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An External Human Heart. |
| Position of Heart| External Structure| Internal Structure| Valves of Heart| Cardiac Cycle| Heartbeat Regulation|
In this section we will deal with the position of the heart, the external and internal structure
of the heart, the valves of the heart, the cardiac cycle and the regulation of heartbeat.
The Position of the Heart.
The heart is situated in the chest cavity behind the breastbone (sternum) with its
pointed end directly slightly towards the left and turned downwards the breatbone. It is
suspended in a hollow between the lungs known as the mediastinum and it is kept in
position by the large blood vessels which enter and leave the heart.
The External Structure of the Heart.
The human heart is a hollow organ and is surrounded by a double membrane, called the pericardium. The space between the two membranes is filled with a watery fluid which prevents friction when the heart beats. At its lower tip the outer membrane of the pericardium is attached to the diaphragm. A transverse and a longitudinal groove are visible on the surface of the heart. These grooves indicate the positions of the inner walls which divide the heart into four chambers, namely the two upper chambers, the atria, and the two larger lower chambers, the ventricles. The coronary arteries and veins are clearly visible on the grooves. Entering the right atrium are the superior vena cava and the inferior vena cava. Entering the left atrium are the four pulmonary veins. From the upper central portion of the heart arise the pulmonary artery with a right and left branch and the aorta with its branches.
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An External Human Heart. |
A muscular septum divides the heart internally into a left and a right half. Each half is
subdivided into two chambers, the atrium (reception) and the ventricle (pump chamber)
. Internally all four chambers have a smooth membranous lining, the endocardium.
The Right half of the Heart.
The right half of the heart is reponsible for the circulation of deoxygenated (oxygen-poor) blood to the lungs so that it may be oxygenated. This circulation is called pulmonary circulation or lesser circulation.
The right atrium is thin walled and is situated at the broad end of the heart. It
serves as a reception room for the deoxygenated (oxygen-poor) blood which receives
the blood from the following blood vessels:
The right ventricle receives the deoxygenated blood from the right atrium, through the right atrio-ventricular opening and then pupms it through the pulmonary artery to the lungs. This opening is guarded by the tricuspid valve which, as the name indicates, consists of three cusps or flaps. When the ventricle contracts the tricuspid valve closes and the blood is forced past the semi-lunar valves into the pulmonary artery. This vessel arises from the upper left corner of the right ventricle. The pulmonary artery conveys blood to the lungs, where it is oxygenated.
The half is reponsible for the circulation of oxygenated blood to all parts of the body, including the heart itself. This circulation is known as greater circulation or systematic circulation.
The left atrium is thin-walled like the right atrium and it situated in the broad end of
the heart. It recieves oxygenated blood from the lungs via four pulmonary veins, two
from each lung.
The left ventricle is characterized by its exceptionally thick muscular wall. Oxygenated blood reaches the left ventricle through the bicuspid or mitral valve. This valve prevents the blood from returning to the left atrium when the left ventricle contracts. In the upper right corner of the left ventricle is the opening to the aorta situated. When the ventricle contracts, oxygenated blood is pumped forcefully through the aorta and all its branches to all parts of the body. Three semi-lunar valves situated at the base of the aorta prevent the blood from returning to the ventricle when it relaxes. The two coronary arteries arise from the aorta just above the semi-lunar valves. Through these arteries the heart muscle itself receives oxygen as well as nutrients.
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Picture of an Internal Heart. |
We have already mentioned the position and function of the valves found in the heart. We shall
now deal with each one of the valves in detail.
The Tricuspid Valve.
This valve controls the opening between the right atrium and the right ventricle, i.e. the
right atrio-ventricular opening. The flaps or cusps of this valve are evaginations of
the inner lining or endocardium of the heart. The cusps are attached by chordae tendinae
(tendon-like cords) to the papillary muscles on the inner wall of the ventricle. When
the right atrium contracts, blood flows freely through the this valve into the right ventricle.
When the right ventricle contracts, the blood is forced upwards and pushes the tricuspid valve
closed. The chordae tendineae prevent the cusps from being pushed through the atrio-
ventricular opening into the atrium. When the right atrium contracts the blood is not forced back
into the venae cavae. The openings to these two vessels are automatically closed
due to the contraction of the muscular wall of the right atrium.
