nephron

A nephron is the functional unit of the kidney, and there are about a million in each one. It has two halves. The renal corpuscle is a knot of capillaries, the glomerulus, sitting inside a cup called Bowman's capsule; blood pressure there forces water and small solutes out of the blood and leaves cells and large proteins behind, which is filtration. The tubule then runs from the capsule through the proximal convoluted tubule, down the loop of Henle, through the distal convoluted tubule and into the collecting duct. Along that run the nephron takes back what the body needs — reabsorption of glucose, amino acids, most of the water and much of the sodium — and adds what it wants gone — secretion of hydrogen ions, potassium and some drugs. What leaves the collecting duct is urine. Two hormones tune the last steps: antidiuretic hormone increases water reabsorption, and aldosterone increases sodium reabsorption.