FREE ESSAY ON HUMAN HEART

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 
There are four main openings into the right atrium; the 
SVC, IVC, coronary sinus, and the atriculo-ventricular 
opening. The larger IVC returns blood from the lower half 
of the body and opens into the lowest part of the right 
atrium, near the septum. The smaller SVC returns blood from 
the upper half of the body and opens into the upper and 
front part of the right atrium. The coronary sinus opens 
into the right atrium between the IVC and 
auriculo-ventricular opening. It returns blood from the 
cardiac muscle of the heart and is protected by a 
semicircular fold of the lining membrane of the atrium, 
called the coronary valve. The auriculo-ventricular opening 
is the large oval aperture of communication between the 
right atrium and ventricle. There are two main valves 
located within the right atrium; the Eustachian valve and 
the coronary valve.3 The Eustachian valve is located 
between the anterior margin of the IVC and the 
auricule-ventricular orifice. It is semilunar in form. The 
coronary valve is a semicircular fold of the lining membrane 
of the right atrium, protecting the orifice of the coronary 
sinus.
The right ventricle is triangular-shaped and extends from 
the right atrium to near the apex. Its anterior surface is 
rounded and convex and forms the larger part of the front of 
the heart. Its posterior surface is flattened, rests on the 
diaphragm muscle, and forms only a small part of this 
surface. Its inner wall is formed by the partition between 
the two ventricles, the septum, and bulges into the cavity 
of the right ventricle. Superiorly, the ventricle forms a 
conical structure called the infundibulum from which the 
pulmonary artery arises. The walls of the right ventricle 
are thinner than those of the left ventricle. The thickest 
part of the wall is at the base and it gradually becomes 
thinner towards the apex. The cavity can contain up to two 
ounces of fluid. 
There are two openings in the right ventricle; the 
auriculo-ventricular opening and the opening of the 
pulmonary artery. The auriculo-ventricular opening is the 
large oval opening between the right atrium and the right 
ventricle. The opening is about an inch in diameter. It is 
surrounded by a fibrous ring, covered by the lining membrane 
of the heart (endocardium), and is larger than the opening 
between the left atrium and the left ventricle. It is 
protected by the tricuspid valve. The opening of the 
pulmonary artery is round and is situated at the top of the 
conus arteriosus, close to the septum. It is on the left 
side and is in front of the auriculo-ventricular opening. 
It is protected by the semilunar valves.3 
There are two main valves associated with the right 
ventricle; the tricuspid valve and the semilunar valves. 
The tricuspid valve consists of three segments of a 
triangular shape, formed by the lining membrane of the heart 
(endocardium). They are strengthened by a layer of fibrous 
tissue and muscular fibers.1 These segments are connected 
by their bases to the auriculo-ventricular orifice, and by 
their sides with one another, so as to form a continuous 
membrane which is attached around the margin of the 
auriculo-ventricular opening. Their free margin and 
ventricular surfaces are attached to many delicate tendinous 
cords called chordae tendinae. The central part of each 
valve segment is thick and strong while the lateral margins 
are thin and indented. The chordae tendinae are connected 
with the adjacent margins of the main segment of the valves. 
The semilunar valves guard the opening of the pulmonary 
artery. They consist of three semicircular folds formed by 
the endothelial lining of the heart and are strengthened by 
fibrous tissue. They are attached by their convex margins 
to the wall of the artery at its junction with the 
ventricle. The straight borders of the valve are unattached 
and are directed upwards in the course of the vessel, 
against the sides of which they are pressed during the 
passage of blood along its canal. The free margin of each 
valve is somewhat thicker than the rest of the valve and is 
strengthened by a bundle of tendinous fibers. During the 
passage of blood along the pulmonary artery, these valves 
are pressed against the sides of its cylinder. During 
ventricular diastole (rest), when the current of blood along 
the pulmonary artery is checked and partly thrown back by 
its elastic walls, these valves become immediately expanded 
and close the entrance of the tube. 3
The left atrium is smaller but thicker than the right 
atrium. It consists of two parts; a principle cavity/sinus 
and an appendix auriculae. The sinus is cuboidal in form 
and is covered in the front by the pulmonary artery and the 
aorta. Internally, it is separated from the right atrium by 
the septum auricularum. Behind the sinus on each side, it 
receives the pulmonary veins. The appendix auriculae in the 
left atrium is narrower and more curved than the same 
structure in the right atrium. Its margins are more deeply 
indented, presenting a kind of foliated appearance. Its 
direction is forwards towards the right side, overlapping 
the root of the pulmonary artery. 
There are two main openings in the left atrium; the 
openings of the four pulmonary veins and the 
atrial-ventricular opening. Two of the four pulmonary veins 
open into the right side of the atrium and two open into the 
left side. The two veins on the left exit into the atrium 
through a common opening. None of the pulmonary veins have 
valves. The atrial-ventricular opening is the large oval 
opening of blood flow between the atrium and the ventricle. 
