The urinary system is responsible for ridding the body of many of its waste products. It also helps to maintain a stable environment by regulating the composition of body fluids. The waste substances are removed from the blood and expelled in urine, which is produced by the kidneys. Together, these two organs receive about one-quarter of the blood pumped out by the heart, and yet they contribute less than 1 per cent to our body weight.
The kidneys filter the blood they receive so that waste substances and excess water pass into the urine. Substances that the body needs stay in the blood. The kidneys each contain huge numbers of nephrons, tiny structures that make urine and fine-tune its composition to maintain a stable environment in the body.
The kidneys make urine, which flows down the ureters to the bladder. The bladder stores urine and expels it from the body via the urethra, a tube that is about 20 cm (8 in) long in men and 4 cm (11/2 in) long in women. Signals from the brain and spinal cord trigger emptying of the bladder.
Each kidney contains about one million glomeruli, tiny clusters of blood capillaries. Each glomerulus forms part of a nephron, one of the kidney’s filtering units. A glomerulus is the part of a nephron where water and waste products are forced out of the blood and carried away via a system of little tubes and ducts as urine.