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The Position
The heart is a hollow muscle that disgorges the blood from the heart
cavities by contracting its muscular layer. In an average lifetime the
heart beats more than three billion times. During this time it pumps
more than 300 million litres of blood through the veins. If you take a
closer look at the heart, it becomes apparent that there are, in fact,
two pumps beating to the same tact; these are separated by the cardiac
wall (septum cardiale).
The heart supports two interdependent circular flows: the right heart
and the left heart. The right heart pumps the blood into the lungs,
when it flows from there back into the heart, the left heart pumps it
around the body. The heart is ordinarily about as big as the closed
fist of its host. It weighs an average of 300 grams, lies between the
two lungs and is limited at the front by the breastbone (sternum) and at the back by the esophagus and large aorta. The base of the heart rests on the diaphragm.
Two thirds of the heart are located on the left side of the chest, a
third on the right. It is slightly inclined, the longitudinal axis
being angled to the left and forwards, which leaves the heart apex very
close to the chest wall on the left side. This means that if you feel
with your fingers from the middle of the collarbone straight down into
the 5th inter-rib space, you will feel the heart apex beat; this is
then the position of the heart apex.
Structure
The heart is divided into a left and a right half by a longitudinal
partition. Each half is divided into two hollow cavities an atrium and
a ventricle. The atria are surrounded only by a weak muscle layer.
Heart Valves
The heart contains 4 valves. These function as shut-off valves and
permit blood flow in one direction only. A distinction is made between
these 4 cardiac valves: there are two atrioventricular valves and two
semilunar valves. The two atrioventricular valves, or AV valves for
short, are so-called because they separate the atria from the ventricle
on each side of the heart. The one on the left is called the mitral
valve and the one on the right the tricuspid valve. Atrioventricular
valves are often likened to sails because of their shape and the
sail-like way they are suspended from threads called chordae tendinae.
The valve between the right atrium and right ventricle consists of
three 'sails' or cusps, hence the name tricuspid valve. The valve
between the left atrium and the left ventricle is composed of two such
'sails'. However, because of its similarity to a bishop's miter, it is
called the mitral valve.
The two semilunar valves are the aortic and pulmonic valves. Their
function is to prevent backflow from the arteries in the ventricles.
Both consist of three cusp-like structures which are so arranged that
the backflow of blood forces them together. This prevents blood from
the arteries flowing back into the heart. The valve which lies between
the left ventricle and the aorta is called the aortic valve. The
pulmonic valve separates the right ventricle and the pulmonary artery
(truncus pulmonalis).
Another way of looking at the heart structure is to consider the valves on the right and left side of the heart:
- right half of the heart: tricuspid valve and pulmonic valve
- left half of the heart: mitral valve and aortic valve
From
its tip down to the roots of the great blood vessels, the heart is
wrapped in a double-walled sac, the pericardium, or pericardial sac.
The outside coat of the pericardial sac is called the fibrous
pericardium. It consists of a tough connective tissue and incorporated
fatty tissue. At its lower point it is fused with the diaphragm and at
the side with the pleura. This gives the pericardium a fixed position.
The pericardium is also not very elastic.
The inside wall
of the heart sac is called the serous pericardium. This is fused with
the heart muscle and is, therefore, often called the "outer heart
skin". It contains two layers, both of which serve to lubricate the
heart in order to prevent friction from occurring during heart
activity. The layer next to the fibrous pericardium is the parietal
layer, and the layer next to the heart is the visceral layer, also
known as the epicardium. The epicardium is very smooth.
Between these two layers exists a small cavity called the pericardial
cavity It is filled with a tiny amount of clear liquid, ordinarily
about 20 to 50 ml. This liquid acts as a lubricant. The beating heart
is in constant motion, whereas the pericardium, in its position, is
only minimally involved in this movement. Due to the presence of the
liquid, the smooth surface of the epicardium can move easily and with
little or no friction against the pericardium.
The heart muscle supplies the whole body with blood. Nutrients and
oxygen are transported to the remotest corners, with the heart itself
being no exception.. The branch-like arteries which supply the
heart muscle are called coronary arteries (cor = heart).
At the point where the great artery, the aorta, leaves the heart, two
main arteries branch away from it, these are responsible for looking
after the heart and are known as the right and left coronary arteries.
They supply the heart with about 300 ml blood per minute. This is about
5 percent of the entire blood volume, although the heart makes up only
0.5 percent of the total body weight. This data refers to the body's
resting state. In times of increased stress, when the heart beats
faster, the amount of blood supplied to the heart muscle also increases.
The right coronary artery or arteria coronaria dextra
(abbr. RCA) provides blood to the right atrium, the right ventricle, a
small part of the ventricle wall and the posterior wall of the heart.
The arteries, which run across the underside side of the heart, are
shown in the illustration in a slightly brighter shade of red.
