Morphology and cellular organisation of the kidney

Each human kidney has 1–1.5 million functional units called nephrons. The nephron is a blind-ended tube, the blind end forming a capsule (Bowman's capsule) around a knot of blood capillaries (the glomerulus). The other parts of the nephron are the proximal tubule, loop of Henle, distal tubule and collecting duct, although in transport terms the nephron has been divided into additional segments (Figure RE1).

The glomeruli, proximal tubules and distal tubules are in the outer part of the kidney, the cortex, whereas the loops of Henle and the collecting ducts extend down into the deeper part, the medulla.

Cortical nephrons possess glomeruli located in the outer two-thirds of the cortex and have very short loops of Henle, which only extend a short distance into the medulla or may not reach the medulla at all. In contrast, nephrons whose glomeruli are in the inner third of the cortex (juxtamedullary nephrons) have long loops of Henle that pass deeply into the medulla. In humans about 15% of nephrons are long-looped, but there are also intermediate types of nephron.

Renal blood supply and vasculature

The kidneys receive 20–25% of the cardiac output but account for only 0.5% of the body weight. Of the blood to the kidney, >90% enters via the renal artery and supplies the renal cortex, which is perfused at about 500 ml 100 g−1 tissue min−1 (100 times greater than resting muscle blood flow).