_Levy. Another useful equation for quantitating the diffusion of molecules across the plasma membrane (or any membrane) is as follows: Equation 1.3 J =−PC( i − Co), where J = the flux or rate of diffusion across the membrane P = is the permeability coefficient Ci = concentration of the molecule inside the cell Co = the concentration of the molecule outside the cell This equation is derived from Fick’s equation ( eq. 1.1 ). P incorporates D, ΔX, A, and the partition coefficient (β). P is expressed in units of velocity (e.g., centimeters per second), and C the units of moles/cm3. Thus the units of flux are moles per square centimeter per second (mol/cm2/ sec). Values for P can be obtained experimentally for any molecule and bilayer.


Physiology_Levy

Physiology_Levy. As noted, the phospholipid portion of the plasma membrane represents an effective barrier to many biologically important molecules. Consequently, diffusion through the lipid phase of the plasma membrane is not an efficient process for movement of these molecules across the membrane. It has been estimated that for a cell 20 µm in diameter, with a plasma membrane composed only of phospholipids, dissipation of a urea gradient imposed across the membrane would take approximately 8 minutes. Similar gradients for glucose and amino acids would take approximately 14 hours to dissipate, whereas ion gradients would take years to dissipate.


Physiology_Levy

Physiology_Levy. As noted previously, the vast majority of biologically important molecules cross cell membranes via specific membrane transporters, rather than by diffusing through the lipid portion of the membrane. Nevertheless, eq. 1.3 can be and has been used to quantitate the diffusion of molecules across many biological membranes. When this is done, the value of the permeability coefficient (P) reflects the properties of the pathway (e.g., membrane transporter or, in some cases, multiple transporters) that the molecule uses to cross the membrane.


Physiology_Levy

Physiology_Levy. Despite the limitations of using diffusion to describe and understand the transport of molecules across cell membranes, it is also important for understanding gas exchange in the lungs (see ), the movement of molecules through the cytoplasm of the cell, and the movement of molecules between cells in the extracellular fluid. For example, one of the physiological responses of skeletal muscle to exercise is the recruitment or opening of capillaries that are not perfused at rest. This opening of previously closed capillaries increases capillary density and thereby reduces the diffusion distance between the capillary and the muscle fiber so that oxygen and cellular fuels (e.g., fatty acids and glucose) can be delivered more quickly to the contracting muscle fiber. In resting muscle, the average distance of a muscle fiber from a capillary is estimated to be 40 µm. However, with exercise, this distance decreases to 20 µm or less.


Physiology_Levy

Physiology_Levy. The electrochemical gradient (also called the electrochemical potential difference) is used to quantitate the driving force acting on a molecule to cause it to move across a membrane. The electrochemical gradient for any molecule (Δµx) is calculated as follows: Equation 1.4 ˜µx = RTln + zFV ,[X]o xm where R = the gas constant T = temperature in degrees Kelvin Ln = natural logarithm [X]i = the concentration of X inside the cell [X]o = the concentration of X outside the cell zx = the valence of charged molecules F = the Faraday constant Vm = the membrane potential (Vm = Vi − Vo)


Physiology_Levy

Physiology_Levy. The electrochemical gradient is a measure of the free energy available to carry out the useful work of transporting the molecule across the membrane. It has two components: One component represents the energy in the concentration gradient for X across the membrane (chemical potential bBy convention, membrane voltages are determined and reported with regard to the exterior of the cell. In a typical cell, the resting membrane potential (Vm) is negative. Positive Vm values can be observed in some excitable cells at the peak of an action potential.


Physiology_Levy

Physiology_Levy. CHAPTER 1 Principles of Cell and Membrane Function difference). The second component (electrical potential difference) represents the energy associated with moving charged molecules (e.g., ions) across the membrane when a membrane potential exits (i.e., Vm ≠ 0 mV). Thus for the movement of glucose across a membrane, only the concentrations of glucose inside and outside of the cell need to be considered (Fig. 1.6A ). However, the movement of K+ across the membrane, for example, would be determined both from the K+ concentrations inside and outside of the cell and from the membrane voltage (see Fig. 1.6B Eq. 1.4 can be used to derive the Nernst equation for the situation in which a molecule is at equilibrium across the membrane (i.e., Δµ = 0): Equation 1.5a Equation 1.5b


Physiology_Levy

Physiology_Levy. Fig. 1.6B Eq. 1.4 can be used to derive the Nernst equation for the situation in which a molecule is at equilibrium across the membrane (i.e., Δµ = 0): Equation 1.5a Equation 1.5b The value of Vm calculated with the Nernst equation represents the equilibrium condition and is referred to as the Nernst equilibrium potential (Ex, the Vm at which there is no net transport of the molecule across the membrane). It should be apparent that the Nernst equilibrium potential quantitates the energy in a concentration gradient and expresses that energy in millivolts. For example, for the cell depicted in


Physiology_Levy

Physiology_Levy. Fig. 1.6B , the energy in the K+ gradient (derived from the Nernst equilibrium potential for K+ [EK+]) is proportional to 90.8 mV (causing K+ to move out of the cell). This is opposite to, and of greater magnitude than, the energy in the membrane voltage (Vm =−60 mV), which causes K+ to enter the cell. As a result, the electrochemical gradient is such that the net movement of K+ across the membrane will be out of the cell. Another way to state this is that the net driving force for K+ (Vm − EK+) is 30.8 mV (driving K+ out of the cell). This is described in more detail in The Nernst equation, at 37° C, can be written as follows by replacing the natural logarithm function with the base 10 logarithm function: Equation 1.6a 61 5. mV [X]i


Physiology_Levy

Physiology_Levy. The Nernst equation, at 37° C, can be written as follows by replacing the natural logarithm function with the base 10 logarithm function: Equation 1.6a 61 5. mV [X]i Equation 1.6b 615. mV [X]E = log o x zx [X]i •Fig. 1.6 Electrochemicalgradientsandcellulartransportofmolecules.A, Becauseglucoseisuncharged,theelectrochemicalgradientisdeterminedsolelybytheconcentrationgradientforglucoseacrossthecellmembrane.Asshown,theglucoseconcentrationgradientwouldbeexpectedtodriveglucoseintothecell.B, BecauseK+ ischarged,theelectrochemicalgradientisdeterminedbyboththeconcentrationgradientandthemembranevoltage(Vm).TheNernstequilibriumpotentialforK+ (EK+ ),calculatedwith eq.1.5a ,is−90.8mV(EK+= Vm atequilibrium).Theenergyintheconcentrationgradient,whichdrivesK+ outofthecell,isthusproportionalto+90.8mV.Themembranevoltageof−60mVdrivesK+ intothecell.Thustheelectrochemicalgradient,ornetdrivingforce,is2.97kJ/mol(equivalentto30.8mV),whichdrivesK+ outofthecell.


