(b) Capillaries. Originally, the arteries are large blood vessels. Soon,
however, they divide into smaller branches. These branches then divide again and
again. With each division, the blood vessels become smaller and smaller. Finally, the
blood vessels are so small that only one red blood cell can pass through at a time.
When they reach this size, the blood vessels are called capillaries. When a red blood
cell enters the capillaries, it is free to perform its primary functions. In the pulmonary
system, red blood cells give up carbon dioxide to the lungs and pick up oxygen. In the
systemic system, red blood cells give oxygen and nutrients to the cells and pick up
carbon dioxide and other waste products.
(c) Veins. Capillaries join together to form larger blood vessels, which
then combine to form even larger blood vessels. These blood vessels are called veins.
Veins carry the blood back to the heart. The veins of the systemic system carry
oxygen-poor blood to the right atrium. The veins of the pulmonary system carry
oxygen-rich blood to the left atrium. The veins are not as thick as the arteries, and they
will collapse when severed. Many veins have valves, which keep blood from flowing
backward (away from the heart). The term "vena" denotes a vein.
c. Blood. Blood is a viscous (thick), reddish fluid. When the blood is
oxygenated (oxygen-rich), it is bright red. When the blood is low in oxygen content, it is
a darker red. When the darker color is seen through a layer of skin tissue, it appears to
be bluish. Blood is composed of fluid and solids.
(1) Plasma. The liquid part of the blood is called plasma. It is straw-colored
(pale yellow) and carries the solid components of the blood such as erythrocytes,
leukocytes, and thrombocytes.
(2) Erythrocytes. Erythrocytes (also called red blood cells or RBC) transport
oxygen from the lungs and nutrients from the small intestine to the cells of the body.
They also transport carbon dioxide and other waste materials from the body's cells to
the lungs and kidneys where the waste products are removed and expelled.
(3) Leukocytes. Leukocytes (also called white blood cells or WBC) assist in
the body's defense against disease by attacking and destroying bacteria and other
foreign particles in the blood and body tissues.
(4) Thrombocytes. Thrombocytes (also called platelets) help to stop
bleeding from a damaged blood vessel. Although thrombocytes normally show no
tendency to coagulate (clot) in the blood, they change character when they approach a
cut or tear in a blood vessel. The thrombocytes then combine to form a soft clot where
the vessel wall is broken. This clot soon hardens to form a plug to stop the loss of