Library · Physiology
Urine formation
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Urine is made in two stages, and the first one is a straining step. It is called glomerular filtration and it happens in the glomerulus, the tuft of capillaries where each of the kidney's filtering units begins. There, hydrostatic pressure, the push of the blood, drives fluid and solutes across the filtration membrane without the kidney spending any energy on it.
That membrane is not a simple strainer: it has three layers. The fenestrated capillary endothelium, full of windows. The basement membrane, negatively charged, which holds back proteins. And the slits between the feet of the podocytes, cells whose extensions leave very narrow gaps between them.
The second stage is giving things back. The proximal convoluted tubule, the first stretch of tube after the glomerulus, recovers nearly all the filtered glucose and amino acids and about 65% of the sodium and water. First the fluid is strained through without spending energy, and only then is what is useful returned.
In brief
Urine formation begins with glomerular filtration: hydrostatic pressure drives fluid and solutes across the filtration membrane without using energy.
That membrane has three layers: the fenestrated capillary endothelium, the negatively charged basement membrane that restricts proteins, and the slits between podocytes.
The proximal convoluted tubule then reabsorbs nearly all the filtered glucose and amino acids and about 65% of the sodium and water.
First the fluid is strained through without spending energy, and only then is what is useful returned.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Renal, StatPearlsPublished
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