Micturition, other excretory organs and disorders

Passing urine. As urine fills the bladder, its walls stretch. Stretch receptors send signals to the central nervous system, which then makes the bladder's smooth muscles contract and the urethral sphincter (a ring of muscle) relax, releasing urine. This is the micturition reflex.

A healthy adult passes 1 to 1.5 litres of urine a day. It is light yellow, slightly acidic (pH about 6.0), and carries away 25-30 g of urea a day. Doctors test urine for clues to disease: glucose in urine (glycosuria) and ketone bodies (ketonuria) are signs of diabetes mellitus.

Other organs also help remove wastes:

• Lungs: remove about 200 mL of CO₂ a minute, and a lot of water.
• Liver: puts bilirubin, biliverdin, cholesterol, broken-down steroid hormones, vitamins and drugs into the bile, which leaves with the faeces.
• Skin: sweat glands remove water, NaCl, a little urea and lactic acid (though sweat's main job is cooling); sebaceous glands remove sterols, hydrocarbons and waxes in an oily secretion, sebum.

When the kidneys fail, urea builds up in the blood: uremia. It can be treated by haemodialysis. Blood is drawn from an artery, mixed with heparin (to stop it clotting), and passed through a long cellophane tube bathed in dialysing fluid. This fluid matches plasma except that it has no nitrogenous wastes, so the wastes diffuse out of the blood into it. The cleaned blood is given anti-heparin and returned through a vein.

The final treatment for kidney failure is a kidney transplant, preferably from a close relative, to reduce the chance of the body rejecting it. Other disorders: renal calculi, kidney stones of crystallised salts like oxalates; and glomerulonephritis, inflammation of the glomeruli.