The filtering is carried out through the capillaries which are present in the Bowman's capsule. Seen to the right of the diagram below.

Water and waste products are separated from the blood by the filters and flow into the tubules. Much of this water is reabsorbed by the tubules and the wastes are concentrated into urine.

The tubules receive a combination of waste materials and chemicals that your body can still use. Your kidneys measure out chemicals like sodium, phosphorus, and potassium and release them back to the blood to return to the body. In this way, your kidneys regulate the body's level of these substances. The right balance is necessary for life, but excess levels can be harmful.

The kidneys remove wastes and extra water from the blood to form urine. Urine flows from the kidneys to the bladder through the ureters.

Now let us take a much closer look at the structure of the capillaries where the filtration takes place.

It is clear that the main barriers to filtration are the gaps between the cells lining the capillary (endothelial cells), the basement membrane and the holes in the cells on the other side of the basement membrane.

Wastes, such as urea and creatinine, must also be removed from the body. Urea and other wastes are made when the body breaks down protein, such as meat. Creatinine is a waste product of the muscles. As blood flows through the blood vessels, small molecules such as waste products squeeze through the holes. These waste products become part of the urine. As kidney function decreases, the levels of urea and creatinine in the blood increase.

Useful substances, such as protein and red blood cells, are too big to pass through the holes in the filter. They stay where they belong--in the blood.

Many of the substances in the blood and body fluid must be kept at the correct level for the body to function properly. For example, sodium and potassium are minerals which come from food. These minerals are needed by the body for good health, but they must be kept at specific levels. When the kidneys are working properly, excess minerals, such as sodium and potassium, are excreted from the body in the urine. The kidneys also help to regulate the levels of other minerals, such as calcium and phosphate, which are important for the formation of bone.

The urine is collected from the tubules in a funnel-like renal pelvis and then flows through a tube called the ureter into the bladder. The bladder stores urine until you urinate. Urine passes out of the body through a tube called the urethra. The kidney normally makes one to two litres of urine every day depending on how much you drink.

The normal kidney has the ability to greatly increase its workload. If one kidney is lost, the other kidney can enlarge and do the work of two.