Written for a science honors project Informative, comprehensive
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 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 border, about one centimeter in thickness, and the inner medulla. The medulla is divided into 8 to 18 cone-shaped masses of tissue named 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 down between each pyramid as the renal columns.
Urine passes through the body in a fairly complex way. The initial site of urine production in the body is the glomerus. The arterial blood pressure drives a filtrate of plasma containing salts, glucose, amino acids, and nitrogenous wastes such...