Essentials of Anatomy and Physiology

The heart pumps blood, which creates blood
pressure, and circulates oxygen, nutrients,
and other substances. The heart is located in
the mediastinum, the area between the
lungs in the thoracic cavity.

Pericardial Membranes—three layers that
enclose the heart (see Fig. 12–1)

1. The outer, fibrous pericardium, made of fibrous
   connective tissue, is a loose-fitting sac that sur-
   rounds the heart and extends over the diaphragm
   and the bases of the great vessels.


2. The parietal pericardium is a serous membrane
   that lines the fibrous pericardium.


3. The visceral pericardium, or epicardium, is a serous
   membrane on the surface of the myocardium.


4. Serous fluid between the parietal and visceral peri-
   cardial membranes prevents friction as the heart
   beats.

Chambers of the Heart (see Fig. 12–2 and
Table 12–1)

1. Cardiac muscle tissue, the myocardium, forms the
   walls of the four chambers of the heart.


2. Endocardium lines the chambers and covers the
   valves of the heart; is simple squamous epithelium
   that is very smooth and prevents abnormal clotting.
   3. The right and left atria are the upper chambers,
   separated by the interatrial septum. The atria
   receive blood from veins.
   4. The right and left ventricles are the lower cham-
   bers, separated by the interventricular septum. The
   ventricles pump blood into arteries.

Right Atrium

1. Receives blood from the upper body by way of the
   superior vena cava and receives blood from the
   lower body by way of the inferior vena cava.


2. The tricuspid (right AV) valve prevents backflow of
   blood from the right ventricle to the right atrium
   when the right ventricle contracts.

Left Atrium

1. Receives blood from the lungs by way of four pul-
   monary veins.


2. The mitral (left AV or bicuspid) valve prevents
   backflow of blood from the left ventricle to the left
   atrium when the left ventricle contracts.


3. The walls of the atria produce atrial natriuretic
   peptide when stretched by increased blood volume
   or BP. ANP increases the loss of Naions and
   water in urine, which decreases blood volume and
   BP to normal.

286 The Heart

STUDY OUTLINE

AGING AND THE HEART

The heart muscle becomes less efficient with age, and
there is a decrease in both maximum cardiac output
and heart rate, although resting levels may be more
than adequate. The health of the myocardium
depends on its blood supply, and with age there is
greater likelihood that atherosclerosis will narrow the
coronary arteries. Atherosclerosis is the deposition of
cholesterol on and in the walls of the arteries, which
decreases blood flow and forms rough surfaces that
may cause intravascular clot formation.
High blood pressure (hypertension) causes the left
ventricle to work harder; it may enlarge and outgrow
its blood supply, thus becoming weaker. A weak ven-
tricle is not an efficient pump, and such weakness may
progress to congestive heart failure; such a progres-

sion may be slow, or may be rapid. The heart valves

may become thickened by fibrosis, leading to heart

murmurs and less efficient pumping. Arrhythmias are

also more common with age, as the cells of the con-

duction pathway become less efficient.

SUMMARY

As you can see, the nervous system regulates the func-

tioning of the heart based on what the heart is sup-

posed to do. The pumping of the heart maintains

normal blood pressure and proper oxygenation of tis-

sues, and the nervous system ensures that the heart

will be able to meet these demands in different situa-

tions. Blood pressure and the blood vessels are the

subjects of the next chapter.