a. Oxygen. Oxygen is in the air filling the alveolus of the lung. The oxygen
passes through the walls of the alveolus and capillary to become dissolved in the
plasma of the blood. Most of the dissolved oxygen is rapidly picked up by the
hemoglobin of the RBCs. Thus, the RBC is the main transporting element for oxygen in
b. Carbon Dioxide. Carbon dioxide is produced during metabolic oxidation
within the individual cell. It passes through the cell membrane and the wall of the
capillary to become dissolved in the plasma of the blood. Through action of an enzyme
in the RBCs, most of the carbon dioxide (CO2) is transformed into bicarbonate ions
10-14. TRANSPORT OF OTHER SUBSTANCES
Other substances, such as the end products of digestion, are also carried by the
blood. They are either dissolved or suspended in the plasma.
The life processes cannot continue in the body cells without sources of energy.
From glucose, energy is released to produce ATP, the driving force of the life processes
of the body.
a. When a specific portion of the cerebral cortex is active, more blood is
delivered to that portion. This is an example of how more blood can be delivered to the
body parts where it is most needed.
b. When the hormone epinephrine (Adrenalin) is secreted by the adrenal gland,
it is delivered to all parts of the body by the cardiovascular system. Among other
effects, epinephrine increases the rate of metabolism of all cells of the body. This helps
to mobilize energy during a "fight-or-flight" stress reaction.
c. In periods when much energy is required, the body can use its stores of fat
as a source of energy. As we have seen in the chapter on the digestive system, the
lymphatic circulatory system picks up the end products of lipid (fat) digestion and carries
them to the cardiovascular system.
(1) This fat is generally deposited throughout the body, particularly the
subcutaneous layer, as yellow fat. In a rapid turnover, the high energy content of the fat
is released for use throughout the body.
(2) In infants, there is often brown fat at the junctions of the major blood
vessels. In periods of high-energy requirements, this brown fat releases energy into the
blood stream immediately.