Urine is composed of unwanted substances that have been filtered from the blood by nephrons, the functional units of the kidneys. The urine formed in the kidneys passes through the ureters and is temporarily stored in the bladder. From here it is emptied, normally under voluntary control, through the urethra. A healthy adult excretes 0.5–2 litres (1–4 pints) of urine each day.
Blood entering the nephron is filtered through a cluster of capillaries called the glomerulus. The filtrate then enters the renal tubule, along which a complex process of secretion and reabsorption occurs. Useful substances such as glucose are reabsorbed; the acidity of the blood is regulated; and water levels are adjusted. The resulting fluid is called urine.
Urine consists of a mixture of waste products and other substances. The mixture is balanced so that the body’s internal environment remains constant. The water content of urine depends on whether there is too much or too little water in the body.
When the bladder is full, nerves in the bladder wall send signals to the spinal cord. Signals are then sent back to the bladder, making it contract and expel urine. In older children and adults, the timing of urination can be regulated because this process is controlled by the brain. Infants lack this control, and the bladder empties when full.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.