Heart
Heart
Heart in anatomy, is the hollow muscular organ that receives blood
from the veins and propels it into and through the arteries. The heart of a
human is about the size of a closed fist. It is situated behind the lower part
of the breastbone, extending more to the left of the midline than to the right.
It is roughly conical in shape, with the base directed upward and to the right
and slightly backward; the apex touches the chest wall between the fifth and
sixth ribs. The heart is held in place principally by its attachment to the
great arteries and veins, and by its confinement in the pericardium, a
double-walled sac with one layer enveloping the heart and the other attached to
the breastbone, the diaphragm, and the membranes of the thorax.
Structure
and Function
·
Blood from the body returns to the right atrium through two large
veins, the superior and inferior venae cavae; in addition the blood that has
supplied the heart muscle is drained directly into the right atrium through the
coronary sinus.
·
Return of venous blood to the right atrium takes place during the
entire heart cycle of contraction and relaxation, and to the right ventricle
only during the relaxation part of the cycle, called diastole, when both right
heart cavities constitute a common chamber; near the end of diastole,
contraction of the right atrium completes the filling of the right ventricle
with blood.
·
Rhythmic
contractions of the right ventricle expel the blood through the pulmonary
arteries into the capillaries of the lung, where the blood receives oxygen.
·
The lung capillaries then empty into the pulmonary veins, which in turn empty into the left atrium.
· Pulmonary
venous return to the left atrium and left ventricle proceeds
simultaneously in the same manner as the venous return to the right heart
cavities.
·
Contraction
of the left ventricle rhythmically propels the blood into the aorta and from there to
all arteries of the body, including the coronary arteries which supply the
heart muscle.
·
The blood forced from the ventricles during systole, or
contraction, is prevented from returning during diastole by valves at the
openings of the aortic and pulmonary arteries.
·
These valves consist of
three semilunar (half-moon-shaped) flaps of membrane, which are curved in
the direction of blood flow and which open readily on pressure in that
direction; when the original pressure subsides, back pressure forces the edges
of the flaps together.
·
The tricuspid valve,
situated between the right atrium and ventricle, is composed of three triangular
flaps of membrane, and the bicuspid or mitral valve, between the left atrium
and ventricle, has two such flaps.
·
The bases of the flaps in both these valves are attached in a
groove at the junction of the atrium and ventricle; the free edge is anchored
by tendinous cords, known as the chordae tendinae, to the muscles of the heart
wall.
·
The flaps remain open until the ventricle fills with blood. When
the ventricle begins to contract, the valve is closed by pressure.
·
The chordae tendinae
prevent inversion of the flaps during this period of systolic pressure.
·
The rate of heartbeat is controlled by the autonomic
nervous system, being accelerated by the sympathetic system and depressed by the
parasympathetic system.
·
Nervous impulses originate rhythmically in a special nervous node,
the sinus, or sinoatrial node, located in the right atrium near the superior
vena cava.
·
The impulses travel along a slender bundle of neuromuscular
fibers, called the bundle of His, or the atrioventricular bundle, which, after
a relay at the atrioventricular node, branches into two bundles, one for each
of the ventricles. Through these specialized fibers the nervous impulse
stimulates, at rapid speed and in a known orderly sequence, contraction of the
heart-muscle fibers.
·
The heart produces two sounds in each cycle of the beat. The first
is rather dull and is caused by vibration of the auriculoventricular valves and
by the contraction of the ventricular muscle fibers; the second is sharp and is
caused by the sudden closing of the aortic and pulmonary valves.
·
In cardiac disease these regular sounds may be replaced or
accompanied by murmurs caused by turbulent blood rushing through abnormal
valves or openings; detection of such murmurs is of great importance in
diagnosis.
Heart
Diseases
Congenital heart defects include persistence of fetal connections
between the arterial and venous circulations, such as the ductus arteriosus, a
vessel normally connecting the pulmonary artery and the aorta only until birth.
Formerly the expectation of life for such infants was extremely
limited; with the advent of early diagnosis and improved techniques of hypothermia, surgery
is often possible in the first week of life and the outlook for these infants
is greatly improved.
Rheumatic heart disease was formerly one of the most serious forms
of heart disease of childhood and adolescence, involving damage to the entire
heart and its membranes. It usually followed attacks of rheumatic
fever. Widespread use of antibiotics effective against the
streptococcal bacterium that causes rheumatic fever has greatly reduced the
incidence of this condition.
Myocarditis is inflammation or degeneration of the heart muscle. Although it
is often caused by various diseases such as syphilis, goiter, endocarditis, or
hypertension, myocarditis may appear as a primary disease in adults or as a
degenerative disease of old age.
Gradual narrowing of the arteries throughout life restricts the
blood flow to the heart muscles. Symptoms of this restricted blood flow can include
shortness of breath, especially during exercise, and a tightening pain in the
chest called angina pectoris. The plaque may become large enough to
completely obstruct the coronary artery, causing a sudden decrease in oxygen
supply to the heart. Obstruction, also called occlusion, can occur when part of
the plaque breaks away and lodges farther along in the artery, a process called
thrombosis. Often found among older persons is cor pulmonale, or pulmonary
heart disease, which usually is the result of a lung ailment, such as emphysema, or a
disease affecting circulation to the lungs, such as arteriosclerosis of the
pulmonary artery. Another condition found in older persons is congestive heart
failure, in which the ventricles pump much less efficiently. The muscular walls
of the ventricles enlarge with the effort to propel more of the blood into the
circulation, giving rise to the large, floppy hearts characteristic of this
syndrome.
Blood Flow of the blood in the
Heart
The electrocardiograph, an instrument for recording the electrical
currents produced by the heart muscle during various phases of contraction, is
an important diagnostic tool. The efficiency of the heart as a pump may be
measured accurately by the use of cardiac catheterization. In this technique a
tube is introduced, through a vein or an artery or both, into the right, left,
or both heart cavities, the pulmonary artery, and the aorta.
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