How urine is formed

Urine is made in three steps: glomerular filtration, reabsorption and secretion.

Filtration. About 1100-1200 mL of blood flows through the kidneys every minute, roughly one-fifth of all the blood the heart pumps. In the glomerulus, blood pressure pushes the blood through a three-layered filter: the wall of the capillary (endothelium), the inner layer of Bowman's capsule, and a basement membrane between them. The capsule's inner cells, the podocytes, wrap around the capillaries with tiny gaps between them, the filtration slits. Like a fine sieve, the filter lets almost everything in plasma pass, except the proteins, which are too big. Because it is so fine, this is called ultra filtration.

The amount of filtrate formed per minute is the glomerular filtration rate (GFR): about 125 mL a minute, which is 180 litres a day. The kidneys keep the GFR steady with a special structure, the juxta glomerular apparatus (JGA): if the GFR falls, its JG cells release renin, which raises it again (more in a later lesson).

But we pass only about 1.5 litres of urine a day. So nearly 99% of the filtrate must be taken back into the blood: this is reabsorption. Useful substances like glucose, amino acids and Na⁺ are reabsorbed actively (using energy). Nitrogenous wastes, and water in the early parts of the tubule, are reabsorbed passively.

Secretion. The tubule cells also actively push some substances into the filtrate: H⁺, K⁺ and ammonia. This keeps the body's balance of ions and its acid-base balance right.