The kidneys, which excrete nitrogenous wastes, are the primary excretory organs, but other organs also function in excretion: The skin excretes water and salts in sweat; the lungs excrete carbon dioxide as a gas; and the liver excretes bile pigments that come from the breakdown of hemoglobin by way of the intestine.
The terms defecation and excretion do not refer to the same process. Defecation refers to the elimination of feces from the body. Excretion, on the other hand, refers to the elimination of metabolic waste products of metabolism. For example, undigested food and bacteria, which make up feces, have never been a part of the functioning of the body, while electrolytes in the urine are excretory substances because they were once metabolites in the body.
The urinary system includes the kidneys and associated structures.
Functions of the Urinary System
The primary functions of the urinary system are carried out by the kidneys:
- The kidneys excrete nitrogenous wastes, such as urea, uric acid, creatinine, and ammonium.
- The kidneys maintain blood volume by regulating the amount of water excreted.
- The kidneys monitor blood composition by regulating electrolyte excretion. Sodium (Na +), is the most significant, but potassium (K+), bicarbonate (HC03 -), and calcium (Ca2+), among others, are also important.
- The kidneys monitor blood pH chiefly by regulating the excretion of certain ions, such as hydrogen (H+).
- The kidneys secrete the enzyme renin, which helps maintain blood pressure.
- The kidneys secrete the growth factor erythropoietin, which stimulates red blood cell production.
