In vertebrates, kidneys are the two major organs of excretion. Excess
water, toxic waste products of metabolism such as urea, uric acid, and inorganic
salts are disposed of by kidneys in the form of urine. Kidneys are also largely
responsible for maintaining the water balance of the body and the pH of the
blood. Kidneys play important roles in other bodily functions, such as
releasing the erythropoietin protein, and helping to control blood pressure.
Kidneys are paired, reddish-brown, bean-shaped structures. They are
about eleven centimeters long. Kidneys are located on each side of spine, just
above the waist. They are loosely held in place by a mass of fat and two layers
of fibrous tissue. It is believed that the kidney first evolved in the original
vertebrates where freshwater organisms needed some means of pumping water from
the body. The kidney became adept at reabsorbing glucose, salts, and other
materials which would have been los...