The 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.
 
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.
 
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. 

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.
 
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. 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.
 
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. 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. 

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.
 
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.
 
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 this 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.
 
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.
 
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.
 
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.
 
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.
 
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.
 
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.