Heart Essay, Research Paper HEART The human heart is a specialized, four-chambered muscle that maintains BLOOD flow in the CIRCULATORY SYSTEM. Located in the thorax, it lies left of the body’s midline, above and in contact with the
Heart Essay, Research Paper
The human heart is a specialized, four-chambered muscle that maintains BLOOD flow in the CIRCULATORY
SYSTEM. Located in the thorax, it lies left of the body’s midline, above and in contact with the
diaphragm. It is situated immediately behind the breastbone, or sternum, and between the lungs, with its
apex tilted to the body cavity’s left side. In most people the apex can be felt during each heart
contraction. At rest, the heart pumps about 59 cc (2 oz) of blood per beat and 5 l (5 qt) per minute,
compared to 120-220 cc (4-7.3 oz) per beat and 20-30 l (21-32 qt) per minute during exercise. The adult
human heart is about the size of a fist and weighs about 250-350 gm (9 oz).
Blood supplies food and oxygen to the cells of the body for their life needs and removes the waste
products of their chemical processes. It also helps to maintain a consistent body temperature, circulate
hormones, and fight infections. The brain cells are very dependent on a constant supply of oxygen. If the
circulation to the brain is stopped, death ensues shortly. Since heart attacks are the number-one cause
of death in the United States, the heart gets a great deal of attention.
The role of the heart was long considered a mystery and often given elevated importance. Some thought it
was the seat of the soul. Others thought it was the center of love, courage, joy, and sadness. Primitive
man must have been aware of the heartbeat and probably recognized the heart as an organ whose malfunction
could cause sudden death.
The Hippocratic De Corde, which probably dates from the time of ARISTOTLE, describes the construction of
the heart’s valves. LEONARDO DA VINCI made exquisite drawings of the heart, but it was not until the
publication of William HARVEY’s De Motu Cordis (1628) that the heart’s specific role in relation to
circulation was widely understood.
STRUCTURE AND FUNCTION OF THE HUMAN HEART
The heart’s wall has three parts. Muscle tissue, or myocardium, is the middle layer. The inner layer, or
endocardium, that lines the inside of the heart muscle consists of a thin layer of endothelial tissue
overlying a thin layer of vascularized connective tissue. The outside of the heart, the epicardium, is in
intimate contact with the pericardium; this serous membrane is a closed sac covering the heart muscle’s
outside wall. Within the sac, a small amount of fluid reduces the friction between the two layers of
tissue. In addition to muscular and connective tissue, the heart muscle contains varying amounts of fatty
tissue, especially on the outside. Both anatomically and functionally, the heart is divided into a left
and a right half by the cardiac septum. Each half contains two separate spaces: the atrium (pl. atria),
or auricle, and the ventricle. The upper reservoirs, or collecting chambers, are the thin-walled atria,
and the lower pumping chambers are the thick-walled ven!
tricles. The total thickness of the ventricular walls is about three times that of the atria; the wall of
the heart’s left half is approximately twice as thick as that of the right half. The thickness of the
heart muscle varies from 2 to about 20 mm (0.1 to 0.8 in). This thickness is correlated with the maximum
pressure that can be attained in each chamber.
FLOW OF BLOOD THROUGH THE HEART
The right atrium receives oxygen-poor blood from two major veins: the superior and inferior vena cava,
which enter the atrium through separate openings. From the right atrium the blood passes through the
tricuspid valve, which consists of three flaps, or cusps, of tissue. This valve directs blood flow from
the right atrium to the right ventricle. The tricuspid valve remains open during diastole, or ventricular
filling; however, when the ventricle contracts, the valve closes, sealing the opening and preventing
backflow into the right atrium. Five cords attached to small muscles (papillary muscles) on the
ventricles’ inner surface prevent the valves’ flaps from being pushed backward. From the right ventricle
blood is pumped through the pulmonary, or semilunar, valve, which has three half-moon-shaped flaps, into
the pulmonary artery. This valve prevents backflow from the artery into the right ventricle. From the
pulmonary artery, blood is pumped to the lungs, where it gives up ca!
rbon dioxide and receives oxygen, and then is returned to the heart’s left side through four pulmonary
veins (two from each lung) to the left atrium and then through the mitral valve, a two-flapped valve also
called a bicuspid valve, to the left ventricle. As the ventricles contract, the mitral valve prevents
backflow of blood into the left atrium, and blood is driven through the aortic valve into the AORTA, the
major artery, which supplies blood to the entire body. The pulmonary valve, like the aortic valve, has a
semilunar shape and a unidirectional function.
The blood supply to the heart muscle is furnished mainly by the CORONARY ARTERIES, which originate from
the aorta immediately after the aortic valve. These vessels pass through the fatty tissue beneath the
pericardium and then branch out into the heart muscle.
The coronary veins transport the deoxygenated blood from the heart muscle to the right atrium. The
heart’s energy supply is almost completely dependent on these coronary vessels. Only the tissues lying
directly beneath the endocardium receive a sufficient amount of oxygen from the blood within the cavities
of the heart.
