Nephron

A nephron is one of the tiny tubular units that carry out the kidney's work; each human kidney contains around a million of them. A nephron has two main parts. The renal corpuscle consists of the glomerulus, a knot of capillaries, cupped inside the glomerular (Bowman's) capsule. The renal tubule leads away from it in sequence: the proximal convoluted tubule, the loop of Henle, which dips down into the medulla, and the distal convoluted tubule, which drains into a collecting duct shared by many nephrons.

Urine forms in three steps. In filtration, blood pressure forces water and small solutes out of the glomerulus into the capsule, while blood cells and most proteins stay behind. In reabsorption, the tubule returns most of the water and useful substances, such as glucose, sodium and amino acids, to the blood; the proximal tubule does much of this work. In secretion, the tubule moves extra wastes and ions from the blood into the fluid. Hormones such as antidiuretic hormone and aldosterone fine-tune the later segments and the collecting ducts, so the kidney can make urine more dilute or more concentrated as needed.

The glomerular filtration rate, the volume of fluid all the nephrons filter each minute, is the standard measure of kidney function.