Sun28November0333PM 6

The renal glomeruli filter blood to produce up to 180 litres of filtrate every 24 hours. Approximately 99% of the filtrate is reabsorbed.

By which one of the following segments of the nephron is the highest proportion of water reabsorbed?

(Please select 1 option)

Collecting ducts

Descending loop of Henle

Distal convoluted tubule Incorrect answer selected

Ascending loop of Henle

Proximal convoluted tubule This is the correct answer

Explanation

Most of the water of the glomerular filtrate is reabsorbed by the proximal convoluted tubule (65-70%) and is termed "obligatory" absorption.

Sodium is the main osmotic driving force for the reabsorption of water and other solutes as it enters the tubular cell via the apical membrane and subsequently actively transported out across the basolateral membrane. The filtrate remains isosmotic.

Loop of Henle:

Descending limb is freely permeable to water. The countercurrent exchange and multiplier arrangements of the vasa recta and the loop of Henle produce a hyperosmolar filtrate as water passively moves into the interstitium in the inner medullar. 20-25% of water is reabsorbed.

Thick ascending limb impermeable to water (Na-K-2Cl co-transporter).

In the distal convoluted tubule 5-10% of the filtrate is further reabsorbed passively down a concentration gradient driven by a Na+/K+ ATPase pump.

In the collecting ducts under the influence of antidiuretic hormone (ADH) a further 1-3% of water can be reabsorbed.

Answer Statistics

1

25%

2

12%

3

11%

4

16%

5

38%

Times answered: 248