Section III. THE HEART AND THE SYSTEMIC CIRCULATION OF BLOOD
Through the action of its very muscular walls, the heart produces the primary
motive force to drive the blood through the arterial system. In humans, the heart is
located just above the diaphragm, in the middle of the thorax, and extending slightly to
the left. It is said that the heart of an average individual is about the size of that
individual's clenched fist.
a. General Construction of the Human Heart. See Figure 2-2 for an
illustration of the human heart.
(1) Chambers. The heart is divided into four cavities known as the
chambers. The upper two chambers are known as the atria, right and left. Each atrium
has an ear-like projection known as an auricle. The lower two chambers are known as
ventricles, right and left. Between the two atria is a common wall known as the
interatrial septum. Between the two ventricles is a common wall known as the
(2) Wall layers. The walls of the chambers are in three general layers.
Lining the cavity of each chamber is a smooth epithelium known as the endocardium.
(Endocarditis is an inflammation of the endocardium.) The middle layer is made up of
cardiac muscle tissue and is known as the myocardium. The outer layer of the heart is
another epithelium known as the epicardium.
(3) Relationship of wall thickness to required pressure levels. A cross-
section of the chambers shows that the atrial walls are relatively thin. The right
ventricular wall is much thicker. The left ventricular wall is three to five times thicker
than that of the right. These differences in wall thickness reflect the amount of muscle
tissue needed to produce the amount of pressure required of each chamber.