Excretion is the removal of waste products made by chemical reactions in the body's cells. The main wastes are carbon dioxide (from respiration), removed by the lungs, and urea (from breaking down excess proteins in the liver), removed by the kidneys.

The urinary system contains two kidneys, two ureters (tubes to the bladder), the bladder (which stores urine) and the urethra (which carries urine out of the body).

Each kidney contains about a million tiny tubes called nephrons. Filtration: blood under high pressure is pushed through a knot of capillaries (the glomerulus) into the start of the nephron. Small molecules such as water, glucose, salts and urea pass into the tubule; this liquid is called the filtrate. Blood cells and proteins are too big and stay in the blood.

Reabsorption: as the filtrate moves along the tubule, all the glucose, most of the water and the salts the body needs are taken back into the blood. What is left (water, urea and excess salts) is urine.

Approximate concentrations (g/L) — Plasma: protein 80, glucose 1.0, urea 0.3. Filtrate: protein 0, glucose 1.0, urea 0.3. Urine: protein 0, glucose 0, urea 20.

The kidneys keep the body's water level balanced. When the blood is too concentrated (too little water), the pituitary gland releases more antidiuretic hormone (ADH), which makes the tubules reabsorb more water. When there is too much water, less ADH is released. This is an example of negative feedback.

If the kidneys fail, a dialysis machine can filter the blood, or the person may receive a kidney transplant.