| In the average adult the kidneys filter the total volume of the blood nearly 300 times a day, producing about 1 litre (2 pints) of urine, passed on four to six occasions. As urine is produced it filters into collecting ducts in the neck of the kidneys and trickles down the ureters into the bladder. As soon as the bladder is half full - its total capacity is about 0.5 litre (1 pint) - stretch receptors in its wall signal the urge to urinate. After the age of about four the sphincter between the bladder and urethra is under voluntary control.
The male urethra, which acts as a passageway for semen as well as urine, is nearly four times as long as the female; this, and the fact that its opening at the tip of the penis is well removed from the anus, explains why men are less susceptible to urinary infections than women. Unfortunately, the short female urethra provides bacteria with a relatively swift invasion route to the bladder.
Urine, the sterile product of the non-stop filtering activities of the kidneys, is 96 per cent water and 4 per cent organic and inorganic solids, and its colour can be a valuable guide not only to the function of the kidneys, but also to the state of the liver and gall bladder. The kidneys regulate the amount of water and also the balance of acid and alkaline constituents in the blood, and on those two things all the chemical functions of the body intimately depend.
Inside each kidney the branches of the renal artery subdivide into a million or so tiny tufts of blood vessels called glomeruli; small molecules - water, salt, various minerals, glucose, and wastes such as urea, a by-product of protein breakdown - squeeze through their walls under pressure and enter a million adjacent tubules called nephrons which selectively reabsorb them; in a healthy kidney, 99 per cent of the water and all the glucose are reabsorbed. This reabsorption process is controlled by hormones made in the pituitary gland at the base of the brain and in the adrenal glands which sit on top of the kidneys. The kidneys themselves produce a number of hormones which help to control blood pressure, stimulate production of red blood cells in the bone marrow, and activate Vitamin D. |
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