It is smaller than the same opening between the right atrium 
and ventricle.3
The left ventricle is longer and more conical shaped than 
the right ventricle. It forms a small part of the left side 
of the anterior surface of the heart and a large portion of 
the posterior surface. It also forms the apex of the heart 
because it extends beyond the right ventricle. Its walls 
are nearly twice as thick as those of the right ventricle. 
They are thickest in the broadest part of the ventricle, 
becoming gradually thinner towards the base and also towards 
the apex, which is the thinnest part of the left ventricle.
There are two main openings in the left ventricle; the 
atrial-ventricular opening and the aortic opening. The 
atrial-ventricular opening is located behind and to the left 
side of the aortic opening. The opening is a little smaller 
than the same opening between the right atrium and 
ventricle. Its position corresponds to the center of the 
sternum. It is surrounded by a dense fibrous ring and is 
covered by the lining membrane of the heart and is protected 
by the mitral valve. The circular aortic opening is located 
in front of and to the right side of the atrial-ventricular 
opening from which it is separated by one of the segments of 
the mitral valve. The opening is protected by the semilunar 
valves. 
There are two valves located within the left ventricle; the 
mitral valve and the semilunar valve. The mitral valve is 
attached to the circumference of the atrial-ventricular 
opening in the same way that the tricuspid valve is attached 
on the opposite side of the heart. The valve contains a few 
muscular fibers, is strengthened by fibrous tissue, and is 
formed by the lining of the heart (endocardium). It is 
larger, thicker, and stronger than the tricuspid, and 
consists of two segments of unequal size. The mitral valves 
are connected to many chordae tendonae. Their attachment is 
the same as on the right side except they are thicker, 
stronger, and less numerous. The semilunar valves surround 
the aortic opening. They are similar in structure and mode 
of attachment to those of the pulmonary artery. However, 
they are larger, thicker, and stronger than those of the 
right side. Between each valve and the cylinder of the 
aorta is a deep depression called the sinuses of Valsalva. 
The depressions are larger than those at the root of the 
pulmonary artery.3
Figure 1: a. Cross sectional view of the heart. b. Top 
view of the heart showing the four valves
Histology of the Layers of the Heart:
The heart and its vessels are surrounded by a conical 
membranous sac called the pericardium. The pericardial sac 
is composed of two layers; the parietal pericardium and the 
visceral pericardium with the space in-between the two being 
called the pericardial cavity. The parietal pericardium is 
composed primarily of compact fibrocollagenous tissue along 
with elastic tissue. It is a fibrous membrane of loose 
irregular connective tissue that is lined internally by a 
mesothelium which is essentially simple squamous epithelium. 
The visceral pericardium forms the internal lining of the 
pericardium and reflects over the outer surface of the 
heart. This reflection forms the outer layer of the 
epicardium. The visceral pericardium is also composed of 
compact fibrocollagenous tissue with elastic tissue but, is 
smooth mesothelium. The pericardial cavity is located 
between the parietal and visceral pericardium and contains 
small amounts of serous fluid.
The heart tissue itself can be subdivided into three 
layers; (from the outside in) epicardium, myocardium, and 
endocardium. The epicardium is the outermost layer of the 
heart and consists of a loose connective tissue of 
fibroblasts, collagen fibers, and adipose tissue. It 
contains a stroma which houses coronary arteries and veins 
that are surrounded by a layer of fat. These coronary 
branches penetrate the myocardium. 
The myocardium contains the main muscle mass of the heart 
and is composed primarily of striated muscle cells. Each of 
the cardiac muscle cells contain one central elongated 
nucleus with some central euchromatin and some peripheral 
heterochromatin. The two atria have a very thin myocardial 
layer which increases greatly in thickness as you go from 
the atria to the right ventricle and into the left 
ventricle. The outer surface of the myocardium, next to the 
epicardium, is not composed of smooth muscle but is very 
smooth in texture. The inner surface of the myocardium is 
rough and is raised into trabeculations. The ventricular 
papillary muscles, which are for the attachment of the 
chordae tendinae, are extensions of the myocardium even 
though they are covered by endocardium. The outer layer of 
the myocardium is superficial bulbospiral and swirls around 
the ventricle in a clockwise fashion. The middle layer is 
circular muscles that are the ventricular constrictors. The 
inner layer, which is deep bulbospiral, swirls around the 
ventricle in a counterclockwise fashion. 
The layer underneath the myocardium is known as the 
enodcardium. It contains a continuous smooth endothelial 
layer that covers all the inner surfaces of the heart, 
including the valves. The outer layer of the endocardium, 
underneath the myocardium, is irregularly arranged 
collagenous fibers that may contain Purkinje fibers/cells. 
The inner part of the endocardium contains more regularly 
arranged collagen and elastic fibers than the outer layer. 
Some myofibroblasts are present in the endocardium which is 
thicker in the atria than in the ventricles. There is a 
subendothelial component of the endocardium underneath the 
endothelium. The component contains fibroblasts, scattered 
smooth muscle cells, elastic fibers, collagen fibers, and an 
amorphous ground substance that contains glycoproteins and 
proteoglycans.
