There are a pair of kidneys that are purplish-brown and are located below the ribs in the middle of the back. Their function is to:
Remove waste from the blood in the form of urine
Keep substances stable in the blood
Make erythropoietin, a hormone which helps make red blood cells
Make vitamin D active
Regulate blood pressure
The kidneys remove waste from the blood through tiny filtering units called nephrons. Each nephron is made up of a ball of small blood capillaries, called a glomerulus. There is also a small tube called a renal tubule, which drains the urine and joins other tubules carrying the urine out of the kidney to the ureter.
Urea, together with water and other wastes, forms the urine.