Glomerular filtration

Urine is made in three main steps, each in its own parts of the nephron: glomerular filtration, reabsorption and secretion.

Filtration of blood by the glomerulus comes first. On average the kidneys filter 1100–1200 mL of blood a minute, roughly one-fifth of what each ventricle pumps out per minute.

The push comes from the blood pressure in the glomerular capillaries. Blood is filtered through three layers: the lining endothelium of the glomerular vessels, the basement membrane, and the epithelium of Bowman's capsule.

The epithelial cells of Bowman's capsule, the podocytes, are arranged intricately so that tiny gaps, filtration slits or slit pores, are left between them.

The filter is so fine that nearly everything in plasma except the proteins passes into the lumen of Bowman's capsule, so the process is called ultrafiltration.

The volume of filtrate formed by the kidneys each minute is the glomerular filtration rate (GFR). In a healthy person GFR is about 125 mL/minute, which is 180 litres a day.

The kidneys regulate GFR themselves. One efficient route is the juxta glomerular apparatus (JGA), a sensitive region formed by modified cells of the DCT and the afferent arteriole where the two touch.

A fall in GFR can make the JG cells release renin, which stimulates glomerular blood flow and so brings GFR back to normal.