More information about the kidneys (child)

Each kidney has an outer layer called the cortex and an inner layer called the medulla. The basic unit of the kidney is called the nephron, and each adult kidney contains hundreds of thousands. The nephron starts with a filtration unit called the glomerulus, which contains capillaries filled with blood from the renal artery. Salts, glucose, water and the waste products of metabolism such as urea and creatinine (in fact everything in the blood except blood cells and large molecules such as proteins) cross the walls of the capillaries and enter the glomerulus. From there they pass into the tubular part of the nephron, which goes from the cortex down into the medulla and back up again, before passing back through the medulla to become what is known as the collecting duct. This drains into the centre of the kidney - the renal pelvis - which is connected to a tube called the ureter. These, running down the back of the abdomen alongside the spine, carry urine from the kidneys to the bladder.

Under normal circumstances the kidneys receive 25 per cent of the cardiac output, which means that in the adult about a litre of blood is filtered every minute. Of that, 999 mls is reabsorbed, and one ml of urine is produced. Besides the salts and water, most of the other filtered constituents are reabsorbed with the exception of urea and creatinine, which are excreted.

The kidneys begin to develop during early embryonic life. The urine they produce drains from the bladder via a tube called the urachus, which passes from the foetus via the umbilicus with the umbilical vessels. Kidney function at birth is only about one 6th that of the adult; it improves during the first year of life and reaches adult levels by about the age of one.

The amount of fluid we lose depends on intake, activity and temperature. In times of excessive exercise, or when it’s very hot and we sweat a lot, a part of the brain called the hypothalamus recognises that the blood is becoming too concentrated and instructs the pituitary gland (see more information on the brain in the head section) to increase its production of a hormone called anti-diuretic hormone (or ADH). This acts on the collecting ducts and more water is reabsorbed, making the urine more concentrated and the blood more dilute. (It has been said that the urinary output of Baghdad is but a puff of dust, but it’s necessary to produce at least half a litre of urine a day in order to remove toxic products). If we take excessive amounts of fluid the brain recognises that the blood is becoming too dilute, and so switches off the production of ADH.

Salt balance is monitored by cells in the cortex of the kidney close to the filtration apparatus. There are sensitive to the amount of salt in the blood and urine, as well as the blood pressure. If they sense the pressure is too low they secrete a hormone called renin, which acts via other hormones to increase the retention of sodium, which causes increased retention of fluid and a resultant increase in the circulating blood volume, and hence the blood pressure. Many drugs, called diuretics, work in different parts of the kidney to increase both salt and fluid loss - effective in patients who are retaining too much fluid, such as those with congestive cardiac failure (see the heart in the chest cavity section), or in those with high blood pressure.

Urine passes from the kidneys down the ureters in small spurts, driven by contractions of the renal pelvis. The ureters in the adult are about 25 centimetres long, and they end in the bladder which lies in the pelvis. They pass through the wall of the bladder at an oblique angle, which means that when the bladder contracts during urination they are closed, to prevent urine passing back up to the kidney (a condition called reflux). The bladder is a hollow muscular organ lined by cells called transitional cells. It has a capacity in the adult of between 250 and 500 millilitres, and is served by nerves from the autonomic nervous system (see more information in muscles and nerves in the limbs section). When there is about 200 mls of urine in the bladder it starts to contract, and the nerves inform us of the need to urinate. If it is socially inconvenient we can override this reflex and the contractions settle down, but once the bladder is filled to capacity the reflex is reactivated, the desire to urinate becomes overwhelming, and we may pass water spontaneously.

The urine leaves the bladder via a tube called the urethra. In the adult male this is about 20 centimetres in length, and initially passes through the prostate gland where it is joined by the two ejaculatory ducts (from the two vas deferens, which carry sperm from the testicles, and glands called the seminal vesicles which produce most of the fluid present in ejaculate) and a number of prostatic ducts which carry the secretions from the prostate gland necessary to nourish the sperm. After the prostate the urethra passes through a voluntary sphincter, and then under the bony symphysis pubis (the bone at the bottom of the abdomen) and into the penis. It ends at the glans - the head of the penis - as an opening called the penile meatus.

In the female the urethra is much shorter, a matter of two to three centimetres. It runs in the anterior vaginal wall and opens behind the clitoris. It has a sphincter which is mainly made up of muscle fibres from the bladder, and which partly depends on the support offered by the muscle of the anterior vaginal wall in order to function properly. During urination pressure in the bladder rises and the tone of the muscles forming the sphincter relaxes: water is passed and the bladder should empty completely.