Kidneys Essay, Research Paper


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.

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