(1) Plasma. Plasma is a protein-containing fluid which constitutes 50 to 60
percent of the blood by weight. Plasma is the liquid medium that carries food to the
cells and waste products away from the cells.
(2) Other blood components. Red blood cells (erythrocytes), white blood
cells (leukocytes), and platelets (thrombocytes) are carried through the circulatory
system suspended in plasma. Red blood cells, white blood cells, and platelets
constitute 40 to 50 percent of blood by weight. Red blood cells are disk-shaped, cell-
like bodies without a nucleus. They contain a substance called hemoglobin.
Hemoglobin combines with oxygen to carry it from the lungs to the tissues. Oxygenated
hemoglobin is red and gives blood its color. White blood cells are colorless, vary in size
and shape, and have a nucleus. They protect the body by destroying foreign
substances, such as bacteria, in the blood and tissues. White blood cells move about
by a "flowing" motion. They squeeze through capillary walls and move through tissues
in pursuit of bacteria. Platelets are smaller than red blood cells. They are round or oval
in shape without a nucleus. Platelets aid in the clotting of blood at wound sites. In an
average person, each cubic millimeter of blood contains about 5,000,000 red blood
cells, 5,000 to 10,000 white blood cells, and 300,000 platelets.
c. Blood Vessels.
(1) Arteries. Blood pumped by the heart is carried to the tissues through a
system of elastic, hollow tubes called arteries (see figure 1-9). This system of arteries is
like a tree with a large trunk. The arteries leave the heart and give off branches which
repeatedly divide and become smaller and smaller. The arteries have a nerve supply
controlled by the autonomic nervous system allowing them to enlarge or constrict.
(2) Capillaries. The arteries branch into billions of tiny vessels called
capillaries. The capillaries have very thin walls through which food and oxygen pass
from the blood to the cells. While food and oxygen are passing through the walls of the
capillaries, another process is going on. This process causes the waste materials and
carbon dioxide from the cells to pass back into the capillaries. Thus, the capillaries
make the necessary exchanges of water, gases, salts, food, and wastes between the
blood and the tissues.
(3) Veins. As carbon dioxide and waste materials enter the capillaries and
as the blood loses oxygen and food, it turns from a bright red to a darker red. Here the
venous system begins. The vessels, now called veins, are no longer elastic and
muscular. Their walls are thin and collapsible. Veins have paired valves to prevent the
backflow of blood. Veins, like arteries, resemble a tree with many branches and
eventually form major trunks leading back to the heart (see figure 1-10). The supply of
blood to any part of the body may be increased or decreased by a change in the rate of
heartbeat or in the size of blood vessels.