Another system, the excretory system, must remove the urea from the blood to prevent too high a concentration of the waste from building up in the blood. The kidneys serve to filter urea from the blood and, also, to re absorb water, simple sugars, amino acids, and salts. The kidneys also help maintain the proper levels of these substances in the blood. Most substances have a kidney threshold level. When the level of a substance exceeds its threshold level, the kidneys do not re absorb the excess substance, but, instead, the excess appears in the urine. Thus, through the removal of nitrogen wastes and other excess substances from the blood and through the re absorption of water, nutrients, and salts, the kidneys play an extremely important function in maintaining homeostasis.
