The kidneys are two glandular structures which extract water and certain soluble waste materials from the blood and pass them by way of the ureters into the bladder in the form of urine. They are also responsible for also by balancing levels of electrolytes in the body, controlling blood pressure, and stimulating the production of red blood cells. The kidneys as well as the sweat glands in the skin are filtration plants which permit the passage of water and wastes through their structure, but prevent the passage of the corpuscles and food materials of the blood. One of the primary ways the kidneys do this is by absorbing the waste products of metabolism (urea, sulphur, hormonal substances etc.) and then eliminating them in urine.