Students should be able to explain the effect on cells of osmotic changes in body fluids. • Water leaves the body via the lungs during exhalation. Water, ions and urea are lost from the skin in sweat. There is no control over water, ion or urea loss by the lungs or skin. Excess water, ions and urea are removed via the kidneys in the urine. If body cells lose or gain too much water by osmosis they do not function efficiently. • The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water. • People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis. Students should know the basic principles of dialysis.