Water reabsorption

Movement of water

Occurs via

Simple diffusion
Through aquaporin in apical membranes of the tubular cells
Through tight junctions

Variability in the intrinsic water permeability across nephron segments is mostly due to variabilities in:

Tight junction, and
Apical membrane (of tubular cells)

Osmotic diuresis

i.e. increased urine flow due to abnormally high amount of any substance in the glomerular filtrate that is reabsorbed incompletely or not at all by proximal tubules.

e.g. glucose

Because of:

1. Presence of high amount of non-sodium solute

--> Its osmotic pressure impairs water reabsorption in proximal tubule

--> Amount of water remains high in the lumen

2. Tight junction in proximal tubule is leaky to sodium

--> Sodium transport is a gradient-limited system

Thus,

Increased water content

--> Luminal sodium concentration slightly lower than interstitial

--> Sodium diffuses back from interstitium into lumen through tight junction

Overall

Increased water content going into LOH
Increased sodium content going into LOH

By segment

Proximal tubule

Water is reabsorbed with sodium

Loop of Henle

Descending limb

Water reabsorption without significant sodium reabsorption

Ascending limb

Both thick and thin ascending limbs are impermeable to water

Sodium reabsorption without water reabsorption

Overall effect of LOH

Fraction of sodium reabsorbed is greater than fraction of water reabsorbed

--> Dilution of the tubular content

Distal convoluted tubule

Impermeable to water

Sodium reabsorption without water reabsorption

--> Further dilution of tubular content

Collecting duct

Water permeability in CCD and MCD
--> Subject to control by ADH

Cortical and outer medullary collecting ducts are impermeable to water without ADH
Inner medullary collecting duct has some water permeability even without ADH

Effect of ADH

Water reabsorption
* via apical aquaporin 2
* in principle cells