The Human Heart

Biology - Histology

The Human Heart.

Abstract:
Dorland\'s Illustrated Medical Dictionary defines the heart
as "the viscus of cardiac muscle that maintains the
circulation of the blood". It is divided into four
cavities; two atria and two ventricles. The left atrium
receives oxygenated blood from the lungs. From there the
blood passes to the left ventricle, which forces it via the
aorta, through the arteries to supply the tissues of the
body. The right atrium receives the blood after it has
passed through the tissues and has given up much of its
oxygen. The blood then passes through the right ventricle
into the lungs where it gets oxygenated. There are four
major valves in the heart; the left atrioventricular valve
(also known as the mitral or bicuspid valve), the right
atrioventricular valve (tricuspid), aortic valve, and the
pulmonary valve. The heart tissue itself is nourished by
the blood in the coronary arteries.2

Position of the Heart Within the Body:
The heart is placed obliquely in the chest. The two atria
are directed upwards and backwards to the right and are at
the level of the fifth through the eight dorsal vertebrae.
The apex of the heart points downwards and forwards to the
left and corresponds to the interspace between the fifth and
sixth ribs, two inches below the left nipple. Its atrial
border corresponds to a line drawn across the sternum on a
level with the upper border of the third costal cartilage.
Its lower border (apex) corresponds to a line drawn across
the lower end of the same bone, near the xiphoid process.
Its upper surface is rounded and convex, directed upwards
and forwards, and formed mainly by the right ventricle and
part of the left ventricle. The posterior surface of the
heart is flattened and rests upon the diaphragm muscle. Of
its two borders, the right is the longest and thinnest, the
left is shorter but thicker and round.

Size:
In an adult, the heart measures about five inches in
length, three and a half inches in the broadest part of its
transverse diameter, and two and a half inches in its
antero-posterior. The average weight in the male varies
from ten to twelve ounces. In the female, the average
weight is eight to ten ounces. The heart will continue to
grow in size up to an advanced period of life. This growth
is more obvious in men than in women.3

Circulation of Blood in an Adult:
The heart is subdivided by a longitudinal muscular septum
into two lateral halves which are named right and left
according to their position. A transverse muscle divides
each half into two cavities. The upper cavity on each side
is called the atria/auricle, and the lower side is called
the ventricle. The right atrium and ventricle form the
venous side of the heart. Dark venous blood is pumped into
the right atrium from the entire body by the superior (SVC)
and inferior vena cava (SVC), and the coronary sinus. From
the right atrium, the blood passes into the right ventricle
and from the right ventricle, through the pulmonary artery
into the lungs.3 Once the blood becomes
oxygenated/arterialized by its passage through the lungs, it
is returned to the left side of the heart by the pulmonary
veins which open into the left atrium. From the left
atrium, the blood passes into the left ventricle where it is
distributed by the aorta and its subdivisions through the
entire body.

Morphology of Each Heart Chamber:
The right atrium is a little longer than the left. Its
walls are also somewhat thinner than the left. The right
atrium is capable of containing about two ounces of fluid.
It consists of two parts, a principle cavity/sinus, and an
appendix auriculae. The sinus is a large
quadrilateral-shaped cavity located between the IVC and the
SVC. Its walls are extremely thin and are connected on the
lower surface with the right ventricle and internally with
the left atrium. The rest of the right atrium is free and
unattached. The appendix auricle is a small conical
muscular pouch. It projects from the sinus forwards and to
the left side, where it overlaps the root of the pulmonary
artery.6
There are four main openings into the right atrium; the
SVC, IVC, coronary sinus, and the atriculo-ventricular
opening.