The function of the excretory system is to remove waste and harmful by-products of normal metabolic processes from the body. If these waste products accumulate, they interfere with the delicate balance of oxygen, water, and nutrients needed for normal cell development. The main by-product of cell metabolism, carbon dioxide, is expelled by respiration, but cellular metabolism also produces a host of salts, fats, and excess chemicals, including carbon, hydrogen, oxygen, and nitrogen. The excretory system filters out those that are harmful and eliminates them so the body can maintain a state of homeostasis.
While the entire body is involved in the process of eliminating waste, the main organs of the excretory system are the kidneys. They are two bean-shaped organs located in the lower back that contain more than a million filtering units called nephrons. As blood passes through the kidneys, the nephrons filter out urea, a nitrogenous carbon compound produced by cell metabolism, and send it through tubes called ureters to the bladder where it is eliminated by urination. If the excretory system is functioning normal, the nephrons recycle beneficial substances, like glucose and proteins, back into the blood stream.
Elimination of waste from digestion is also a function of the excretory system, and the liver plays a role by decomposing worn out blood cells into bile pigments and passing them to the alimentary canal where they can be eliminated by defecation. The liver also produces urea and detoxifies the body by eliminating cholesterol and harmful toxins. The skin plays a role in maintaining a healthy internal balance by eliminating salts and other water soluble contaminants through perspiration. It is not considered part of the excretory system per se, because the process is more one of secretion than it is of active excretion.
Since the function of the excretory system is vital to maintaining health, kidney disease, or renal failure, is life-threatening. Symptoms include changes in urination, swelling in the legs, shortness of breath, fatigue, and skin rashes. If one kidney becomes diseased, it is possible for the body to function normally if the other is healthy. If both fail, a patient must rely on survival dialysis, which is external cleansing of the blood, unless a transplant can be performed. The liver's role in the excretory system is also vital, and, though it is a resilient organ, patients with acute liver failure suffer a high mortality rate.