Physiology_Levy

Physiology_Levy. These are the most common forms of the Nernst equation in use. In these equations, it is apparent that for a univalent ion (e.g., Na+ , K+ , Cl−), a 10-fold concentration gradient across the membrane is equivalent in energy to an electrical potential difference of 61.5 mV (at 37° C), and a 100-fold gradient is equivalent to an electrical potential difference of 123 mV. Similarly, for a divalent ion (e.g., Ca++), a 10-fold concentration gradient is equivalent to a 30.7-mV electrical potential difference, because z in eqs. 1.6a and 1.6b is equal to 2. When the net movement of a molecule across a membrane occurs in the direction predicted by the electrochemical gradient, that movement is termed passive transport. Thus for the examples given in


Physiology_Levy

Physiology_Levy. When the net movement of a molecule across a membrane occurs in the direction predicted by the electrochemical gradient, that movement is termed passive transport. Thus for the examples given in Fig. 1.6 , the movement of glucose into the cell and the movement of K+ out of the cell would be considered passive transport. Transport that is passive is sometimes referred to as either “downhill transport” or “transport with the electrochemical gradient.” In contrast, if the net movement of a molecule across the membrane is opposite to that predicted by the electrochemical gradient, that movement is termed active transport, a process that requires the input of energy (e.g., ATP). Active transport is sometimes referred to as either “uphill transport” or “transport against the electrochemical gradient.”


Physiology_Levy

Physiology_Levy. In the various classes of plasma membrane transport proteins, the movement of H2O through water channels is a passive process (see later discussion), as is the movement of ions through ion channels and the transport of molecules via uniporters (e.g., transport of glucose via GLUT-1). The ATPase-dependent transporters can use the energy in ATP to drive active transport of molecules (e.g., Na+,K+-ATPase, H+-ATPase, or ABC transporters). Because the transport is directly coupled to the hydrolysis of ATP, it is referred to as primary active transport. Solute carriers that couple movement of two or more molecules (e.g., 3Na+,Ca++ antiporter) often transport one or more molecules (one Ca++ molecule in this example) against their respective electrochemical gradient through the use of the energy in the electrochemical gradient of the other molecule or molecules (three Na+ in this example). When this occurs, the molecule or molecules transported against their electrochemical gradient are said


Physiology_Levy

Physiology_Levy. in the electrochemical gradient of the other molecule or molecules (three Na+ in this example). When this occurs, the molecule or molecules transported against their electrochemical gradient are said to be transported by secondary active transport mechanisms (


Physiology_Levy

Physiology_Levy. Fig. 1.7 The movement of water across cell membranes occurs by the process of osmosis. The movement of water is passive, with the driving force for this movement being the osmotic pressure difference across the cell membrane. Fig. 1.8 illustrates the concept of osmosis and the measurement of the osmotic pressure of a solution. Osmotic pressure is determined by the number of solute molecules dissolved in the solution. It is not dependent on such factors as the size of the molecules, their mass, or their chemical nature (e.g., valence). Osmotic pressure (π), measured in atmospheres (atm), is calculated by van’t Hoff’s law as follows: Equation 1.7 ˜= nCRT, CHAPTER 1 Principles of Cell and Membrane Function


Physiology_Levy

Physiology_Levy. Equation 1.7 ˜= nCRT, CHAPTER 1 Principles of Cell and Membrane Function K+ Na+ 3Na+ Na+, K+ -ATPase 3Na+-Ca++ antiporter IntracellularconcentrationsExtracellularconcentrations3Na+ Na+: 145 mEq/L K+: 4 mEq/L Glucose: 5 mmol/L Ca++: 2.5 mEq/L (ionized) Na+: 12 mEq/L K+: 120 mEq/L Glucose: 2 mmol/L Ca++: 0.001 mEq/L (ionized) Ca++ Secondary active transport of Ca++ Primary active transport of Na+ and K+ Passive transport Na+ channel K+ channel Glucose uniporter Glucose 2K+ ATP •Fig. 1.7 Examplesofseveralmembranetransporters,illustratingprimaryactive,passive,andsecondaryactivetransport.Seetextfordetails.ATP,adenosinetriphosphate.


Physiology_Levy


Physiology_Levy

Physiology_Levy. Thus,iftheintracellularglucoseconcentrationwas2mmol/L,thecellcouldlowertheextracellularglucoseconcentrationtoapproximately0.02mmol/L.However,byincreasingthenumberofNa+ ionstransportedwithglucosefromonetotwo,SGLT-1cangenerateanearly10,000-foldglucosegradient: 615. mV Again,iftheintracellularglucoseconcentrationis2mmol/L,SGLT-1couldremovevirtuallyallglucosefromeitherthelumenofthesmallintestineorthelumenoftheproximaltubule(i.e.,theluminalglucoseconcentration≅ 0.0002mmol/L). where molecules at 37°C can exert an osmotic pressure of 2.54 n = number of dissociable particles per molecule × 10−2 atm, as calculated with eq. 1.7 and the following C = total solute concentration values: R = gas constant n = 1 T = temperature in degrees Kelvin C = 0.001 mol/L


Physiology_Levy

Physiology_Levy. For a molecule that does not dissociate in water, such as R = 0.082 atm L/mol K glucose or urea, a solution containing 1 mmol/L of these T = 310 °K •Fig. 1.8


Physiology_Levy

Physiology. 4thed.St.Louis:Mosby;2006.)

Physiology_Levy. Physiology. 4thed.St.Louis:Mosby;2006.)


