Excretion is the removal of waste products which are made by chemical reactions in the body (products of metabolism). An example is carbon dioxide being a waste product of respiration. The carbon dioxide is excreted through the lungs. Another example is the kidneys excreting water and salt. The kidneys contain hundreds of tubules. The tubules filter water, glucose, urea and salt from the blood. Then glucose, water and some salts are selectively reabsorbed. Then the waste products (water, urea and salt) pass to the bladder as urine.
Water is lost from the body through the lungs and the skin. The water balance in the body is kept level by the kidneys (see above). When the body is warm, the skin loses water through seating to cool the body down. The lungs also give off more water in warm air. The urine is more concentrated to conserve water. On a cold day the urine would be less concentrated as the body would not need to lose as much water.