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.