Physiology_Levy

Physiology_Levy. Because 1 atm equals 760 mm Hg at sea level, π for this solution can also be expressed as 19.3 mm Hg. Alternatively, osmotic pressure is expressed in terms of osmolarity (see the following section). Regardless of the molecule, a solution containing 1 mmol/L of the molecule therefore exerts an osmotic pressure proportional to 1 mOsm/L. For molecules that dissociate in a solution, n of eq. 1.7 will have a value other than 1. For example, a 150-mmol/L solution of NaCl has an osmolarity of approximately 300 mOsm/L because each molecule of NaCl dissociates into a Na+ and a Cl− ion (i.e., n = 2). If dissociation of a molecule into its component ions is not complete, n will not be an integer. Accordingly, osmolarity for any solution can be calculated as follows: Equation 1.8 Osmolarity = concentration × number of dissociable particles mOsm/L = mmol/L × number of particles


Physiology_Levy

Physiology_Levy. Equation 1.8 Osmolarity = concentration × number of dissociable particles mOsm/L = mmol/L × number of particles The terms osmolarity and osmolality are frequently confused and incorrectly interchanged. Osmolarity refers to the osmotic pressure generated by the dissolved solute molecules in 1 L of solvent, whereas osmolality is the number of molecules dissolved in 1 kg of solvent. For a dilute solution, the difference between osmolarity and osmolality is cNaCl does not completely dissociate in water. The value for n is 1.88 rather than 2. However, for simplicity, the value of 2 is most often used.


Physiology_Levy

Physiology_Levy. insignificant. Measurements of osmolarity are temperature dependent because the volume of the solvent varies with temperature (i.e., the volume is larger at higher temperatures). In contrast, osmolality, which is based on the mass of the solvent, is temperature independent. For this reason, osmolality is the preferred term for biologic systems and is used throughout this book. Because the solvent in biological solutions and bodily fluids is water, and because of the dilute nature of biological solutions and bodily solutions, osmolalities are expressed as milliosmoles per kilogram of water (mOsm/kg H2O).


Physiology_Levy

Physiology_Levy. The tonicity of a solution is related to the effect of the solution on the volume of a cell. Solutions that do not change the volume of a cell are said to be isotonic. A hypotonic solution causes a cell to swell, whereas a hypertonic solution causes a cell to shrink. Although related to osmolality, tonicity also accounts for the ability of the molecules in solution to cross the cell membrane.


Physiology_Levy

Physiology_Levy. Consider two solutions: a 300-mmol/L solution of sucrose and a 300-mmol/L solution of urea. Both solutions have an osmolality of 300 mOsm/kg H2O and therefore are said to be isosmotic (i.e., they have the same osmolality). When red blood cells—which for the purpose of this illustration also have an intracellular fluid osmolality of 300 mOsm/kg H2O—are placed in the two solutions, those in the sucrose solution maintain their normal volume, whereas those placed in urea swell and eventually burst. Thus the sucrose solution is isotonic and the urea solution is hypotonic. The differential effect of these solutions on


Physiology_Levy

Physiology_Levy. CHAPTER 1 Principles of Cell and Membrane Function red blood cell volume is related to the permeability of the red blood cell plasma membrane to sucrose and urea. The red blood cell membrane contains uniporters for urea. Thus urea easily crosses the cell membrane (i.e., the cell is permeable by urea), driven by the concentration gradient (i.e., extracellular urea concentration > intracellular urea concentration). In contrast, the red blood cell membrane does not contain sucrose transporters, and sucrose cannot enter the cell (i.e., the cell is impermeable by sucrose).


Physiology_Levy

Physiology_Levy. To exert an osmotic pressure across a membrane, a molecule must not cross the membrane. Because the red blood cell membrane is impermeable by sucrose, it exerts an osmotic pressure equal and opposite to the osmotic pressure generated by the contents within the red blood cell (in this case, 300 mOsm/kg H2O). In contrast, urea is readily able to cross the red blood cell membrane, and it cannot exert an osmotic pressure to balance that generated by the intracellular solutes of the red blood cell. Consequently, sucrose is termed an effective osmole, whereas urea is an ineffective osmole. To take into account the effect of a molecule’s ability to permeate the membrane on osmotic pressure, it is necessary to rewrite eq. 1.7 as follows: Equation 1.9 ˜e = ˙(nCRT), where σ is the reflection coefficient (or osmotic coefficient) and is a measure of the relative ability of the molecule to cross the cell membrane, and Πe is the “effective osmotic pressure.”


Physiology_Levy

Physiology_Levy. For a molecule that can freely cross the cell membrane, such as urea in the preceding example, σ= 0, and no effective osmotic pressure is exerted (e.g., urea is an ineffective osmole for red blood cells). In contrast, σ= 1 for a solute that cannot cross the cell membrane (in the preceding example, sucrose). Such a substance is said to be an effective osmole. Many molecules are neither completely able nor completely unable to cross cell membranes (i.e., 0 <σ< 1) and generate an osmotic pressure that is only a fraction of what is expected from the molecules’ concentration in solution. Oncotic pressure is the osmotic pressure generated by large molecules (especially proteins) in solution. As illustrated in


Physiology_Levy

Physiology_Levy. Oncotic pressure is the osmotic pressure generated by large molecules (especially proteins) in solution. As illustrated in Fig. 1.9 , the magnitude of the osmotic pressure generated by a solution of protein does not conform to van’t Hoff’s law. The cause of this anomalous relationship between protein concentration and osmotic pressure is not completely understood, but it appears to be related to the size and shape of the protein molecule. For example, the correlation to van’t Hoff’s law is more precise with small, globular proteins than with larger protein molecules.


Physiology_Levy

Physiology_Levy. The oncotic pressure exerted by proteins in human plasma has a normal value of approximately 26 to 28 mm Hg. Although this pressure appears to be small in relation to osmotic pressure (28 mm Hg ≅ 1.4 mOsm/kg H2O), it is an important force involved in fluid movement across capillaries (see •Fig. 1.9 Relationshipbetweentheconcentrationofplasmapro-teinsinsolutionandtheosmoticpressure(oncoticpressure)theygenerate.Proteinconcentrationisexpressedingramsperdeciliter.Normalplasmaproteinconcentrationisindicated.Notehowtheactualpressuregeneratedexceedsthatpredictedbyvan’tHoff’slaw.80 60 40 20 0 2 6 10 14 Protein (g/dl) Predicted by van’t Hoff’s law Normal plasma ActualOsmotic pressure (g/dL)


Physiology_Levy

Physiology_Levy. The total concentration of all molecules in a solution can also be measured as specific gravity. Specific gravity is defined as the weight of a volume of solution divided by the weight of an equal volume of distilled water. Thus the specific gravity of distilled water is 1. Because biological fluids contain a number of different molecules, their specific gravities are greater than 1. For example, normal human plasma has a specific gravity in the range of 1.008 to 1.010.


