The function of the kidneys is to maintain the water and salt balance of the body, to regulate the excretion of many substances in the urine and to remove waste products such as urea.

Each kidney has a million microscopic small filtration units called nephrons.

Every day, 1500 litres of blood flow through the kidneys. About 160 litres of primary urine is formed through filtration. Note that the kidneys filter much more fluid than the amount of urine that is excreted (about 1,5 liters per day).

Glomerular filtration is the first step in making urine. It is the process that your kidneys use to filter excess fluid and waste products from blood plasma to the kdiney's renal tubule, so they may be eliminated from your body.

The artery that enters the kidney branches into a cluster of tiny capillaries, which form a structure known as a glomerulus. Within the glomerulus, the blood pressure is so high that some of the blood plasma is pushed through the walls of the capillaries. This primary urine contains water, salts, sugar and urea.

Blood cells and blood proteins are so large in molecular size that they do not pass the "filter" in a healthy kidney but remain in the bloodstream.

Reabsorption is the process where water and solutes, such as glucose and bicarbonate, are transferred from the renal tubule back into blood plasma. The reabsorption of water and specific solutes occurs to varying degrees over the entire length of the renal tubule.

Even after filtration has occurred, the tubules continue to secrete additional substances into the tubular fluid.

Excretion is the term used for the substances that are removed from the body with urine. Excretion is therefore the end result of the above three processes. The urine is made up of water, salts and urea. It continues its journey from the kidneys to the renal pelvis. From the renal pelvis, the urine travels down a narrow tube called the ureter into the bladder. The bladder slowly fills up with urine, which empties from the body through another small tube called the urethra.