Selective Reabsorption in the Nephron

Useful substances enter the filtrate

The glomerulus filters blood into Bowman’s capsule. Small substances, including water, glucose, salts and urea, enter the fluid called filtrate. Filtration does not separate useful glucose from waste urea, so the nephron must recover substances the body needs before the remaining fluid becomes urine. This return from the filtrate to the surrounding blood is called selective reabsorption.

Glucose is reabsorbed in the first tubule

Filtrate flows from Bowman’s capsule into the proximal convoluted tubule. Glucose moves through the cells lining this tubule and back into nearby blood capillaries. It is reabsorbed by active transport, which uses energy released by respiration and can move glucose against its concentration gradient. The tubule cells have many mitochondria to provide this energy.

At a normal blood glucose level, all the filtered glucose is reabsorbed, so urine does not normally contain glucose. If blood glucose is very high, as it can be in diabetes, more glucose may enter the filtrate than the tubule can reabsorb. Some can then remain in the fluid and appear in urine. Glucose in urine is therefore a possible sign of diabetes, not proof of it on its own.

Water returns to the blood

Water is also reabsorbed from the filtrate. As useful dissolved substances are taken back into the blood, water can follow by osmosis through partially permeable membranes. Water is reabsorbed along the nephron, including the proximal convoluted tubule, the loop of Henle and the collecting duct. The amount returned from the collecting duct varies with the body’s need to conserve water; the role of ADH in controlling this is covered separately.

The fluid left after reabsorption contains water, urea and excess dissolved salts. It becomes urine. Thus, a healthy kidney can remove waste while keeping glucose and returning much of the filtered water to the blood.