Reabsorption

Each day, the kidneys filter about 150 liters of blood, while only about 1.5 liters of urine is actually expelled from the body. Reabsorption thus recovers a large proportion of the water filtered by the kidneys and plays a critical role in maintaining the water balance of the body.

Mechanism

Reabsorption is driven by active sodium transport from the lumen into the blood by the Na⁺/K⁺ATPase enzyme in the basolateral membrane of the epithelial cells. The resultant sodium gradient causes water and other solutes in the filtrate to follow the sodium ions into the peritubular capillaries via osmosis.

Renal tubules consist of a proximal tubule, loop of Henle, and distal tubule. These different sections are responsible for reabsorbing different substances, at different rates. Most reabsorption (about 60-70% of sodium and almost all of the glucose and amino acids) occurs in the first section, the proximal convoluted tubule. Glucose, amino acids, inorganic phosphate, and some other solutes are reabsorbed via secondary active transport through cotransport channels driven by the sodium gradient. In this section, solutes are reabsorbed isotonically, as water follows the solutes being transported into the blood, such that the osmotic potential of the fluid leaving the proximal convoluted tubule is the same as that of the initial glomerular filtrate. Further on, absorption is not isotonic, as in the descending section of the loop of Henle, water is absorbed, but not solutes, so the filtrate becomes more concentrated. Finally, the thick ascending limb of the loop of Henle and distal nephron together recover the other 30-40% of sodium and other solutes.