World Library  
Flag as Inappropriate
Email this Article


Article Id: WHEBN0000275216
Reproduction Date:

Title: Hemodynamics  
Author: World Heritage Encyclopedia
Language: English
Subject: Cardiovascular physiology, Cell-free marginal layer model, Cardiac output, Pathology, Intensive care medicine
Collection: Cardiovascular Physiology, Computational Fluid Dynamics, Fluid Mechanics
Publisher: World Heritage Encyclopedia


Hemodynamics or hæmodynamics (-dynamics + hemo-) is the fluid dynamics of blood flow. The circulatory system is controlled by homeostatic mechanisms, much as hydraulic circuits are controlled by control systems. Hemodynamic response continuously monitors and adjusts to conditions in the body and its environment. Thus hemodynamics explains the physical laws that govern the flow of blood in the blood vessels. The relationships can be challenging because blood vessels are complex, with many ways for blood to enter and exit under changing conditions.


  • Cardiac output and flow rate 1
  • Blood pressure across the circulatory system 2
  • Determinants of vascular resistance 3
  • Turbulence 4
  • Wall tension 5
  • Venous capacitance 6
  • Monitoring 7
  • Etymology and pronunciation 8
  • See also 9
  • Notes and references 10
  • Bibliography 11
  • External links 12

Cardiac output and flow rate

Diagram of the circulatory system

The heart is the driver of the circulatory system, pumping blood through rhythmic contraction and relaxation. The rate of blood flow out of the heart (often expressed in L/min) is known as the cardiac output (CO).

Blood being pumped out of the heart first enters the aorta, the largest artery of the body. It then proceeds to divide into smaller and smaller arteries, then into arterioles, and eventually capillaries, where oxygen transfer occurs. The capillaries connect to venules, and the blood then travels back through the network of veins to the right heart. The micro-circulation — the arterioles, capillaries, and venules —constitutes most of the area of the vascular system and is the site of the transfer of O2, glucose, and enzyme substrates into the cells. The venous system returns the de-oxygenated blood to the right heart where it is pumped into the lungs to become oxygenated and CO2 and other gaseous wastes exchanged and expelled during breathing. Blood then returns to the left side of the heart where it begins the process again.

In a normal circulatory system, the volume of blood returning to the heart each minute is approximately equal to the volume that is pumped out each minute (the cardiac output).[1] Because of this, the velocity of blood flow across each level of the circulatory system is primarily determined by the total cross-sectional area of that level. This is mathematically expressed by the following equation:

v = Q/A


  • v = velocity (cm/s)
  • Q = blood flow (ml/s)
  • A = cross sectional area (cm2)

Blood pressure across the circulatory system

The blood pressure in the circulation is principally due to the pumping action of the heart.[2] The pumping action of the heart generates pulsatile blood flow, which is conducted into the arteries, across the micro-circulation and eventually, back via the venous system to the heart. During each heartbeat, systemic arterial blood pressure varies between a maximum (systolic) and a minimum (diastolic) pressure.[3] In physiology these are often simplified into one value, the mean arterial pressure (MAP), which is calculated as follows:

MAP ≈ 23(BPdia) + 13(BPsys)
Note: BPdia weighted more heavily since the heart spends two thirds of the heart beat cycle in the diastolic.


  • MAP = Mean Arterial Pressure
  • BPdia = Diastolic blood pressure
  • BPsys = Systolic blood pressure


The relationship between pressure, flow, and resistance is expressed in the following equation:[1]

Flow = Pressure/Resistance

When applied to the circulatory system, we get:



  • CO = cardiac output (in L/min)
  • MAP = mean arterial pressure (in mmHg), the average pressure of blood as it leaves the heart
  • RAP = right atrial pressure (in mmHg), the average pressure of blood as it returns to the heart
  • TPR = total peripheral resistance (in mmHg * min/L)

A simplified form of this equation assumes right atrial pressure is approximately 0:


The ideal blood pressure in the brachial artery, where standard blood pressure cuffs measure pressure, is <120/80 mmHg. Other major arteries have similar levels of blood pressure recordings indicating very low disparities among major arteries. In the innominate artery, the average reading is 110/70 mmHg, the right subclavian artery averages 120/80 and the abdominal aorta is 110/70 mmHg.[5] The relatively uniform pressure in the arteries indicate that these blood vessels act as a pressure reservoir for fluids that are transported within them.


