The Human Heart Essay, Research Paper

Biology – Histology

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.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.