Renal

Question 1

What is ultrafiltration and how does it produce glomerular filtrate?

Answer 1

• The efferent arteriole which takes blood away from the glomerulus is smaller in diameter than the afferent arteriole which takes blood into the glomerulus, so blood in the glomerulus is under high pressure.
• The high pressure forces liquid and small molecules in the blood out of the capillaries and into the Bowman’s capsule. This glomerular filtrate is made up of water, glucose and mineral ions.

Question 2

What is the function of the proximal convoluted tubule?

Answer 2

Reabsorbs Na+, Cl- and water

Question 3

How is water reabsorbed in the proximal convoluted tubule?

Answer 3

1. Sodium ions are actively transported out of the cells lining the proximal convoluted tubule into blood capillaries which carry them away. This is achieved by the NA+/K+ ATPase pumps in the basolateral membrane of the cells. The sodium ion concentration of these cells is therefore lowered.
2. Sodium ions now diffuse down a concentration gradient from the lumen of the proximal convoluted tubule into the epithelial lining cells through carrier proteins by facilitated diffusion. Cl- accompanies Na+ to ensure electrical neutrality.
3. These carrier proteins are of specific types, each of which carries another molecule (glucose or amino acids or chloride ions etc) along with the sodium ions. This is co-transport.
4. The movement of ions makes the water potential in the cells lower than in the tubule so water moves down its concentration gradient by osmosis into the cell.
5. The molecules that have been transported into the cells then diffuse into the blood.

At the end of the proximal convoluted tubule:

• > 90% of filtered Na+, Cl- and H2O has been recaptured
• All filtered K+ is reabsorbed
• All filtered glucose is reabsorbed here
• All filtered amino acids are absorbed here.

Question 4

Describe how the loop of Henle generates an osmotic gradient

Answer 4

• The descending limb which is narrow with thin walls that are highly permeable to water.
• The ascending limb which is wider with thick walls that are impermeable to water.

1. Sodium ions are actively transported out of the ascending limb of the loop of Henle using ATP provided by the many mitochondria in the cells of its wall.
2. This creates a lower water potential (high ion concentration) in the region of the medulla between the two limbs (called the interstitial region).
3. The walls of the descending limb are permeable to water so it passes out of the filtrate by osmosis, into the interstitial space. This water enters the blood capillaries in this region by osmosis and is carried away.
4. The filtrate progressively loses water in this way as it moves down the descending limb lowering its water potential. It reaches its lowest water potential at the tip of the hairpin.
5. At the base of the ascending limb, sodium ions diffuse out of the filtrate and as it moves up the ascending limb these ions are also actively pumped out and therefore the filtrate develops a progressively higher water potential.
6. In the interstitial space between the ascending limb and the collecting duct there is a gradient of water potential with the highest water potential (lowest concentration of ions) in the cortex and an increasingly lower water potential (higher concentration of ions) the further into the medulla one goes.
7. The collecting duct is permeable to water and so as the filtrate moves down it, water passes out of it by osmosis. This water passes by osmosis into the blood vessels that occupy this space and it carried away.
8. As water passes out of the filtrate its water potential is lowered. However, the water potential is also lowered in the interstitial space and so water continues to move out by osmosis down the whole length of the collecting duct. The counter-current multiplier ensures that there is always a water potential gradient drawing water out of the tubule.

Don’t need to remember this whole process just need to know that the loop of Henle generates and osmotic gradient.

Question 5

How is the water permeability of the collecting duct regulated.

Answer 5

• If the collecting duct is made permeable to water then water can leave down its concentration gradient and enter vasa recta capillaries so be retained in circulation. Urine produced is small in volume and concentration.
• Permeability is due to the presence of aquaporins in the membrane.
• ADH binds to a receptor on the collecting duct cell and through a G-protein coupled receptor increases the aquaporins in the membrane, thus the permeability to water.
• ADH is secreted from cells in the posterior pituitary in response to:
  
  • increase in osmolarity (blood more concentrated) detected by osmoreceptors in hypothalamus
  • decrease in blood volume detected by volume receptors in CVS.

Question 6

How does a decrease in sodium cause the release of Renin?

Answer 6

1. Decrease in body sodium thus plasma volume causing increased activity of renal sympathetic nerves which are reflexly activated via baroreceptors.
2. Decreased blood pressure in the kidneys is detected by infrarenal baroreceptors which are stretched less so secrete more renin.
3. Macula densa sense decreased sodium concentration in the tubular fluid

Question 7

How does Renin increase absorption of Na in collecting ducts?

Answer 7

• Renin is an enzyme secreted by the kidneys
• Once in the blood it splits into Angiotesin I and Angiotensinogen
• Angiotensin I is converted to Angiotensin II under the action of ACE (Angiotensin-converting enzyme)
• Angiotensin II stimulates the release of the hormone Aldosterone from the adrenal cortex
• This induces the synthesis of Na transporters in the collecting duct thus increases sodium absorption