L06-Renal tubular function 2

Question 1

What are the three most important solutes for water reabsorption?

Answer 1

Na+, cl- and HCO3-

Question 2

What is the key function of the thick ascending loop of henle in terms of water reabsorption?

Answer 2

It has the function of creating a hyperosmolar interstitual space in the medulla that drives water loss from the thin decending limb.

Question 3

By what method is water reabsorbed in the thin decending limb?

Answer 3

It is fully permeable to water and osmotic force causes water to leave. This is due to the hyperosmolarity created by the thick ascending limb.

Question 4

How does water reabsorption occur in the thick ascending limb of the loop of henle?

Answer 4

There are Na/k/cl cotransporters (abbreviated NKCC2) that act to move these ions out of the filtrate and create a hyperosmotic environment that draws water out of the filtrate. K+ recycling through the apical membrane ensures that the cotransporter is kept going.

Question 5

What is furosemide and how does it work?

Answer 5

It is a ‘loop’ diuretic which acts in the ascending limb of the loop of henle. It blocks the NKCC2 transporter hence reducing the reabsorption of water.
It is commonly used in cardiac and renal failures but has the side effect of hypokalaemia which can casue cardiac arrhythmias.

Question 6

What are thiazides and how do they work? Give an example

Answer 6

Act in the distal tubule to block the Na/cl cotransporter. This has moderate diuretic effects, used as an antihypertensive. hydrochlorothiazide is an example

Question 7

What is spironolactone and how does it work?

Answer 7

Acts in the collecting tubules to block the effects of aldosterone. This results in reduced Na reabsorption and hence has diuretic effects. It is used in heart failure as it is K+ sparing diuretic (blocking aldosterone K+ secretion) so doesnt have the cardiac arrhythmia side effects.

Question 8

What is the urea counter current multiplication and how does it aid water reabsorption?

Answer 8

the late distal tubule is impermeable to urea so as water leaves this increases urea concentration. As it moves into the collecting duct this is permeable to urea so it moves out down it concentration gradient and back into the loop of henle. This movement in a countercurrent fashion draws more water out of the loop. ADH also increases the number of urea transporters. This is facilitated diffusion through UTA1 channels