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Kidney structure and ultrafiltration IB biology HL. My IB exam is tomorrow... yay!

The Kidney has what two functions?

Excretion and Osmoregulation.


The removal from the body of the waste products of metabolic pathways.


The control of the water balance of the blood, tissue, or cytoplasm of a living organism.

What is the structure of the kidney?

Cortex, Medulla, Renal artery, Renal vein, Pelvis of the kidney, and Ureter (carries urine to the bladder). Note that the cortex and the medulla of the kidney contain many narrow tubes called nephrons.

What is the structure of the glomerulus?

Basement membrane (the filter), Podocytes, fenestrated wall of capillary, blood plasma, and red blood cells.

What are Podocytes?

Strangely shaped cells with finger-like projections which wrap around capillaries in the glomerulus and provide support.

What is Ultrafiltration?

is the method by which the glomerulus filters out most solutes from the blood plasma, while retaining most proteins.

Explain Ultrafiltration

1. Particles with a relative molecular mass of less than 65,000 can pass through the fenestrated membranes of the capillaries, which allows for notably large particles to pass through.
2. This and the relatively high blood pressure in the glomerulus allows for an environment in which solutes can be easily taken out of the blood.

What are the three principle parts of the glomerulus that allow for Ultrafiltration?

1. pores between the cells in the walls of the capillaries that are small enough to keep blood cells out but let fluid in
2. the basement membrane covers and supports the capillaries, composed of negatively charged glycoproteins which prevent plasma proteins from being filtered out.
3. podocytes form the inner wall of the Bowman's capsule, provide narrow gaps through which small molecules are prevented from being let out.

Selective re-absorption

Every moment large volumes of glomerular filtrate are being produced. This filtrate contains not only waste products, but substances that the body may need as well. These substances must be re-absorbed back into the blood and this happens through selective re-absorption in the proximal convoluted tubule.

How is selective re-absorption done?

1. The wall of a nephron consists of a single layer of cells. In the PCT the cells have microvilli projecting into the lumen, giving a large surface area for absorption.
2. Pumps in the membrane re-absorb useful substances by ACTIVE TRANSPORT, using ATP produced by mitochondria in the cells.
3. All of the glucose in the filtrate is re-absorbed. About 80% of the mineral ions, including sodium is re-absorbed.
4. Active transport of solutes makes the total solute concentration higher in the cells of the wall than in the filtrate in the tubule.
5. Water therefore moves from the filtrate to the cells and on into the adjacent blood capillary by osmosis. About 80% of the water in the filtrate is re-absorbed, leaving 20% of the original volume to flow on into the loop of Henle.

The loop of Henle

creates an osmotic concentration gradient in the fluid immediately surrounding it, making it possible to produce urine which is more osmotically concentrated than blood plasma. The overall effect of the loop of Henle is to increase the solute concentration of the medulla.

Descending limbs

are permeable to water but not to sodium ions.

Ascending limbs

are permeable to sodium ions but not to water. Pump sodium ions from the filtrate into the medulla by active transport, creating a high solute concentration in the medulla.

Osmoregulation in the collecting duct ( If the water content of the blood is too low)

If the water content of the blood is too low:
1. The pituitary gland secretes ADH.
2. This hormone makes the cells of the collecting duct produce membrane channels called aquaporins, which makes the collecting duct permeable to water.
3. As the filtrate passes down the collecting duct through the medulla, the high solute concentration of the medulla causes most of the water in the filtrate to be reabsorbed by osmosis.
4. A small volume of concentrated urine is produced.

Osmoregulation in the collecting duct ( If the water content of the blood is too high)

If the water content of the blood is too high:
1. ADH is not secreted.
2. Aquaporins are broken down and the collecting duct becomes less permeable to water.
3. Little water is reabsorbed as the filtrate passes down the collecting duct.
4. And a large volume of dilute urine is produced.
5. The urine produced by the collecting ducts drains into the renal pelvis and down to the ureter in the bladder.

Outline the structure and action of kidney dialysis machines

1. Blood enters machine from patient's vein and runs through partially permeable tubules and into the dialysis chamber.
2. The tube allows nitrogenous waste to diffuse from the blood and into the dialysis fluid. Urea diffuses through the membrane, as dialysis fluid contains none, removing it from the body.
3. Water and solutes will then be added to the blood if necessary by diffusion from the dialysis fluid which runs across the semipermeable tubules containing the circulating blood.
4. The blood is then run through an air bubble trap before it is reintroduced to the vein.

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