Kidneys In vertebrates, the kidneys are the two main organs of excretion. Excess water, toxic waste products of metabolism such as urea, uric acid and inorganic salts are eliminated by the kidneys in the form of urine. The kidneys are also largely responsible for maintaining the body's water balance and blood pH. The kidneys play an important role in other bodily functions, such as releasing the protein erythropoietin and controlling blood pressure. The kidneys are paired, reddish-brown, bean-shaped structures. They are about eleven centimeters long. The kidneys are located on each side of the spine, just above the waist. They are held in place loosely by a mass of fat and two layers of fibrous tissue. The kidney is believed to have first evolved in early vertebrates, where freshwater organisms needed a means to pump water from the body. The kidney became adept at reabsorbing glucose, salts, and other materials that would have been lost if simply pumped out of the body by a simple organ. The cut surface of the kidney reveals two distinct areas: the cortex, a dark band along the outer edge, about a centimeter thick, and the inner medulla. The medulla is divided into 8 to 18 cone-shaped masses of tissue called renal pyramids. The apex of each pyramid, the papilla, extends into the renal pelvis, through which urine is released from the kidney tissue. The cortex arches over the bases of the pyramids (cortical arches) and extends between each pyramid as renal columns. Urine passes through the body in a fairly complex way. The initial site of urine production in the body is the glomerus. Blood pressure pushes through the glomerus a filtrate of plasma containing salts, glucose, amino acids, and nitrogenous wastes such as urea and a small amount of ammonia. Proteins and fats are filtered out of the plasma to remain in the normal bloodstream. Plasma is now called gloular filtrate. One hundred to one hundred and forty milliliters of this filtrate are formed every minute! The filtrate passes along a convoluted tibula. Most of the water contained and part of the dissolved materials are reabsorbed through the walls of the tibula and return to the blood. Water, sodium, chloride, bicarbonate, and all glucose are reabsorbed into the bloodstream, but products such as urea and ammonia remain in the tibula. During the final stage of the passage process, most of the remaining filtrate is selectively reabsorbed until only about 1% of the original filtrate is excreted as urine. The urine is eventually collected in the kidneys. Urine is collected