THE CIRCULATORY SYSTEM
The circulatory system consists of the heart, blood vessels, and blood. The
circulatory system brings oxygen and nutrients to the body's cells and carries away
waste products. The circulatory system is also called the cardiovascular system
("cardio-" means heart; "-vascular" means vessels.)
a. Heart. The heart (figure 1-1) is a strong, muscular organ that, by its rhythmic
contractions, acts as a force pump maintaining blood circulation. The heart is about the
size of a fist and is located in the lower left-central part of the chest cavity.
Figure 1-1. The human heart (front view).
Layers. The heart consists of three layers.
(a) The myocardium is the middle layer. It is composed of the actual
heart muscles. ("Myo-" means muscle; "cardium" means heart.)
(b) The pericardium is the outer layer. It is a double-walled sac that
surrounds the heart muscles. ("Peri-" means around.)
(c) The endocardium is the inner layer. It forms the inner lining of the
four chambers. ("Endo-" means within.)