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 as urea and a small amount of ammonia through the
glomerus. Proteins and fats are filtered out of the plasma, to remain in the
normal blood stream. The plasma is now called glorular filtrate. One-hundred
to one-hundred-forty milliliters of this filtrate are formed each minute!

The filtrate passes along a convoluted tibule. The majority of the
water content and some of the dissolved materials are reabsorbed through the
walls of the tibule 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 tibule. 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 extroardinary organ. Without it\'s processes, human
life would be virtually impossible.

Category: Science