Physiology_Levy

Physiology_Levy. Thespecificgravityofurineissometimesmeasuredinclinicalsettingsandusedtoassesstheurineconcentratingabilityofthekidneys.Thespecificgravityofurinevariesinproportiontoitsosmolality.However,becausespecificgravitydependsbothonthenumberofmoleculesandontheirweight,therelationshipbetweenspecificgravityandosmolalityisnotalwayspredictable.Forexample,inpatientswhohavereceivedaninjectionofradiocontrastdye(molecularweight> 500g/mole)forx-raystudies,valuesofurinespecificgravitycanbehigh(1.040to1.050),eventhoughtheurineosmolalityissimilartothatofplasma(e.g.,300mOsm/kgH2O). The plasma membrane is a lipid bilayer composed of phospholipids and cholesterol, into which are embedded a wide range of proteins. One class of these membrane proteins (membrane transport proteins or transporters) is involved in the selective and regulated transport of molecules into and out of the cell. These transporters include water channels (aquaporins), ion channels, solute carriers, and ATP-dependent transporters.


Physiology_Levy

Physiology_Levy. The movement of molecules across the plasma membrane through ion channels and solute carriers is driven by chemical concentration gradients and, chemical concentration gradients and electrical potential differences (charged molecules only). The electrochemical gradient is used to quantitate this driving force. ATP-dependent transporters use the energy in ATP to transport molecules across the membrane and often establish the chemical Alberts B, et al. Essential Cell Biology. 4th ed. New York: Garland Science; 2014. Altenberg GA, Ruess L. Mechanisms of water transport across cell membranes and epithelia. In: Alpern R, Moe O, Kaplan M, eds. Seldin and Giebisch’s The Kidney—Physiology and Pathophysiology. 5th ed. New York: Academic Press; 2013. Hediger MA, et al. The ABCs of membrane transporters in health and disease (SLC series): introduction. Mol Aspects Med. 2013; 34:95-107.


Physiology_Levy

Physiology_Levy. Hediger MA, et al. The ABCs of membrane transporters in health and disease (SLC series): introduction. Mol Aspects Med. 2013; 34:95-107. and electrical gradients that then drive the transport of other molecules through channels and by the solute carriers. Water movement through aquaporins is driven by an osmotic pressure difference across the membrane.


Physiology_Levy

Physiology_Levy. • Transport across the membrane is classified as passive or active. Passive transport is the movement of molecules as expected from the electrochemical gradient for that molecule. Active transport represents transport against the electrochemical gradient. Active transport is further divided into primary active and secondary active transport. Primary active transport is directly coupled to the hydrolysis of ATP (e.g., ATP-dependent transporters). Secondary active transport occurs with coupled solute carriers, for which passive movement of one or more molecules drives the active transport of other molecules (e.g., Na+-glucose symporter, Na+-H+ antiporter). Pawlina W. Histology: A Text and Atlas with Correlated Cell and Molecular Biology. 7th ed. Alphen aan den Rijn, The Netherlands: Wolters Kluwer; 2016. Rojeck A, et al. A current view of mammalian aquaglyceroporins. Annu Rev Physiol. 2008;70:301-327.


Physiology_Levy

Physiology_Levy. Rojeck A, et al. A current view of mammalian aquaglyceroporins. Annu Rev Physiol. 2008;70:301-327. Ruess L, Altenberg GA. Mechanisms of ion transport across cell membranes. In: Alpern R, Moe O, Kaplan M, eds. Seldin and Giebisch’s The Kidney—Physiology and Pathophysiology. 5th ed. New York: Academic Press; 2013. Upon completion of this chapter, the student should be able to answer the following questions: 1. What is steady-state balance, and, with water balance as an example, what are the elements needed to achieve steady-state balance? 2. What are the volumes of the body fluid compartments, and how do they change under various conditions? 3. How do the body fluid compartments differ with regard to their composition? 4. What determines the resting membrane potential of cells? 5. How do cells regulate their volume in isotonic, hypotonic, and hypertonic solutions? 6.


Physiology_Levy

Physiology_Levy. 4. What determines the resting membrane potential of cells? 5. How do cells regulate their volume in isotonic, hypotonic, and hypertonic solutions? 6. What are the structural features of epithelial cells, how do they carry out vectorial transport, and what are the general mechanisms by which transport is regulated?


Physiology_Levy

Physiology_Levy. ormal cellular function requires that the intracellular composition—with regard to ions, small molecules, water, pH, and a host of other substances—be maintained within a narrow range. This is accomplished by the transport of many substances and water into and out of the cell via membrane transport proteins, as described in . In addition, each day, food and water are ingested, and waste products are excreted from the body. In a healthy individual, these processes occur without significant changes in either the volume of the body fluid compartments or their composition. The maintenance of constant volume and composition of the body fluid compartments (and their temperature in warm-blooded animals and humans) is termed homeostasis. The human body has multiple systems designed to achieve homeostasis, the details of which are explained in the various chapters of this book. In this chapter, the basic principles that underlie the maintenance of homeostasis are outlined. In addition, the


Physiology_Levy

Physiology_Levy. homeostasis, the details of which are explained in the various chapters of this book. In this chapter, the basic principles that underlie the maintenance of homeostasis are outlined. In addition, the volume and composition of the various body fluid compartments are defined.


Physiology_Levy

Physiology_Levy. Concept of Steady-State Balance The human body is an “open system,” which means that substances are added to the body each day and, similarly, substances are lost from the body each day. The amounts added to or lost from the body can vary widely, depending on the environment, access to food and water, disease processes, and even cultural norms. In such an open system, homeostasis occurs through the process of steady-state balance.


