IGCSE Biology - Excretion in Humans

Excretion and the kidneys

Excretion

• The removal from organisms of toxic materials, the waste products of metabolism (chemical reactions in cells including respiration) and substances in excess of requirements. Substances should include carbon dioxide, urea and salts.

• The function of the kidney:

In the glomerulus, the blood pressure causes part of the blood plasma to leak across the wall of the capillary. The fluid that has filtered through consists mostly of water with dissolved salts, glucose, urea and uric acid. This filtrate is then collected in the renal tubule. Capillaries, which surround the tubule, absorb substances back into the blood which the body needs. First, all of glucose is reabsorbed, with much of the water. Next, some of the salts are taken back in order to keep the correct concentration in the blood.

•  Urea is formed in the liver from excess amino acids.

• Alcohol, drugs…