The kidneys are a pair of vital organs that can be found in all vertebrate animals. They provide several functions: the excretion of some of the blood's waste products; the regulation of water, mineral and pH levels in the body; and the release of hormones and enzymes, including renin, erythropoietin and 'activated' vitamin D.

Human beings usually have two kidneys (though you can survive with less than a full set), which are situated in the small of the back, with one kidney either side of the spine. They are plumbed into your circulatory system, with the renal arteries being the primary branches off the aorta and the renal veins emptying back into the vena cava to return the filtered blood to the heart. The waste water and filtered salts and nitrates leave the kidney through a third vessel, the ureter, which leads down to the bladder, where we can (usually) dispose of it at will.

The kidney itself is shaped like a bean (hence the name kidney bean) with a niche halfway down one side - the 'hip' or pelvis of the kidney - where the renal artery, renal vein and ureter enter and leave the organ. Internally the kidney has two distinct parts, the inner medulla and the outer cortex.

The Cortex

The artery enters the kidney and branches through the medulla to reach the outer edges of the cortex. In the cortex the blood vessels branch further into tiny capillaries just as they do elsewhere in the body. However, in the kidney, these capillaries form millions of microscopic knotted balls called glomeruli. The capillaries of the glomerulus are 100 times more porous than those elsewhere in the body. The glomerulus is encapsulated by a partially permeable sheath called the renal corpuscle, and it is these tiny sheathed knots of blood vessels, glomeruli and corpuscles, that form the cortex, the kidney's outer layer.

As blood passes through the capillaries, both water and the substances dissolved in it, pass from the blood through the membrane of the corpuscle due to the extreme high pressure of the blood in these small vessels, and the porous nature of the tissue and it is this extraction process which is function of the cortex.

The Medulla

From the corpuscle, the liquid, which is a fairly weak solution of glucose, different salts, nitrates, water, passes into a thin tubule called the nephron. These long microscopic tubes form the body of the medulla, and empty their contents into the ureter. By this time the liquid waste is urine, a concentrated solution of waste salts and urea.

The function of the nephrons which make up the medulla is to reabsorb into the blood the bulk of the water from the solution, and to reabsorb those still valuable salts and sugars back into the blood.

The Nephron

There are around a million nephrons in each kidney. The nephron is then a single long tube with one closed end, which forms the Bowman's Capsule around the glomerulus, and one open end from which it drains its filtrate. This tiny capillary tube has three recognisable sections - the coiled proximal and distal tubules separated by the loop of Henle. The function of the nephron is osmosis and active transport of ions to and from the nephric filtrate across the selectively permeable cell membrane of the nephron itself - one of those phenomena of the human body, along with memory, which we simply do not understand. Selective transport means certain substances are specifically selected and transported across through the cells of the tubule into what is often a higher concentration gradient.

In the proximal Convoluted tubule a massive amount of active reabsorption takes place. Over 90% of the filtrated uric acid, 70% of the inorganic salts, and all of the glucose and amino acids are reabsorbed into the blood here. It is here that levels of angiotensin and parathyriod hormone, secreted elsewhere in the body, control the amount of sodium and phosphate ions reabsorbed, and thereby controlling blood pressure. With such a massive transport of ions back into the blood the osmotic potential is once again raised in the capillaries surrounding the tubule and the larger part of the water in the filtrate is also reabsorbed through osmosis.

The loop of Henle has a descending and ascending section, where an ingenious switch in the direction of the surrounding capillaries means that active transport of sodium from the later ascending section of the loop is absorbed into the blood vessels which then move on with a raised osmotic potential to the earlier descending section of the loop, where this stronger solution causes further water uptake by osmosis than had been available in the proximal tubule.

In the distal tubule these processes continue, and though 98% of the sodium has been reabsorbed by this point it is the delicate control of the final 2% which controls the blood's final sodium level, therefore its water content, and therefore blood pressure! This fine trigger mechanism is controlled in the distal tubule by the hormone aldosterone. As well as actively transporting ions back into the blood the distal tubule also transports further ions from the blood for excretion, in a process called 'tubular secretion'. Here there is an ion for ion uptake of hydrogen and potassium ions from the blood for each sodium ion reabsorbed. The secretion and excretion of these ions controls the pH of the blood to limits between 7.3 and 7.4 (as a result, the final pH of the urine can be anywhere between pH 4.5 and 8.5 - ouch).

Hormone Production and Release in the Kidney

As the kidney controls the volume and concentration of the blood it is not surprising that is also involved in the regulation of blood pressure. As the blood enters the kidney almost directly from the heart, it still maintains the high pressure it has in the aorta, the kidney is then very sensitive to variations in that pressure. Blood pressure can be varied or maintained through control of both the volume of blood in the body, and the contraction/relaxation of the arteries. This is done by the release of the amino acid peptide renin, and its release forms part of a metabolic cycle with the release of ADH and the re-absorption of water into the blood.

Similarly the kidney release two other vital chemicals. Erythropoietin, or EPO, stimulates the bone marrow to make red blood cells. Red blood cells contain an iron based substance called haemoglobin which transports oxygen around the body within the cells to where it is needed. The kidney also helps maintain the bones by releasing calcitrol, the activated form of vitamin D, into the blood. This maintains the normal chemical balance of calcium in the body keeping the bones maintained and healthy.

Kidney Problems

Perhaps the most painful problem one might encounter is kidney stones, often said to be 'more painful than childbirth'.

A common complaint is 'a cold in the kidney'. This is in fact a folk term for an umbrella of problems and infections which cause increased urine production or otherwise stimulate the desire to urinate.

Other, less common problems, are caused by occlusions of the kidneys major vessels, renal artery stenosis and PJU stenosis which can lead to a swelling of the kidney, hydronephrosis.

Kidney Dialysis

When one or both kidneys have failed completely or are temporarily malfunctioning their operation must be undertaken artificially by 'dialysis', an artificial kidney or 'kidney machine' filtering the blood, usually overnight. These machines can keep people alive until a donor organ becomes available, or until their own kidneys recover, but they can only perform the kidney's excretion functions and not perform the delicate ionic and hormonal balancing acts described above. Clearly a patient having to undergo dialysis attached to a machine for up to 10 hours a day, from between four and seven times a week, is taxed physically and mentally, being dependant on this treatment, restricting their ability to travel away from their dialysis machine and cutting a large chunk of time from their weekly routine. New portable dialysis machines are being developed. Dialysis must continue regularly until a transplant can be performed.

Kidney Transplants

The kidneys were the first major organs to be successfully transplanted, the first such operation being conducted in 1954. Having been perfected back in the 1970s these operations have become almost routine these days. The ability to perform the operations is restricted only by the number of available suitable donors. To this end the kidney donor card scheme was introduced. Donor cards now cover all organs suitable for transplantation.