d. Heart Action.
(1) Variations in the individual heart rate depend on sex, age, and physical
activity, as well as on other factors. However, the heart normally beats about 70 times
per minute. The body of the adult normally contains 4 1/2 to 5 1/2 quarts of blood.
(2) Each heartbeat, or cardiac cycle, consists of a wave of ventricular
contractions called systole followed by a wave of relaxation called diastole. Each wave
starts at the atria and moves across the ventricles. When a chamber is in systole, blood
is being forced out; when it is in diastole, blood is entering. The systolic pressure is the
pressure during the contraction phase, the diastolic pressure is the pressure during
relaxation, and the pressure between the two pulses is the pulse pressure.
3-41. THE BLOOD VESSELS
a. The vessels carrying blood from the heart to all parts of the body are the
arteries. They branch out extensively through the body, becoming progressively smaller
and ending in minute vessels called arterioles. From the arterioles, the blood passes
through a network of microscopic thin-walled channels called the capillaries. From
here, the blood is collected into venules and then returned to the heart through veins of
progressively larger size.
b. Arteries are thick-walled and never empty. Veins are thin-walled and collapse
when there is no air. The force of muscles adjacent to veins aids in the forward
propulsion of blood toward the heart. Valves spaced frequently along the large veins
prevent the backflow of blood. Both arteries and veins are lined with endothelium. The
capillary walls are composed of a single layer of endothelial cells.
3-42. SYSTEMIC CIRCULATION
The systemic circulation follows an orderly pattern in which blood from the heart
is pumped through the major arteries and its branches to all body tissues (except the
lungs) and returns to the heart through major veins and their tributaries.
a. Via the Major Arteries. The aorta is the primary arterial trunk line in the
systemic circulatory system. It arises from the left ventricle, ascends superiorly, arches
over the left lung, and branches to supply blood to the head and arms. It descends
through the thoracic and abdominal cavities, along the spinal column to the level of the
fourth lumbar vertebra. Here, it bifurcates into the right and left common iliac arteries,
which supply the lower extremities. Major arteries supplying other parts of the body
branch out from the aorta at specific locations. For descriptive purposes, the aorta may
be divided into four regions: the ascending aorta, the arch of the aorta, the thoracic
aorta, and the abdominal aorta. The major arteries are shown in figure 3-25.