Tubular Reabsorption

Of the roughly 180 litres of filtrate produced daily, about 99% has to be reclaimed by the tubules, since it still carries glucose, amino acids, most of the water and most of the ions the body needs - this recovery is called selective reabsorption, and each tubule segment specialises in a different part of it. In the proximal convoluted tubule, glucose, amino acids, lactate and most of the water are reabsorbed; sodium is actively pumped out via the Na+-K+ pump, with water, urea and uric acid following passively, so this segment alone recovers the bulk of the filtrate before it even reaches the loop of Henle. The loop of Henle then does the work of concentrating the fluid: its thin descending limb is permeable to water (thanks to aquaporin channels) but not to salts, so as water leaves, Na+ and Cl- become more concentrated in the remaining fluid; its thick ascending limb reverses this - it is impermeable to water but actively pumps Na+, Cl- and K+ out into the surrounding tissue, which dilutes the tubular fluid again while building up the salt concentration of the medulla around it. By the time the now-hypotonic fluid reaches the distal convoluted tubule, reabsorption becomes need-based and hormonally regulated: the DCT recovers more water and bicarbonate (to help regulate blood pH) while secreting potassium, and t …