Physiology_Levy

Physiology_Levy. To illustrate the concept of steady-state balance, consider a river on which a dam is built to create a synthetic lake. Each day, water enters the lake from the various streams and rivers that feed it. In addition, water is added by underground springs, rain, and snow. At the same time, water is lost through the spillways of the dam and by the process of evaporation. For the level of the lake to remain constant (i.e., steady-state balance), the rate at which water is added, regardless of source, must be exactly matched by the amount of water lost, again regardless of route. Because the addition of water is not easily controlled and the loss by evaporation cannot be controlled, the only way to maintain a constant level of the lake is to regulate the amount that is lost through the spillways. To understand steady-state balance as it applies to the human body, the following key concepts are important. 1.


Physiology_Levy

Physiology_Levy. To understand steady-state balance as it applies to the human body, the following key concepts are important. 1. There must be a “set point” so that deviations from this baseline can be monitored (e.g., the level of the lake in the preceding example, or setting the temperature in a room by adjusting the thermostat). 2. The sensor or sensors that monitor deviations from the set point must generate “effector signals” that can lead to changes in either input or output, or both, to maintain the desired set point (e.g., electrical signals to adjust the spillway in the dam analogy, or electrical signals sent to either the furnace or air conditioner to maintain the proper room temperature). 3. “Effector organs” must respond in an appropriate way to the effector signals generated by the set point monitor (i.e., the spillway gates must operate, and the furnace or air conditioner must turn on). 4.


Physiology_Levy

Physiology_Levy. 4. The sensitivity of the system (i.e., how much of a deviation from the set point is tolerated) depends on several factors, including the nature of the sensor (i.e., how much of a deviation from the set point is needed for the sensor to detect the deviation), the time necessary for generation of the effector signals, and how rapidly the effector organs respond to the effector signals. 1. 2. 1. 2. 3. 4. •Fig. 2.1 Whole-Body Steady-State Water Balance. Seetextfordetails.ADH,antidiuretichormone(alsocalledargininevasopressin);CNS,centralnervoussystem;GI,gastrointestinal. It is important to recognize that deviations from steady-state balance do occur. When input is greater than output, a state of positive balance exists. When input is less than output, a state of negative balance exists. Although transient periods of imbalance can be tolerated, prolonged states of positive or negative balance are generally incompatible with life.


Physiology_Levy

Physiology_Levy. Fig. 2.1 illustrates several important concepts for the maintenance of steady-state water balance (details related to the maintenance of steady-state water balance are presented in ). As depicted in Fig. 2.1 , there are multiple inputs and outputs of water, many of which can vary but nevertheless cannot be regulated. For example, the amount of water lost through the lungs depends on the humidity of the air and the rate of respiration (e.g., low humidity and rapid breathing increase water loss from the lungs). Similarly, the amount of water lost as sweat varies according to ambient temperature and physical activity. Finally, water loss via the gastrointestinal tract can increase from a normal level of 100 to 200 mL/day to many liters with acute diarrhea. Of these inputs and outputs, the only two that can be regulated are increased ingestion of water in response to thirst and alterations in urine output by the kidneys (see Chapter 35).


Physiology_Levy

Physiology_Levy. Chapter 35). Water balance determines the osmolality of the body fluids. Cells within the hypothalamus of the brain monitor body fluid osmolality for deviations from the set point (normal range: 280-295 mOsm/kg H2O). When deviations are sensed, two effector signals are generated. One is neural and relates to the individual’s sensation of thirst. The other is hormonal (antidiuretic hormone, also called arginine vasopressin), which regulates the amount of water excreted by the kidneys. With appropriate responses to these two signals, water input, water output, or both are adjusted to maintain balance and thereby keep body fluid osmolality at the set point. Volumes and Composition of Body Fluid Compartments


Physiology_Levy

Physiology_Levy. Unicellular organisms maintain their volume and composition through exchanges with the environment they inhabit (e.g., sea water). The billions of cells that constitute the human body must maintain their volume and composition as well, but their task is much more difficult. This challenge, as well as its solution, was first articulated by the French physiologist Claude Bernard (1813-1878). He recognized that although cells within the body cannot maintain their volume and composition through exchanges with the environment, they can do so through exchanges with the fluid environment that surrounds them (i.e., the extracellular fluid). Bernard referred to the extracellular fluid as the milieu intérieur (“the environment within”). He also recognized that the organ systems of the body are designed and function to maintain a constant milieu interieur or a “constant internal environment.” This in turn allows all cells to maintain their volume and composition through exchanges with the


Physiology_Levy

Physiology_Levy. designed and function to maintain a constant milieu interieur or a “constant internal environment.” This in turn allows all cells to maintain their volume and composition through exchanges with the extracellular fluid as a result of membrane transport (see


Physiology_Levy

Physiology_Levy. Transport by the epithelial cells of the gastrointestinal tract, kidneys, and lungs are the body’s interface with the CHAPTER 2 Homeostasis: Volume and Composition of Body Fluid Compartments external environment and control both the intake and excretion of numerous substances, as well as water. The cardiovascular system delivers nutrients to and removes waste products from the cells and tissues and keeps the extracellular fluid well mixed. Finally, the nervous and endocrine systems provide regulation and integration of these important functions.


Physiology_Levy

Physiology_Levy. To provide background for the study of all organ systems, this chapter presents an overview of the normal volume and composition of the body fluid compartments and describes how cells maintain their intracellular composition and volume. Included is a presentation on how cells generate and maintain a membrane potential, which is fundamental for understanding the function of excitable cells (e.g., neurons and muscle cells). Finally, because epithelial cells are so central to the process of regulating the volume and composition of the body fluids, the principles of solute and water transport by epithelial cells are also reviewed. Definition and Volumes of Body Fluid Compartments


Physiology_Levy

Physiology_Levy. Definition and Volumes of Body Fluid Compartments Water makes up approximately 60% of the body’s weight; variability among individuals is a function of the amount of adipose tissue. Because the water content of adipose tissue is lower than that of other tissue, increased amounts of adipose tissue reduce the fraction of water in the total body weight. The percentage of body weight attributed to water also varies with age. In newborns, it is approximately 75%. This decreases to the adult value of 60% by the age of 1 year. Fig. 2.2, total body water is distributed between two major compartments, which are divided by the cell membrane. The intracellular fluid (ICF) compartment is the larger compartment, and contains approximately two thirds of the total body water. The remaining third is contained in the extracellular fluid (ECF) compartment. Expressed as percentages of body weight, the volumes of total body water, ICF, and ECF are as follows:


