3-39. THE CARDIOVASCULAR SYSTEM
The cardiovascular system consists of the heart and the arteries, the veins, and
the capillaries. It is concerned with the movement of blood through the body.
3-40. THE HEART
a. General. The heart (figures 3-23 and 3-24) is the major organ of the
circulatory system. It is hollow, muscular, and somewhat cone-shaped, slightly larger
than the closed fist. In the average male, it weighs about 300 grams. The heart lies
obliquely in the lower two-thirds of the thoracic cavity, with the apex directed downward
and the bulk of its mass to the left of the midsagittal line. It may be felt pulsating
between the fifth and sixth ribs, about 2.5 cm below the left nipple.
(1) Located in the mediastinum, the heart is enclosed in a dual-layered,
loose-fitting sac called the pericardium. Between the outer fibrous layer and the inner
serous layer is a serous liquid, the pericardial fluid, which lubricates the smooth outer
surface of the heart. The superior portion, or base, of the heart consists of two
receiving chambers, the atria. The main body extends into the apex and consists of two
larger chambers, the ventricles. Blood from all parts of the body, except from the lungs,
enters the right atrium through three veins: the coronary sinus (which collects blood
from veins of the heart) and the inferior and superior vena cava (which collect blood
from the other veins of the systemic circulation). Blood from the lungs enters the left
atrium through the pulmonary veins. Two arteries emerge from the ventricles. From the
right ventricle, the pulmonary artery carries blood to the lungs, and from the left
ventricle; the aorta distributes blood to all other parts of the body.
(2) As the heart beats, backflow of blood is prevented by the action of
valves: the tricuspid valve between the right atrium and ventricle, the mitral valve
(bicuspid valve) between the left atrium and ventricle, and the semilunar valves, which
guard the outlets of the ventricles.
c. Circulation of Blood Through the Heart. You may follow the arrows in
figure 3-23 to illustrate the following discussion: deoxygenated blood from the body
tissue enters the right atrium through the superior and inferior venae cavae. It then is
passed through the tricuspid valve into the right ventricle. The blood leaves this
chamber through the pulmonary semilunar valve and is passed by the right and left
pulmonary arteries to the lungs, where it receives a fresh supply of oxygen and releases
carbon dioxide. The oxygenated blood is returned to the heart in the right and left
pulmonary veins, which open into the left atrium. From the left atrium, the blood is
passed through the bicuspid (or mitral) valve into the left ventricle. This ventricle is the
largest and strongest chamber of the heart, and from it the oxygenated blood passes
through the aortic semilunar valve into the aorta, from which it is sent to all parts of the
body, except to the lungs.