A common perception of the urinary system is that it rids the body of waste materials in the form of urine and, well, that is about it. Although the urinary system—composed mainly of the paired kidneys and ureters, and an unpaired bladder—do in fact perform this function, there are other vital functions that are especially critical to the maintenance of a stable environment in the body—homeostasis. Almost all these functions are performed exclusively by the paired kidneys.
They regulate water and electrolyte balance. The body always ensures that the excretion of water and electrolytes (e.g. sodium, potassium and calcium) must be precisely matched to their intake, so that they remain at a constant level in tissues.
Maintaining the acid-base balance of the blood and body fluids is another function of the kidneys—along with the lungs and body fluid buffers. The human body is very sensitive to even slight changes in its pH level; so these mechanisms exist to regulate it.
High blood pressure has several life-threatening complications associated with it; similarly low blood pressure is not an amiable condition. The kidney—through a hormone system—plays an important role in maintaining blood pressure within normal range.
The kidneys stimulate the body to make red blood cells, by releasing a hormone called erythropoietin. That is why people with kidney problems or those who have had them removed might develop anemia (too few red blood cells).
Vitamin D (gotten either from sunlight exposure or diets) is initially in its inactive form. The kidney (with the liver) is responsible for producing the active form of this vitamin. Vitamin D is essential for healthy bones and teeth.
Finally, the kidney also has the capacity to produce glucose during prolonged fasting. It does this through a complex process referred to as gluconeogenesis.