b. Blood Vessels. The blood vessels of the cardiovascular system carry the
blood away from the heart and back to the heart again.
(1) Blood circulation systems. Since the heart is divided into two parts (the
right half consisting of the right atrium and the right ventricle and the left half consisting
of the left atrium and the left ventricle), it is not surprising to find that there are actually
two blood circulatory systems--the systemic and the pulmonary.
(a) Systemic. The systemic (general) circulatory system is the larger of
the two systems. It takes the blood pumped by the left ventricle to all parts of the body
and returns the blood to the right atrium. The blood is rich in oxygen when it leaves the
heart through the left ventricle and is low in oxygen content when it returns to the right
(b) Pulmonary. The pulmonary circulatory system takes the blood
pumped by the right ventricle to the lungs and returns the blood to the left atrium. The
blood is oxygen-poor when it leaves the right ventricle and oxygen-rich when it returns
to the left atrium.
(2) Types of blood vessels. Both the systemic and the pulmonary
circulatory systems are composed of three major types of blood vessels: arteries,
capillaries, and veins.
(a) Arteries. Arteries carry blood pumped by the ventricles away from
the heart. The arteries of the systemic circulatory system carry oxygenated (oxygen-
rich) blood to body tissues. Pulmonary arteries carry deoxygenated (oxygen-poor)
blood to the lungs. Arteries have the capacity to constrict and dilate. This constricting
and dilating helps regulate blood pressure.
(b) Capillaries. The arteries are large blood vessels that divide into
smaller branches. These branches then divide again and again. Each time, 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 blood cell enters the capillaries, it is free to
perform its primary functions. In the pulmonary system, the red blood cells give up
carbon dioxide to the lungs and pick up oxygen. In the systemic system, blood gives
oxygen, nutrients, and fluids to the cells and picks up carbon dioxide and other waste
(c) Veins. Capillaries join together to form larger blood vessels. These
blood vessels combine to form even larger blood vessels. This process continues until
large blood vessels called veins are formed. Veins carry 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.