Answer
Distal to the proximal convoluted tubule (PCT), each kidney tubule descends into the medulla, turns and then ascends as the ascending limb (thin, then, thick ) of the nephron loop ( Loop of Henle).
The thin descending limb of the Loop of Henle (loop) is permeable to water but relatively impermeable to ions of solutes -- Na+, K+, Cl= etc. Therefore, water leaves the thin descending limb and is reabsorbed into the ISF of the interstitium of the medulla.
Water continues to leave the thin descending limb of the loop because of what is called a counter current mechanism: As soon as filtrate enters the tubule, water is absorbed. This continues even to the bottom of the tube because of the increasing osmotic gradient with the interstitium all along this limb-- to the bottom.
The ascending limb does not absorb water because it lacks *aquaporins; its job is to excrete ions into the medullary interstitium to maintain the solute concentration gradient of the countercurrent mechanism.
Work Step by Step
A kidney tubule starts as a cuplike structure called Bowman's capsule. This structure houses a tuft of capillaries --the glomerulus. Blood is brought into this capillary bed by an afferent arteriole and leaves by an efferent arteriole.
Distal to the capsule, the tubule twists a number of times to form the proximal convoluted tubule (PCT) in the cortex. Beyond the PCT, the tubule descends into the medulla as the thin descending limb of the loop (permeable to water).
In the medulla this thin limb of the loop turns and travels upward as the ascending limb of the loop. The ascending loop comprises two structurally different segments -- the thin ascending loop, and the thick ascending (cortical) loop. The thin ascending loop is permeable to ions (Na+, Cl- etc) which passively diffuse out of the lower portion of the ascending limb into the tissue of the medulla.
Salts (ions of salts are removed from the thick segment of the ascending limb also, but by active transport of carrier molecules.
Water permeability is very low in the thick ascending limb but rises in the collecting ducts.
Notes.
Aquaporins are molecules that form pores in membranes to let water pass through.
Counter current mechanisms use energy.
