Page 139 - 84 human physiolofy part-1

The Cardiovascular System

to the ventricles, and not the other way. The two semilunar (SL) valves are present in the arteries
leaving the heart; they prevent blood from flowing back into the ventricles. The sound heard in a heart
beat is the heart valves shutting. The right AV valve is also called the tricuspid valve because it has
three flaps. It is located between the right atrium and the right ventricle. The tricuspid valve allows
blood to flow from the right atrium into the right ventricle when the heart is relaxed during diastole.
When the heart begins to contract, the heart enters a phase called systole, and the atrium pushes blood
into the ventricle. Then, the ventricle begins to contract and blood pressure inside the heart rises. When
the ventricular pressure exceeds the pressure in the atrium, the tricuspid valve snaps shut. The left AV
valve is also called the bicuspid valve because it has two flaps. It is also known as the mitral valve due
to the resemblance to a bishop's mitre (a type of hat). This valve prevents blood in the left ventricle
from flowing into the left atrium. As it is on the left side of the heart, it must cope with a lot of strain
and pressure; this is why it is made of only two cusps, as there is less to go wrong. There are two
remaining valves called the Semilunar Valves. They have flaps that resemble half moons. The
pulmonary semilunar valve sits between the right ventricle and the pulmonary trunk. The aortic
semilunar valve sits between the ventricle and the aorta.

Subvalvular Apparatus

The chordae tendinae are attached to papillary muscles that cause tension to better hold the valve.
Together, the papillary muscles and the chordae tendinae are known as the subvalvular apparatus. The
function of the subvalvular apparatus is to keep the valves from prolapsing into the atria when they
close. The subvalvular apparatus have no effect on the opening and closing of the valves. This is
caused entirely by the pressure gradient across the valve.

Complications With The Heart

The most common congenital abnormality of the heart is the bicuspid aortic valve. In this
condition, instead of three cusps, the aortic valve has two cusps. This condition is often undiagnosed
until the person develops calcific aortic stenosis. Aortic stenosis occurs in this condition usually in
patients in their 40s or 50s, an average of 10 years earlier than in people with normal aortic valves.
Another common complication of rheumatic fever is thickening and stenosis (partial blocking) of the
mitral valve. For patients who have had rheumatic fever dentist are advised to prophylactally
administer antibiotics prior to dental work to prevent bacterial endocarditis that occurs when bacteria
from the teeth enter the circulation and attach to damaged heart valves.

Passage of Blood Through the Heart

While it is convenient to describe the flow of the blood through the right side of the heart and then
through the left side, it is important to realize that both atria contract at the same time and that both
ventricles contract at the same time. The heart works as two pumps, one on the right and one on the left
that works simultaneously. The right pump pumps the blood to the lungs or the pulmonary circulation
at the same time that the left pump pumps blood to the rest of the body or the systemic circulation.
Venous blood from systemic circulation (deoxygenated) enters the right atrium through the superior
and inferior vena cava. The right atrium contracts and forces the blood through the tricuspid valve
(right atrioventricular valve) and into the right ventricles. The right ventricles contract and force the
blood through the pulmonary semilunar valve into the pulmonary trunk and out the pulmonary artery.

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