Semi-lunar valves of the Pulmonary Artery.
Situated at the base of the pulmonary artery and being turned inwards into the artery, these
valves allow blood to flow upwards into the artery and prevent from falling back into the
right ventricle when the latter relaxes. Blood flowing back causes the semi-lunar valves to
close.
The Bicuspid Valve.
This valve allows blood to flow freely from the left atrium into the left ventricle, but prevents
the blood from flowing back into the left atrium when the ventricle contracts. In structure and
functioning this valve is the same as the tricuspid valve, except that it has only two
triangular cusps.
Semi-lunar valve of the Aorta.
Three semi-lunar valves, situated at the base of the aorta, allow blood to be forced from
the left ventricle into the aorta. The same valves prevent blood from returning to the ventricle
between contractions. When the left atrium contracts the blood is not forced back into the four
pulmonary veins because their openings are automatically closed when the atrium
contracts.
The Cardiac Cycle.
The heart serves as a central pump organ. Because the heart muscles contract and relax
rhythmically the heart is able to circulate the blood through the whole body. The contraction
of any part of the heart is referred to as systole while the relaxation is called
diastole. In a normal healthy person the heart beats at an average of 70 to 80 times per
minute. The duration of one cycle is approximately 0.8 of a second. During one cycle
a whole series of changes take place in the heart, which are collectively referred to as the
cardiac cycle. Thus, the succession of events during each complete heartbeat is known as the
heart cycle or cardiac cycle. The cardiac cycle may be represented as follows:
Atrial Systole.
The two atria contract simultaneously for about 0.1 of a second. The openings from the
two venae cavae, the coronary vein and from the four pulmonary veins are closed by the
contraction of the atria and the blood remaining in the atria is pumped into the ventricles
through the atrio-ventricular openings.
Ventricular Systole.
The two ventricles contract immediately thereafter for about 0.3 of a second. The
tricuspid and bicuspid valves are closed which prevents the blood from flowing back into the
atria. The blood is pumped from the right ventricle into the pulmonary artery and from the
left ventricle into the aorta. During the ventricular systole the atria are in
diastole.
General Diastole.
Immediately hereafter both ventricles relax. Bothe atria and ventricles are now relaxed and in a of general diastole which lasts for about 0.4 of a second. Because of a general decrease in pressure in the ventricles there is a tendency for the blood to flow back from the pulmonary artery and the aorta into the ventricles. This is however prevented by the three semi-lunar valves in each of these blood vessels. During general diastole the blood flows from the two venae cavae and four pulmonary veins into the two atria and from them directly into the two ventricles.
The next cardiac cycle can now start. The stroke volume of the heart, that is the volume pumped
during each cycle (or stroke), is about 70ml. In an exercised heart the stroke volume is usually
larger than normal, the heart beat is slower and the period of rest is more than 0.4 of a second.
The Regulation of Heartbeat.
The heart has the ability to contract rhythmically without the stimulus of a nerve impulse . The rate at which the heart beats is controlled by the autonomic nervous system. The cardiac centre, i.e. the part of the nervous system controlling the heartbeat, is situated at the base of the brain, in the medulla oblongata. One nerve, vagus nerve, slows down the heart rate while another nerve, accelerator nerve, accelarates the heartbeat. Both nerves terminate in a small mass of neuromuscular tissue situated in the wall of the right atrium near the point where the superior vena cava enters the heart. The mass of tissue is called the sino-atrial node, S-A node or pacemaker.
A similar mass of neuromuscular tissue, the atrio-ventricular node, or A-V node, is situated in the wall between the left and right atria. From this node a bundle of cardiac muscle, called the bundle of His, extends into the septum between the two ventricles where it forms two branches, one leading to the wall of the right ventricle and the other continuing in the wall of the left ventricle. These two branches give rise to a network of muscle fibres, called Purkinje fibres, which are modified cardial muscle fibres which conduct impulses much faster than ordinary cardiac muscles. In summary, the contraction of the heart takes place as follows: