(1) The arteries carry blood away from the chambers of the heart.
(2) The veins carry blood to the chambers of the heart.
(3) Capillaries are extremely thin-walled vessels having only the intimal layer
through which exchanges can take place between the blood and the tissue cells.
c. Relationships. Arteries and veins are largest where they are closest to the
heart. Away from the heart, they branch into smaller and smaller and more numerous
vessels. The branching continues until the smallest arteries (arterioles) empty into the
capillaries. The capillaries in turn are drained by the venules of the venous system.
d. Valves. Within the heart and the veins are structures known as valves.
Valves function to insure that the blood flows in only one direction.
9-7. THE HEART
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 9-2 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
ATRIUM = hall
AURICLE = ear-like flap
VENTER = belly
SEPTUM = fence
(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.