The kidneys are one of the major organs of the body. People usually have two kidneys. They are found on either side of the spine with their top part inside the lower ribs, below the diaphragm, inside the abdominal cavity.
Kidneys are bean shaped, and in adults are about 7 - 10 cms (2½ - 4 inches) long and 2.5 - 4 cms (1 – 2 inches) wide. Each kidney weighs about 150 gms (about 5 ounces).
Each kidney receives blood from the main artery of the body via an offshoot called the renal artery. As the blood flows through the kidneys it passes through tiny tubules calleds nephrons. There are over a million nephrons in each kidney. Each nephron contains a microscopic filter that continuously filters out toxins in the blood from food waste and removes excess water. This also contributes to the control of acid levels in the blood. The blood leaves the kidneys via the renal vein. The waste that is filtered from the blood is called urine. The urine is passed from the kidney to the bladder via a fine tube called the ureter. The urine is then passed out of the body via an opening controlled by muscle in the bladder called the urethra. Kidneys normally produce about 1 to 2 litres (approximately 2 – 4 pints) of urine a day. Less urine is produced at night.
The kidneys produce a hormone called erythropoietin – or EPO for short – that stimulates the bone marrow to produce red blood cells.
Whenever the kidneys detects a decrease of blood pressure in the nephrons they produce a hormone called renin. Renin triggers a chain of actions in the body that leads to an increase in blood pressure.
The kidneys make active vitamin D which allows calcium to be absorbed from the gut into the bloodstream.