Understanding kidney disease

Humans have a right and a left kidney, which are oval organs about the size of a fist that lie to either side of the spine. The primary function of the kidneys is to filter waste products, excess sodium, and water from the blood, and help eliminate them from the body.

Each kidney is supplied with blood by a large artery. In healthy adults, the two kidneys together receive approximately 20% of the blood pumped by the heart, which translates into one litre of blood per minute to the kidneys.

Each kidney contains approximately one million nephrons, which have a tubule (small tube) and a filtering unit called a glomerulus. In the glomerulus, blood passes through very specialized tiny blood vessels, called capillaries. The blood pressure within the capillaries forces much of the watery component of the blood to exit into the space surrounding these capillaries and into the kidney tubules. The cells and protein of the blood are unable to cross into the tubules. Water and salt are reabsorbed by the tubules and the waste that remains forms urine. The urine drains in the middle of the kidney, then into the bladder via the ureter and out of the body through the urethra.

During the course of this filtration process other substances, such as medications, are also passed into the urine.

Your kidneys provide more filter power than your body needs. This means that even one kidney or parts of two kidneys are enough to filter waste products from your blood.

Your kidneys play a critical role to keep your body healthy. Your kidneys:

  • regulate the level of water in your body
  • remove wastes such and urea produced from the breakdown of protein and creatinine from the breakdown of muscle
  • help to regulate the levels of minerals in the body such as sodium, potassium, calcium, and phosphorous
  • remove medications from your body
  • produce the hormone erythropoietin, which controls the rate of production of red blood cells
  • activate vitamin D in the body


Written and reviewed by the MediResource Clinical Team