The Human Heart

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