Blood Functions. Excretion
The blood carries the waste products of cellular metabolism
to the excretory organs. The removal of carbon dioxide
via the lungs has been described above. Water produced
by the oxidation of foods or available from other sources
in excess of needs is excreted by the kidneys as the
solvent of the urine. Water derived from the blood also
is lost from the body by evaporation from the skin and
lungs and in small amounts from the gastrointestinal
tract. The water content of the blood and of the body
as a whole remains within a narrow range because of
effective regulatory mechanisms, hormonal and other,
that determine the urinary volume. The concentrations
of physiologically important ions of the plasma, notably
sodium, potassium, and chloride, are precisely controlled
by their retention or selective removal as blood flows
through the kidneys. Of special significance is the
renal (kidney) control of acidity of the urine, a major
factor in the maintenance of the normal pH of the blood.
Urea, creatinine, and uric acid are nitrogen-containing
products of metabolism that are transported by the blood
and rapidly eliminated by the kidneys. The kidneys clear
the blood of many other substances, including numerous
drugs and chemicals that are taken into the body. In
performing their excretory function, the kidneys have
a major responsibility for maintaining the constancy
of the composition of the blood. (See also renal system.)
The liver is in part an excretory organ. Bilirubin (bile
pigment) produced by the destruction of hemoglobin is
conveyed by the plasma to the liver and is excreted
through the biliary ducts into the intestinal tract.
Other substances, including certain drugs, also are
removed from the plasma by the liver.
