When the proteins combine with oxygen, a waste is produced called as nitrogenous wastes. The solid wastes are difficult to be removed from the body unlike the gaseous wastes like carbondioxide and water vapour, hence solid wastes are to be converted into soluble waste product such as urea because nitrogenous wastes cannot be converted to gaseous form by the body efficiently and exhaled it.
The major function of the urinary system is to remove urea from the blood stream, so that it does not accumulate in the body and become toxic.
Urea is formed in the liver from ammonia, which in turn is derived from the breakdown of simple proteins in the body cells. The urea is carried in the bloodstream to the kidneys, where it passes with water, salts and acids out of the bloodstream and into the kidney tubules as urine. Urine then travels down the uterus into the bladder and out of the body.
Besides removing urea from the blood, another important function of the kidneys is to maintain the proper balance of water, salts and acids in the body fluids.
Salts such as sodium, potassium and some acids are known as electrolytes. Electrolytes are necessary for the proper functioning of the muscle and nerve cells. The kidney adjusts the amount of water and electrolytes by secreting some substances into the urine and holding back others in the bloodstream for use in the body.
In addition to forming urine and eliminating it from the body, the kidneys also act as endocrine organs, secreting into the bloodstream substances that act at some distant site in the body.
Example of the kidneys’ endocrine function include the secretion of rennin, a substance important in the control of blood pressure, and erythropoietin, a hormone that regulates the production of red blood cells.
The kidneys also secrete an active form of vitamin D, necessary for the absorption of calcium from the intestine. In addition, hormones such as insulin and parathyroid hormone are degraded and extracted from the bloodstream by the kidneys.