Metabolism: Metabolism consists of all the chemical reactions that occur within the cells of the body. Some of the products of metabolism are used by the cells. However, some of the products of cellular metabolism are toxic to the cells. These toxic waste products must be first removed from the cells, and then eventually from the body before they reach concentrations which are toxic to the body cells.

Excretion: Excretion is the removal from the body of the toxic waste products of metabolism.

In higher animals, the excretory system is the organ system which removes waste products from the blood and then removes them from the body. The kidneys are the primary organs of the excretory system which remove cellular waste products from the blood and in turn produce urine, which contains these waste products. The urine and its wastes are then stored in the bladder before traveling through the urethra (the passageway which carries urine from the bladder to outside the body) to exit the body.

In terms of maintaining the body at a constant state, the kidneys also play a major role. By varying the amount of water and/or salt they filter from the blood, they can maintain the percentage of water and salt remaining the blood within narrow limits. This then keeps the levels of water and salts in the fluids bathing the tissues at near constant levels.

When there is too much water (or too little salts) in the blood, the kidneys filter more water out of the blood and less salt. This reduces the water content of blood, and thus boost the salt content of the blood.

When there is too little water in the blood or too much salt, the kidneys filter less water from the blood but filter more salt out of the blood. This creates urine that has low water content but a high concentration of salts.