What do the kidneys do?

The kidneys are the organs that filter and purify the blood, ridding the body of excess water, salts and waste product. The blood entering the kidneys is filtered through a series of coils of small blood vessels, which look like balls of wool under a microscope.m This filter (called glomerulus) allows water and chemicals to pass through, but holds back the useful proteins and blood cells.

The filtered fluid is then processed as it passes down a long tube (called the tubule). By the time it reaches the end of the tubule only the excess water, salts, acids and waste products are left. This fluid, now called urine, passes into the pelvis of the kidney and flows down a long muscular tube (called ureter) into the bladder.

Another important function of the kidneys is to produce three essential messenger substances :

• Activated vitamin D, which is necessary for calcium absorption from the bowel and healthy bone formation
• Renin, which controls blood pressure
• Erythropoetin, which stimulates the production of red blood cells, which carry oxygen around the body

In chronic kidney disease, less of the messenger substances are being produced.