Physiology_Levy

Physiology_Levy. Total body water = 0 6. × (body weight) ICF = 0 4. × (body weight) ECF = 0 2. × (body weight) The ECF compartment is further subdivided into interstitial fluid and plasma. The ECF also includes fluid contained within bone and dense connective tissue, as well as the cerebrospinal fluid. The interstitial fluid surrounds the cells in the various tissues of the body and makes up three fourths of the ECF volume. Plasma is contained within the vascular compartment and represents the remaining fourth of the ECF. In some pathological conditions, additional fluid may accumulate in what is referred to as a third space. Third-space collections of fluid are part of the ECF; an aIn these and all subsequent calculations, it is assumed that 1 L of fluid (e.g., ICF and ECF) has a mass of 1 kg. Although, 1 L of the ICF and ECF has a mass of slightly more than 1 kg, this simplification allows conversion from measurements of body weight to volume of body fluids.


Physiology_Levy

Physiology_Levy. Total body water (TBW) 0.6 x body weight 42 L Extracellular fluid (ECF) 0.2 x body weight 14 L Intracellular fluid (ICF) 0.4 x body weight 28 L Cell membrane Interstitial fluid 3/4 of ECF 10.5 L Plasma 1/4 of ECF 3.5 L •Fig. 2.2 Relationship Between the Volumes of the Various Body Fluid Compartments. Theactualvaluesshownareforanindividualweighing70kg.(ModifiedfromLevyMN,KoeppenBM,StantonBA.Berne & Levy’s Principles of Physiology.4thed.St.Louis:Mosby;2006.) example is the accumulation of fluid in the peritoneal cavity (ascites) of individuals with liver disease. Movement of Water Between Body Fluid Compartments Fig. 2.2 , water moves between the ICF and ECF compartments across the plasma membranes of cells, and it moves between the vascular (plasma) and interstitial compartments across capillary walls. The pathways and driving forces for this water movement are different across cell membranes, in comparison to the capillary walls.


Physiology_Levy

Physiology_Levy. Movement of water between the ICF and ECF compartments, across cell membranes, occurs through aquaporins expressed in the plasma membrane (see Chapter 1).


Physiology_Levy

Physiology_Levy. The driving force for this water movement is an osmotic pressure difference. The osmotic pressure of both the ICF and ECF is determined by the molecules/ions present in these fluids. For simplicity, these can be divided into (1) molecules of low molecular weight (e.g., glucose) and ions (e.g., Na+) and (2) macromolecules (e.g., proteins). The osmotic pressures of both the ICF and ECF are in the range of 280 to 295 mOsm/kg H2O. For the ECF, the low-molecular-weight molecules and ions account for nearly all of this pressure because the osmotic pressure contributed by proteins is only 1 to 2 mOsm/kg H2O. The molecules/ions contributing to the osmotic pressure within the cell are less well understood, but they also include low-molecular-weight molecules (e.g., glucose), ions (e.g., Na+), and macromolecules (e.g., proteins). The fact that cell volume remains constant when ECF osmolality is constant means that the osmotic pressure inside the cells is equal to that of the ECF. If an osmotic


Physiology_Levy

Physiology_Levy. macromolecules (e.g., proteins). The fact that cell volume remains constant when ECF osmolality is constant means that the osmotic pressure inside the cells is equal to that of the ECF. If an osmotic pressure difference did exist, the cells would either swell or shrink, as described in the section “Nonisotonic Cell Volume Regulation.”


Physiology_Levy

Physiology_Levy. Movement of water between the vascular (plasma) compartment and the interstitial fluid compartment occurs across the capillary wall. The amount of water that moves across the capillary wall, and the mechanism of the water movement varies depending on the capillary. For example, in the capillary sinusoids of the liver, endothelial cells are often separated by large gaps (discontinuous capillary). As a result, water and all components of the plasma (and some cellular elements) can pass easily across the wall. Other capillaries are lined by endothelial cells that contain fenestrations that are up to 80 to 100 nm in diameter (e.g., in the kidneys). These fenestrations allow all components of the plasma (only cellular elements of blood cannot pass through the fenestrations) to move across the capillary wall. Some capillaries (e.g., in the brain) form a relatively tight barrier to water and small molecules and ions, and water movement occurs through small pores on the endothelial cell


Physiology_Levy

Physiology_Levy. the capillary wall. Some capillaries (e.g., in the brain) form a relatively tight barrier to water and small molecules and ions, and water movement occurs through small pores on the endothelial cell surface or through clefts between adjacent endothelial cells. These pores and clefts allow water and molecules smaller than 4 nm to pass. In addition, a small amount of water traverses the capillary wall via pinocytosis by endothelial cells.


Physiology_Levy

Physiology_Levy. The driving forces for fluid (water) movement across the capillary wall are hydrostatic pressure and oncotic pressure (i.e., osmotic pressure generated by proteins). Collectively, these are called the Starling forces. Capillary fluid movement is discussed in detail in in brief, hydrostatic pressure within the capillary (as a result of the pumping of the heart and the effect of gravity on the column of blood in the vessels feeding a capillary) is a force that causes fluid to move out of the capillary. Hydrostatic pressure in the surrounding interstitial tissue opposes the effect of the capillary hydrostatic pressure. The oncotic pressure of the plasma in the capillary tends to draw fluid from the interstitium into the capillary. The oncotic pressure of the interstitial fluid opposes this. Thus the amount of fluid moving across the wall of the capillary is determined as follows: Equation 2.1


Physiology_Levy

Physiology_Levy. Equation 2.1 Kf = filtration constant (measure of surface area + in πi = interstitial fluid oncotic pressure. Depending on the magnitude of these forces, fluid may move out of the capillary or into the capillary. The compositions of the various body fluid compartments differ; however, as described later, the osmolalities of the fluid within these compartments are essentially identical. Thus the compartments are in “osmotic equilibrium.” In addition, any change in the osmolality of one compartment quickly causes water to redistribute across all compartments, which brings them back into osmotic equilibrium. Because of this rapid redistribution of water, measuring the osmolality of plasma or serum, which is easy to do, reveals the osmolality of the other body fluid compartments (i.e., interstitial fluid and intracellular fluid).


