b. Nerve Supply. The nerve supply to the heart is from two sets of nerves
originating in the medulla of the brain. The nerves are part of the involuntary
(autonomic) nervous system. One set, the branches from the vagus nerve, keeps the
heart beating at a slow, regular rate. The other set, the cardiac accelerator nerves,
speeds up the heart. The heart muscle has a special ability; it contracts automatically,
but the nerve supply is needed to provide an effective contraction for blood circulation.
Within the heart muscle itself, there are special groups of nerve fibers that conduct
impulses for contraction. These groups make up the conduction system of the heart.
When the conduction system does not operate properly, the heart muscle contractions
are uncoordinated and ineffective. The impulses within the heart muscle are tiny
electric currents, which can be picked up and recorded by the electrocardiogram, the
The blood vessels are the closed system of tubes through which the blood flows.
The arteries and arterioles are distributors. The capillaries are the vessels through
which all exchange of fluid, oxygen, and carbon dioxide takes place between the blood
and tissue cells. The capillaries are the smallest of these vessels but are of greatest
importance functionally in the circulatory system. The venules and veins are collectors,
carrying blood back to the heart.
a. The Arteries and Arterioles. The system of arteries and arterioles is like a
tree, with the large trunk, the aorta, giving off branches that repeatedly divide and
subdivide. Arterioles are very small arteries, about the diameter of a hair. By way of
comparison, the aorta is more than one inch in diameter. An artery wall has a layer of
elastic, muscular tissue that allows it to dilate and constrict. When an artery is cut, this
wall does not collapse, and bright red blood escapes from the artery in spurts.
b. Capillaries. Microscopic in size, capillaries are so numerous that there is at
least one or more near every living cell. A single layer of endothelial cells forms the
walls of a capillary. Capillaries are the essential link between arterial and venous
circulation. The vital exchange of substances between the capillary blood and the
tissue cells takes place through the capillary wall. Blood starts its route back to the
heart as it leaves the capillaries.
c. Veins. Veins have thin walls and valves. Formed from the inner vein lining,
these valves prevent blood from flowing back toward the capillaries. Venules, the
smallest veins, unite into veins of larger and larger size as the blood is collected for
return to the heart. The superior vena cava, collecting blood from all regions above the
diaphragm and the inferior vena cava, collecting blood from all regions below the
diaphragm, return the venous blood to the right atrium of the heart. Superficial veins lie
close to the surface of the body and can be seen through the skin.