Blood Functions. Nutrition
Each substance required for the nutrition of every
cell in the body is transported by the blood: the precursors
of carbohydrates, proteins, and fats; minerals and salts;
vitamins and other accessory food factors. These substances
must all pass through the plasma on the way to the tissues
in which they are used. The materials may enter the
bloodstream from the gastrointestinal tract, or they
may be released from stores within the body or become
available from the breakdown of tissue.
The concentrations of many plasma constituents, including
glucose and calcium, are carefully regulated, and deviations
from the normal may have adverse effects. One of the
regulators of the blood sugar is insulin, a hormone
released into the blood from glandular cells in the
pancreas. Ingestion of a carbohydrate meal is followed
by increased production ofinsulin, which tends to keep
the blood sugar from rising excessively as the carbohydrates
are broken down into their constituent sugar molecules.
But an excess of insulin may severely reduce the blood
sugar, causing a reaction that, if sufficiently severe,
may include coma and even death. Glucose is transported
in simple solution, but some substances requirespecific
binding proteins (proteins with which the substances
form temporary unions) to convey them through the plasma.
Iron and copper, essential minerals, have special and
necessary transport proteins. Nutrient substances may
be taken up selectively by the tissuesthat require them.
Growing bones use large amounts of calcium, and bone
marrow removes iron from plasma for hemoglobin synthesis.
