What excretion means
Excretion is the removal from the body of the toxic waste products of metabolism (chemical reactions in cells) and substances in excess of requirements. The main excretory organs are the kidneys, which remove urea, excess water and excess salts, and the lungs, which remove carbon dioxide.
Remember
- Excess amino acids cannot be stored, so the liver breaks them down by deamination, forming urea.
- Urea travels in the blood to the kidneys to be removed.
How a nephron works
Each kidney contains about a million tiny tubules called nephrons. Blood at high pressure enters a knot of capillaries, the glomerulus, inside a Bowman's capsule.
Key idea
Ultrafiltration: high blood pressure forces small molecules (water, glucose, salts, urea) out of the glomerulus into the capsule. Large molecules such as proteins and blood cells are too big and stay in the blood.
Selective reabsorption then follows along the tubule: all the useful glucose, most of the water and needed salts are reabsorbed back into the blood. Urea and excess water and salts remain, forming urine, which passes down the ureter to the bladder.
Controlling water (osmoregulation)
The hormone ADH (antidiuretic hormone) controls water reabsorption. When the blood is too concentrated, more ADH is released, more water is reabsorbed, and a small volume of concentrated urine is produced.
Worked example
A person drinks 2 litres of water quickly. The blood becomes dilute, so less ADH is released. Less water is reabsorbed in the tubules, so a large volume of dilute urine is produced, returning the blood to normal concentration.