REGULATION OF THE HEARTBEAT
The heart muscle pumps the blood through the body by means of rhythmical contractions (systole) and
dilations (diastole). The heart’s left and right halves work almost synchronously. When the ventricles
contract (systole), the valves between the atria and the ventricles close, as the result of increasing
pressure, and the valves to the pulmonary artery and the aorta open.
When the ventricles become flaccid during diastole and the pressure decreases, the reverse process takes
place: through the valves between the atria and the ventricles, which are now open again, blood is drawn
from the atria into the ventricles, and the valves to the pulmonary artery and the aorta close.
At the end of diastole the atria also contract and thus help to fill the ventricles. This is followed by
systole. The electrical stimulus that leads to contraction of the heart muscle originates in the heart
itself, that is, in the sinoatrial node (SA node), or pacemaker. This node, which lies just in front of
the opening of the superior vena cava, measures no more than a few millimeters. It consists of heart
cells that emit regular impulses. Because of this spontaneous discharge of the sinus node, the heart
muscle is automated, and a completely isolated heart can contract on its own, as long as its metabolic
processes remain intact. The electrical stimulus from the SA node becomes propagated regularly over the
muscle cells of both atria and reaches the atrioventricular node (AV node), which lies on the border
between the atria and the ventricles. The stimulus continues into the bundle of His. This bundle proceeds
for about a centimeter and then divides into a left and a right!
bundle branch. The two bundle branches lie along the two sides of the heart’s septum and then proceed
toward the apex. The small side branches that come off are the Purkinje fibers, which conduct the
stimulus to the muscle cells of the heart’s ventricles.
The Purkinje fibers differ from the cardiac muscle cells and conduct the stimuli more rapidly. However,
the AV node conducts the stimulus relatively slowly. As a result, the heart chambers contract regularly
and evenly during systole, and ventricular contraction does not coincide with that of the atria; so the
pumping function is well-coordinated. Potentially, the whole conduction system is able to discharge
spontaneously and can take over the function of the SA node. The rate at which the cells of the SA node
discharge under normal circumstances is externally influenced through the autonomic nervous system, which
sends nerve branches to the heart. Through their stimulatory and inhibitory influences they determine the
resultant heart rate. In adults at rest this is between 60 and 74 beats a minute. In infants and young
children it may be between 100 and 120 beats a minute. Tension, exertion, or fever may cause the rate of
a healthy heart to vary between 55 and 200 beats a minu!
The output of the heart is expressed as the amount of blood pumped out of the heart each minute: the
heart minute-volume (HMV). This is the product of the heart rate and the stroke volume (SV), the amount
of blood pumped out of the heart at each contraction.
EVOLUTION OF THE HEART
The hearts of primitive vertebrates apparently had only one atrium and one ventricle. Since their body
temperature and metabolic rate fluctuated with the environmental temperature, they did not need as
efficient a circulatory system as mammals and birds. The two-chamber heart is retained by modern fish,
but oxygen-rich blood does not mix with oxygen-poor blood, because the blood is aerated at the gills and
goes directly into systemic circulation, not to the heart. As the primitive lung evolved in amphibians,
two circulatory systems arose. The problem of mixing oxygenated and deoxygenated blood was resolved in a
number of amphibians such as the FROG, in which the single atrium is divided into two separate chambers.
Thus there is only a slight mixing of the bloods in these three-chambered hearts. This adaptation appears
to help the frog when it is under water, since the skin provides oxygen when the lungs cannot be used. In
SIRENS a partial division takes place in the ventricle !
As animals became larger and more active on land, they needed more pressure to provide faster flow. The
sides of the heart were separated when a septum formed to divide the ventricle into two chambers. Birds
and mammals have completely separate chambers and have more blood per tissue weight and more pressure,
because the tissues of birds and mammals (warm-blooded vertebrates) require a constant perfusion of
oxygen-rich blood in order to maintain their high metabolic rates and constant body temperature.
The closure of the heart valves and the contraction of the heart muscle produce sounds that can be heard
through the thoracic wall by the unaided ear, although they can be amplified by means of a STETHOSCOPE.
The sounds of the heart may be represented as lubb-dupp-pause-lubb-dupp-pause. The lubb sound indicates
the closing of the valves between the atria and ventricles and the contracting ventricles; the dupp sound
indicates the closing of the semilunar valves. In addition, there may also be cardiac murmurs, especially
when the valves are abnormal. Some heart murmurs, however, may also occur in healthy persons, mainly
during rapid or pronounced cardiac action. The study of heart sounds and murmurs furnishes valuable
information regarding the condition of the heart muscle and valves. The heart sounds are recorded with
the aid of sensitive microphones (phonocardiography), so that anomalies of the heart or the valves can be
analyzed. The conduction of the contraction stimulus can!
also be recorded on the body surface by an ELECTROCARDIOGRAPH. This measures the differences in
potential (in microvolts) that exist between a number of fixed points on the limbs and the chest wall.
The electrocardiogram (cardiogram, ECG) that is obtained in this way furnishes information about the
rhythm of the heart, the conduction of the stimulus, and the condition of the heart muscle. Other methods
that have been devised to examine the heart are the mechanical recording of the heartbeat,
echocendiography and radioisotopes, X-ray analysis of the heart’s form and movements, and X-ray contrast
studies of the blood flow through the heart and the coronary vessels.
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