The human kidney and the nephron

The human excretory system has a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. The kidneys are reddish-brown, bean-shaped, and lie against the back wall of the abdomen, between the last thoracic and third lumbar vertebrae. Each is about 10-12 cm long, 5-7 cm wide, 2-3 cm thick, and weighs 120-170 g.

On the inner, curved-in side of each kidney is a notch, the hilum, where the ureter, blood vessels and nerves enter. Inside it is a funnel-shaped space, the renal pelvis, with branches called calyces. Under a tough outer capsule, the kidney has an outer cortex and an inner medulla. The medulla is divided into cone-shaped medullary pyramids that point into the calyces, and the cortex dips down between the pyramids as the Columns of Bertini.

Each kidney has nearly one million tiny filtering units, the nephrons: the functional units of the kidney. Each nephron has two parts:

• The glomerulus: a knot of tiny blood capillaries formed by an incoming vessel (the afferent arteriole) and drained by an outgoing one (the efferent arteriole).
• The renal tubule, which starts as a double-walled cup, Bowman's capsule, around the glomerulus. Then come the coiled proximal convoluted tubule (PCT), the hairpin loop of Henle (with a descending and an ascending limb), and the distal convoluted tubule (DCT), which opens into a collecting duct.

The glomerulus together with Bowman's capsule is the Malpighian body (renal corpuscle). The Malpighian body, PCT and DCT lie in the cortex; the loop of Henle dips into the medulla.

Most nephrons have short loops that barely enter the medulla: cortical nephrons. Some have very long loops running deep into the medulla: juxta medullary nephrons. The efferent arteriole forms a network of capillaries around the tubule (peritubular capillaries), and a thin U-shaped vessel, the vasa recta, runs alongside the loop of Henle. The vasa recta is well developed in juxta medullary nephrons, but reduced or absent in cortical nephrons.