Physiology_Levy

Physiology_Levy. As described later, Na+ is a major constituent of the ECF. Because of its high concentration in comparison with other molecules and ions, Na+ (and its attendant anions, primarily Cl− and HCO3 −) is the major determinant of the osmolality of this compartment. Accordingly, it is possible to obtain an approximate estimate of the ECF osmolality by simply doubling the sodium concentration [Na+]. For example, if a blood sample is obtained from an individual, and the [Na+] of the serum is 145 mEq/L, its osmolality can be estimated as follows: Equation 2.2 In contrast to water, the movement of ions across cell membranes is more variable from cell to cell and depends on the presence of specific membrane transport proteins (see the section “


Physiology_Levy

Physiology_Levy. In contrast to water, the movement of ions across cell membranes is more variable from cell to cell and depends on the presence of specific membrane transport proteins (see the section “ Composition of Body Fluid Compart ”). Consequently, in trying to understand the physiology of fluid shifts between body fluid compartments, it can be assumed that while water moves freely between the compartments, there is little net movement of solutes. For most situations, this is a reasonable assumption. To illustrate the physiologic characteristics of fluid shifts, consider what happens when solutions containing various amounts of NaCl are added to the ECF. Example 1: Addition of Isotonic Sodium Chloride to the Extracellular Fluid


Physiology_Levy

Physiology_Levy. Addition of an isotonic NaCl solution (e.g., intravenous infusion of 0.9% NaCl: osmolality ≈ 290 mOsm/kg H2O) bSome exceptions do exist. The cerebrospinal fluid is part of the ECF, but its osmolality is slightly higher than that of the ECF elsewhere in the body. Also, regions within the kidney can have osmolalities that are either less than or greater than that of the ECF. However, these volumes are small (≈150 mL) in comparison with the total volume of the ECF (≥12 L). cSerum is derived from clotted blood. Thus serum differs from plasma by the absence of clotting factors. With regard to osmolality and the concentrations of other molecules and ions, the osmolality and concentrations in plasma and serum are virtually identical. dFluids are usually administered intravenously. When electrolyte solutions are infused by this route, equilibration between plasma and interstitial fluid is rapid (i.e., minutes) because of the high permeability of many capillary walls for water and


Physiology_Levy

Physiology_Levy. electrolyte solutions are infused by this route, equilibration between plasma and interstitial fluid is rapid (i.e., minutes) because of the high permeability of many capillary walls for water and electrolytes. Thus these fluids are essentially added to the entire ECF. eA 0.9% NaCl solution (0.9 g NaCl/100 mL) contains 154 mmol/L of NaCl. Because NaCl does not dissociate completely in solution (i.e., 1.88 Osm/mol), the osmolality of this solution is 290 mOsm/kg H2O, which is very similar to that of normal ECF.


Physiology_Levy


Physiology_Levy

Physiology_Levy. Eq.2.2 givesthebestestimateoftheeffectiveosmolalityoftheserum. *Theureaconcentrationinplasmaismeasuredasthenitrogenintheureamolecule,orbloodureanitrogen(BUN). to the ECF increases the volume of this compartment by the volume of fluid administered. Because this fluid has the same osmolality as does the ECF, and therefore the ICF, there is no driving force for fluid movement between these compartments, and the volume of the ICF remains unchanged. Although Na+ can cross cell membranes, it is effectively restricted to the ECF by the activity of the Na+,K+-ATPase, which is present in the plasma membrane of all cells (see the section “ Ionic Composition of Cells”). Therefore, there is no net movement of the infused isotonic NaCl solution into cells. Example 2: Addition of Hypotonic Sodium Chloride to the Extracellular Fluid


Physiology_Levy

Physiology_Levy. Ionic Composition of Cells”). Therefore, there is no net movement of the infused isotonic NaCl solution into cells. Example 2: Addition of Hypotonic Sodium Chloride to the Extracellular Fluid Addition of a hypotonic NaCl solution to the ECF (e.g., intravenous infusion of 0.45% NaCl; osmolality ≅ 145 mOsm/kg H2O) decreases the osmolality of this fluid compartment, which results in the movement of water into


Physiology_Levy

Physiology_Levy. the ICF. After osmotic equilibration, the osmolalities of the ICF and ECF are again equal but lower than before the infusion, and the volume of each compartment is increased. The increase in ECF volume is greater than the increase in ICF volume. Example 3: Addition of Hypertonic Sodium Chloride to the Extracellular Fluid Addition of a hypertonic NaCl solution to the ECF (e.g., intravenous infusion of 3% NaCl: osmolality ≅ 1000 mOsm/kg H2O) increases the osmolality of this compartment, which results in the movement of water out of cells. After osmotic equilibration, the osmolalities of the ECF and ICF are again equal but higher than before the infusion. The volume of the ECF is increased, whereas that of the ICF is decreased. Composition of Body Fluid Compartments


Physiology_Levy

Physiology_Levy. Composition of Body Fluid Compartments The compositions of the ECF and ICF differ considerably. The ICF has significantly more proteins and macromolecules than the ECF. There are also differences in the concentrations of many ions. The composition of the ICF is maintained by the action of a number of specific cell membrane transport proteins. Principal among these transporters is the Na+,K+adenosine triphosphatase (Na+,K+-ATPase), which converts the energy in ATP into ion and electrical gradients, which can in turn be used to drive the transport of other ions and molecules by means of ion channels and solute carriers (e.g., symporters and antiporters).


Physiology_Levy

Physiology_Levy. The compositions of the plasma and interstitial fluid compartments of the ECF are similar because those compartments are separated only by the capillary endothelium, a barrier that ions and small molecules can permeate. The major difference between the interstitial fluid and plasma is that the latter contains significantly more protein. Although this differential concentration of protein can affect the Fluidandelectrolytedisordersareobservedcommonlyinclinicalpractice(e.g.,inpatientswithvomitingordiarrhea,orboth).Inmostinstances,thesedisordersareself-limited,andcorrectionofthedisorderoccurswithoutneedforintervention.However,moresevereorprolongeddisordersmaynecessitatefluidreplacementtherapy.Suchtherapymaybeadministeredorally,withspecialelectrolytesolutions,orintravenously,withfluid.


