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In the presence of vasopressin the greatest fraction of filtered water is re-absorbed from which part of the nephron:

A Proximal tubule
B Distal tubule

C Loop of Henle
D Collecting duct

Ans. A Proximal tubule
  1. Proximal tubular reabsorption: Normally about 65 percent Q

Of the filtered load of sodium and water and a slightly lower percentage of filtered chloride are reabsorbed by the proximal tubule before the filtrate reaches the loop of Henle.

  1. Loop of Henle: The descending limb of the loop of Henle is permeable to water Q, but the ascending limb is impermeable.
  2. Na+ K+ and Cl- are co—transported out of the thick segment of the ascending limb. Therefore the fluid in the descending limb of the loop of Henle becomes hypertonic Q as water moves into the hypertonic interstitium. In the ascending limb it becomes more dilute and when it reaches the top it is hypotonic to plasma because of movement of Na &cr out of tubular lumen. In passing through 1 of Henle another 15% of the filtered water is removed, so approximately 20% of filtered water enters the distal tubule Q
  3. Distal tubule impermeable to water, and continued removal of solute in excess of solvent further dilutes the tubular fluid. About 5% o the filtered water is removed in this segment.
  4. Collecting duct have two portions: a cortical portion and medullary portion. The changes in the osmolality and volume in the collecting duct depends on the amount of the vasopressin acting on the duct.
  5. In the presence of vasopressin:

a. Cortical collecting duct In the presence of vasopressin to produce maximal antidiuresis, water moves enough out of the hypotonic fluid entering the cortical collecting duct into the interstitium of the cortex and the tubular fluid becomes isotonic. In this fashion as mud as 10% of the filtered water removed.

b. Medullary collecting duct

The isotonic fluid then enters medullary collecting duct. An additional 4.7% or more of the filtrate is reabsorbed into the hypertonic interstitium of medulla.

In humans, the osmolality of urine may reach 1400 mosm/kg of H2O almost 5 times the osmolality of plasma, with a total of 99.7%f of the filtered water being reabsorbed and only 0.3% appearing in the urine.

  1. In the absence of vasopressin:

When the vasopressin is absent, the collecting duct epithelium is relatively impermeable to water. The impermeability of the distal portions of the nephron is not absolute, along with the salt that is pumped out of the collecting duct, about 2% of the filtered water is absorbed in the absence of vasopressin 13% of the filtered water is excreted.

Thus, we have seen that in the presence of vasopressin, water reabsorption isQ

a. Proximal tubule 65%

b. Loop of Henle20%

c. [ descending limb]

d. Distal tubule 5%

e. Cortical collecting duct 10%

f. Medullary collecting duct —

g. Thus 99.7% of filtered water is reabsorbed and 0.3% is excreted.

h. In the absence of vasopressin

i. Proximal tubule 65%

j. Loop of Henle 20%

k. Distal tubule 2%


l. Thus 87% is reabsorbed and 13% is excreted.

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