The valves of the heart are attached to the cardiac 
skeleton and consist of chondroid (a material resembling 
cartilage). The base of each valve is supported by a 
fibrocollagenous ring. Each valve also has a dense 
fibrocollagenous central plate that is covered by simple 
squamous epithelium. Chordae tendonae connect with the 
valves at the edge of each cusp as well as underneath each 
cusp at one end and they attach to papillary muscles in the 
ventricles at the other end. Endocardial endothelium 
completely covers the papillary muscles, valves, and the 
chordae tendonae. The junctions between the cusps of each 
valve are known as commissures.
The conducting system of the heart consists of four main 
components; the sinuatrial node (SA), the atrioventricular 
node (AV), the bundle of his, and the Purkinje fibers/cells. 
All the parts of this conducting system are composed of 
modified cardiac muscle cells. The SA node is located in 
the right atrium, at the point where the superior vena cava 
enters. The small muscle fibers of the SA node contain a 
central nodal artery and desmosomes. The muscle fibers do 
not contain intercalated discs. The AV node is located in 
the medial wall, in front of the opening of the coronary 
sinus and above the tricuspid ring. Its small muscle fibers 
are more regularly arranged than those of the SA node. The 
AV node contains a rich nerve and blood supply. The bundle 
of his has a right (single bundle) and a left (branched 
bundle) bundle branch located underneath the endocardium. 
It is histologically similar to the other components of the 
conducting system. The Purkinje fibers/cells can be found 
in clusters of about six cells which are located under the 
endocardium in the ventricles. The cytoplasm of Purkinje 
fibers appears pale under the microscope and contains many 
glycogen granules.7 
Physiology of the Heart:
The principle function of the heart and circulatory system 
is to provide oxygen and nutrients and to remove metabolic 
waste products from tissues and organs of the body. The 
heart is the pump that provides the energy necessary for 
transporting the blood through the circulatory system in 
order to facilitate the exchange of oxygen, carbon dioxide, 
and other metabolites through the thin-walled capillaries. 
The contraction of the heart produces changes in pressures 
and flows in the heart chambers and blood vessels. The 
mechanical events of the cardiac cycle can be divided into 
four periods; late diastole, atrial systole, ventricular 
systole, and early diastole.6 
In late diastole, the mitral and tricuspid valves are open 
and the pulmonary and aortic valves are closed. Blood flows 
into the heart throughout diastole thus filling the atria 
and ventricles. The rate of filling declines as the 
ventricles become distended, and the cusps of the 
atrioventricular valves start to close. The pressure in the 
ventricles remains low throughout late diastole.8 
In atrial systole, contraction of the atria forces 
additional blood into the ventricles, but approximately 70 
percent of the ventricular filling occurs passively during 
diastole. Contraction of the atrial muscle that surrounds 
the openings of the superior and inferior vena cava and 
pulmonary veins, narrows their orifices and the inertia of 
the blood moving towards the heart tends to keep blood in 
the heart. However, there is some regurgitation of blood 
into the veins during atrial systole.2&5 
At the start of ventricular systole, the AV valves close. 
The muscles of the ventricles initially contract relatively 
little, but intraventricular pressure rises sharply as the 
muscles squeezes the blood in the ventricle. This period of 
isovolumetric ventricular contraction lasts about 0.05 
seconds until the pressures in the ventricles exceed the 
pressure in the aorta and in the pulmonary artery, and the 
aortic and pulmonary valves (semilunar valves) open. During 
this isovolumetric contraction, the AV valves bulge into the 
atria, causing a small but sharp rise in atrial pressure. 
When the semilunar valves open, the phase of ventricular 
ejection begins. Ejection is initially rapid, but slows 
down as systole progresses. The intraventricular pressure 
rises to a maximum and then declines somewhat before 
ventricular systole ends. Late in systole, the aortic 
pressure is actually higher than the ventricular pressure, 
but for a short period, momentum keeps the blood moving 
forward. The AV valves are pulled down by the contractions 
of the ventricular muscle, and the atrial pressure drops.5 
In early diastole, after the ventricular muscle if fully 
contracted, the already falling ventricular pressure drops 
even more rapidly. This is the period known as 
protodiastole and it lasts about 0.04 seconds. It ends when 
the momentum of the ejected blood is overcome and the 
semilunar valves close. After the valves are closed, 
pressure continues to drop rapidly during the period of 
isovolumetric relaxation. Isovolumetric relaxation ends 
when the ventricular pressure falls below the atrial 
pressure and the AV valves open, thus allowing the 
ventricles to fill. Again, filling is rapid at first, then 
slows as the next cardiac contraction approaches. Atrial 
pressure continues to rise after the end of ventricular 
systole until the AV valves open, upon which time it drops 
and slowly rises again until the next atrial systole.6,2,&4
Summary:
The heart is arguably the most vital organ the human body 
possesses. Without the heart, none of the tissues in the 
body would receive the vital oxygen necessary for them to 
maintain survival. Heart disease is the number one killer 
of people in America today. Due to this disturbing fact, it 
is no wonder such a large percentage of the fellowships 
granted by the National Institutes of Health go towards 
heart related illnesses.
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