Physiology_Levy

Physiology_Levy. Patientswhosebodyfluidsarehyperosmoticneedhypotonicsolutions.ThesesolutionsmaybehypotonicNaCl(e.g.,0.45%NaCl)or5%dextroseinwater(D5W).AdministrationoftheD5WsolutionisequivalenttotheinfusionofdistilledwaterbecausethedextroseismetabolizedtoCO2andwater.AdministrationofthesefluidsincreasesthevolumesofboththeICFandECF.Inaddition,patientswhosebodyfluidsarehypotonicneedhypertonicsolutions.ThesearetypicallyNaCl-containingsolutions(e.g.,3%or5%NaCl).ThesesolutionsexpandthevolumeoftheECFbutdecreasethevolumeoftheICF.Otherconstituents,suchaselectrolytes(e.g.,K+)ordrugs,canbeaddedtointravenoussolutionstotailorthetherapytothepatient’sfluid,electrolyte,andmetabolicneeds. distribution of cations and anions between these two compartments by the Gibbs-Donnan effect (see the section “ ” for details), this effect is small, and the ionic compositions of the interstitial fluid and plasma can be considered to be identical. Maintenance of Cellular Homeostasis


Physiology_Levy

Physiology_Levy. Maintenance of Cellular Homeostasis Normal cellular function requires that the ionic composition of the ICF be tightly controlled. For example, the activity of some enzymes is pH dependent; therefore, intracellular pH must be regulated. In addition, the intracellular composition of other electrolytes is similarly held within a narrow range. This is necessary for the establishment of the membrane potential, a cell property especially important for the normal function of excitable cells (e.g., neurons and muscle cells) and for intracellular signaling (e.g., intracellular [Ca++]; see for details). Finally, the volume of cells must be maintained because shrinking or swelling of cells can lead to cell damage or death. The regulation of intracellular K+ 3.5-5.0mEq/L120-150mEq/L Ca++ 2.1-2.8(total)mmol/L 1.1-1.4(ionized)≈10−7M(ionized)


Physiology_Levy

Physiology_Levy. K+ 3.5-5.0mEq/L120-150mEq/L Ca++ 2.1-2.8(total)mmol/L 1.1-1.4(ionized)≈10−7M(ionized) Pi1.0-1.4(total)mmol/L 0.5-0.7(ionized)0.5-0.7(ionized) *Ca++ andPi(H2PO4−/HPO4−2)areboundtoproteinsandotherorganicmolecules.Inaddition,largeamountsofCa++ canbesequesteredwithincells.LargeamountsofPiarepresentincellsaspartoforganicmolecules,suchasadenosinetriphosphate(ATP). composition and cell volume is accomplished through the activity of specific transporters in the plasma membrane of the cells. This section is a review of the mechanisms by which cells maintain their intracellular ionic environment and their membrane potential and by which they control their volume. Ionic Composition of Cells


Physiology_Levy

Physiology_Levy. The intracellular ionic composition of cells varies from tissue to tissue. For example, the intracellular composition of neurons is different from that of muscle cells, both of which differ from that of blood cells. Nevertheless, there are similar patterns, and these are presented in . In comparison with the ECF, the ICF is characterized by a low [Na+] and a high [K+]. This is the result of the activity of the Na+,K+-ATPase, which transports 3 Na+ ions out of the cell and 2 K+ ions into the cell for each ATP molecule hydrolyzed. As discussed later in this chapter, the activity of the Na+,K+-ATPase not only is important for establishing the cellular Na+ and K+ gradients but also is involved in determining, indirectly, the cellular gradients for many other ions and molecules. Of importance is that the cellular K+ gradient generated by the activity of the Na+,K+-ATPase is a major determinant of the membrane voltage because of the leak of K+ out of the cell through K+selective channels


Physiology_Levy

Physiology_Levy. is that the cellular K+ gradient generated by the activity of the Na+,K+-ATPase is a major determinant of the membrane voltage because of the leak of K+ out of the cell through K+selective channels (see the section “


Physiology_Levy

Physiology_Levy. Membrane Potential”). Thus the Na+,K+-ATPase converts the energy in ATP into ion gradients (i.e., Na+ and K+), and a voltage gradient (i.e., membrane voltage). The Na+,K+-ATPase–generated ion and electrical gradients are used to drive the transport of other ions and molecules into or out of the cell ( Fig. 2.3 ). For example, as described in , a number of solute carriers couple the transport of Na+ to that of other ions or molecules. The Na+-glucose and Na+–amino acid symporters use the energy in the Na+ electrochemical gradient, directed to bring Na+


Physiology_Levy

Physiology_Levy. CHAPTER 2 Homeostasis: Volume and Composition of Body Fluid Compartments •Fig. 2.3 Cell Model Depicting How Cellular Gradients and the Membrane Potential (Vm) Are Established. (1)TheNa+,K+-ATPasedecreasestheintracellular[Na+]andincreasestheintracellular[K+].SomeK+ exitsthecellviaK+-selectivechannelsandgeneratestheVm(cell’sinterioriselectricallynegative).(2)TheenergyintheNa+ electrochemicalgradientdrivesthetransportofotherionsandmoleculesthroughtheuseofvarioussolutecarriers.(3)TheVmdrivesCl− outofthecellviaCl−-selectivechannels.(4)TheCa++-ATPaseandthe3Na+-Ca++ antiportersmaintainthelowintracellular[Ca++].


Physiology_Levy

Physiology_Levy. into the cell, to drive the secondary active cellular uptake of glucose and amino acids. Similarly, the inwardly directed Na+ gradient drives the secondary active extrusion of H+ from the cell and thus contributes to the maintenance of intracellular pH. The 3Na+-Ca++ antiporter, along with the plasma membrane Ca++-ATPase, extrudes Ca++ from the cell and thus contributes to the maintenance of a low intracellular [Ca++]. In addition, the membrane voltage drives Cl− out of the cell through Cl−-selective channels, thus lowering the intracellular concentration below that of the ECF. As described previously, the Na+,K+-ATPase and K+-selective channels in the plasma membrane are important determinants of the membrane potential (Vm) of the cell. For all cells within the body, the resting Vm is oriented with the interior of the cell electrically negative in relation to the ECF. However, the magnitude of the Vm can vary widely.