More information about the kidneys (female)

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 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 about a litre of blood is filtered every minute. Of that, 999 mls is reabsorbed, and 1 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.

As well as losing fluid in urine, we lose some in the faeces and by evaporation from the skin and in the breath. In general we lose about 800 mls in evaporation and about one and a half litres via urination over 24 hours. 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 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 concentrated and the blood more dilute. (It has been said that the urinary output of Baghdad is but a puff of dust, but it is 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. Alcohol interferes with both the production and function of ADH, which means that we pass a greater volume than we drink, and become dehydrated; one of the features of a hangover.

Salt balance is monitored by cells in the cortex of the kidney close to the filtration apparatus. They 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 section), or in those with high blood pressure.

Urine produced by the kidneys travels down the ureters in small spurts, driven by contractions of the renal pelvis. The ureters are about 25 centimetres long and they end in the urinary bladder, a hollow muscular organ situated in the pelvis. They enter the bladder at an oblique angle, which means that when the bladder contracts in order to empty during urination, the bottom of the ureter is closed, and urine cannot pass back up (a process called reflux) and cause damage to the kidney. The bladder has a muscle layer, called the detrusor muscle, and a lining of cells called transitional cells. Under normal circumstances the adult bladder has a maximum capacity of between 250 and 500 millilitres. The nerve supply is from branches of the autonomic nervous system which monitor the filling pressure and inform us of the need to pass water once there’s about 200ml present. If it is socially inconvenient we can override the sensation and the bladder contractions die down, but once the bladder has expanded to its maximum capacity the reflex becomes overwhelming, and we are likely to urinate spontaneously. Under conditions such as fear, excitement, and anxiety the reflex is stimulated much more readily, when there may be only a small amount of urine present in the bladder, which is why we need to go to the loo when we’re nervous.

The urine leaves the bladder via a tube called the urethra, which in the female is about two and a half centimetres long. It runs in the anterior vaginal wall and opens about two centimetres 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.


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