Invertebrates, 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 the 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 glomerulus. The arterial blood pressure drives a filtrate of plasma containing salts, glucose, amino acids, and nitrogenous wastes such as urea and a small amount of ammonia through the glomerulus. Proteins and fats are filtered out of the plasma, to remain in the normal bloodstream. The plasma is now called glomerular filtrate. One-hundred to one-hundred-forty milliliters of this filtrate are formed each minute!
The filtrate passes along a convoluted tubule. The majority of the water content and some of the dissolved materials are reabsorbed through the walls of the tubule and back into the blood. Water, sodium, chloride, bicarbonate, and all glucose are reabsorbed into the bloodstream, yet products such as urea and ammonia remain in the tubule. During the final stage of the passage process, most of the remaining filtrate is selectively reabsorbed until only about one percent of the original filtrate is to be excreted as urine.
Urine is eventually collected in the kidneys. The urine is collected in the renal pelvis, a funnel-like structure, which is contained inside the kidneys. The urine then passes into a hollow tube, called the ureter, which is forty to forty-five centimeters long. The ureter extends downward, emptying into the urinary bladder. A single tube called the urethra eventually eliminates urine from the bladder.
When excessive amounts of fluid are lost from the body, or when the blood pressure of the body falls below normal, the kidneys release the enzyme renin into the blood. This enzyme promotes the formation of angiotensin. Within minutes, the angiotensin causes vasoconstriction. Vasoconstriction raises blood pressure, and stimulates the secretion of aldosterone, eventually bringing the body’s fluid levels to equilibrium. The kidney is an extraordinary organ. Without its processes, human life would be virtually impossible.