Kidneys
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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