The arm–leg (blood pressure) gradient is the difference between the blood pressure measured in the arms and that measured in the legs. It is normally less than 10 mm Hg,[6] but may be increased in e.g. coarctation of the aorta.[6]

Determinants of vascular resistance

Resistance is also related to vessel radius, vessel length, and blood viscosity, as indicated by the Hagen–Poiseuille equation.[7] The equation is as follows:

  • P: pressure drop/gradient
  • µ: viscosity
  • Q: flow rate of the blood in the vessel
  • r: radius of the vessel








If the blood viscosity increases (gets thicker), the result is an increase in arterial pressure. Certain medical conditions can change the viscosity of the blood. For instance, anemia (low red blood cell concentration), reduces viscosity, whereas increased red blood cell concentration increases viscosity. It had been thought that aspirin and related "blood thinner" drugs decreased the viscosity of blood, but instead studies found that they act by reducing the tendency of the blood to clot.[10]


Blood flow is also affected by the smoothness of the vessels, resulting in either turbulent (chaotic) or laminar (smooth) flow. Smoothness is reduced by the buildup of fatty deposits on the arterial walls.


The equation for this dimensionless relationship is written as:[7]

  • ρ: density of the blood
  • v: mean velocity of the blood
  • μ: viscosity of blood


Wall tension

Components of cylinder stress.




  • P is the blood pressure
  • t is the wall thickness
  • r is the inside radius of the cylinder.





  • t is the radial thickness of the cylinder
  • l is the axial length of the cylinder

Venous capacitance

Veins are described as the "capacitance vessels" of the body because over 70% of the blood volume resides in the venous system. Veins are more compliant than arteries and expand to accommodate changing volume.[12]


An anesthetic machine with integrated systems for monitoring of several hemodynamic parameters, including blood pressure and heart rate.

Hemodynamic monitoring is the observation of hemodynamic parameters over time, such as blood pressure and heart rate. Blood pressure can be monitored either invasively through an inserted blood pressure transducer assembly (providing continuous monitoring), or noninvasively by repeatedly measuring the blood pressure with an inflatable blood pressure cuff.

Etymology and pronunciation

The word hemodynamics () uses combining forms of hemo- and dynamics, thus "the dynamics of blood". The vowel of the hemo- syllable is variously written according to the ae/e variation.

See also

Notes and references

  1. ^ a b Costanzo, Linda S. (2003). Physiology. Board Review Series (3rd ed.). Philadelphia: Lippincott Williams and Wilkins. pp. 73–113.  
  3. ^ "Normal Blood Pressure Range Adults". Health and Life. 
  12. ^ Lough, Mary (2015-04-15). Hemodynamic Monitoring: Evolving technologies and clinical practice (1 ed.). St. Louis, Missouri: Elsevier Mosby. p. 25.  


  • Berne RM, Levy MN. Cardiovascular physiology. 7th Ed Mosby 1997
  • Rowell LB. Human Cardiovascular Control. Oxford University press 1993
  • Braunwald E (Editor). Heart Disease: A Textbook of Cardiovascular Medicine. 5th Ed. W.B.Saunders 1997
  • Siderman S, Beyar R, Kleber AG. Cardiac Electrophysiology, Circulation and Transport. Kluwer Academic Publishers 1991
  • American Heart Association
  • Otto CM, Stoddard M, Waggoner A, Zoghbi WA. Recommendations for Quantification of Doppler Echocardiography: A Report from the Doppler Quantification Task Force of the Nomenclature and Standards Committee of the American Society of Echocardiography. J Am Soc Echocardiogr 2002;15:167-184
  • Peterson LH, The Dynamics of Pulsatile Blood Flow, Circ. Res. 1954;2;127-139
  • Hemodynamic Monitoring, Bigatello LM, George E., Minerva Anestesiol, 2002 Apr;68(4):219-25
  • Claude Franceschi; Paolo Zamboni Principles of Venous Hemodynamics Nova Science Publishers 2009-01 ISBN Nr 1606924850/9781606924853
  • WR Milnor: Hemodynamics, Williams & Wilkins, 1982
  • B Bo Sramek: Systemic Hemodynamics and Hemodynamic Management, 4th Edition, ESBN 1-59196-046-0

External links

  • Hemodynamic Society
  • Learn hemodynamics
  • Educational Particle Image Velocimetry (e-PIV) - resources and demonstrations
