(3) Transport. Blood functions as the chief means of transport for materials
within the body. Since most substances are not absorbed by the formed elements, it is
the plasma that functions as a solvent for most materials transported by the blood to the
capillaries in all parts of the body.
(4) Antibodies. While antigens are frequently carried on the surface of red
cells, antibodies, both natural and acquired, are transported by the plasma portion of the
(5) Blood volume. The maintenance of a proper blood volume is one of the
functions of plasma, the fluid portion of blood. Plasma must be at a high enough level
to fill the chambers of the heart in the pumping mechanism. The plasma portion must
not exceed an optimum level, or the heart will be unable to handle the excess fluid and
thus will become waterlogged.
(6) Gases. About 60 carried in the form of bicarbonate (HCO3). Small
fractions of oxygen and carbon dioxide are carried as gases dissolved in the plasma.
(7) Waste. Waste products from cellular metabolism are carried away by
the plasma portion of the blood, as well as the lymphatic system.
(8) Nutrients. The nutrients that nourish the cells and provide them with
substances necessary for life are carried in the plasma portion of blood. These
products are carried from the point of manufacture to the capillaries and on to the
(9) Hormones. Hormones are carried exclusively by the plasma of the
bloodstream to the different tissues. Blood is the only means of transport for hormones.
GENERAL DESCRIPTION OF THE CIRCULATORY SYSTEM (Figure 21)
a. Heart. The heart is a hollow, muscular organ situated in the thorax, and
extending approximately from the third rib to just about the sixth rib, and with about
twothirds of its mass to the left of the midline. The heart is the pumping unit whereby
blood is circulated throughout the body (figure 2-1). The heart is enclosed in a double
walled fibroserous sac called the pericardium. The pericardial fluid lubricates the
pericardium and prevents friction during movement of the heart.
b. Blood Vessels. The blood vessels are composed of arteries (figure 22),
veins (figure 23), and capillaries. Arteries are the vessels that convey the blood away
from the pumping chambers of the heart. Veins have thinner walls than arteries and are
the vessels that return blood to the pumping chambers of the heart. Capillaries, the
smallest vessels, provide the means for exchanges between the blood and tissues.
Capillaries form a network in the tissues and are characterized by a very thin wall.