also a twisting or wringing action of the left ventricle that causes the apex of the heart to
hit against the inner surface of the chest wall--the apex beat.
(5) The stroke volume is the amount of blood forced out of each ventricle in
one contraction. The cardiac output is the volume of blood pumped out of the ventricles
(RT into the lungs, LT into the systemic circulation) in one minute (expressed in liters
per minute). These volumes will change according to the needs of the body.
10-27. CARDIAC VALVES
Valves are structures that ensure that fluids will pass through them in only one
direction. That is, a valve will open to allow fluids to pass in one direction but will close
to prevent fluids from passing in the other direction. There are two sets of cardiac
valves--the atrioventricular (AV) valves and the semilunar valves. Although the two sets
of valves are quite different in design, they both function passively in response to the
flow of the blood.
a. AV Valves. The AV valves are found between the atria and the ventricles.
The AV valves consist of flaps, known as cusps. The outer margin of each flap is
attached to the inner surface of a fibrous ring. The inner edge of each flap is free.
(1) On the right side is the tricuspid valve. On the left side is the mitral
valve. ("Might is never right.")
(a) Thus, the tricuspid valve is between the right atrium and the right
ventricle. It is named for its three cusps.
(b) The mitral valve is located between the left atrium and the left
ventricle. Since it has two cusps, it is sometimes called the "bicuspid" valve.
(2) The contraction of the atrial walls forces the blood from the atria through
the AV valves and into the ventricles (atrial systole).
(3) When the atria relax (atrial diastole) and the ventricles contract, the
pressure would tend to drive the blood back into the atria. However, each opening is
sealed when the cusps of each AV valve meet in the valve center. This prevents blood
from flowing further back into the atria.
(4) A special anatomic arrangement helps prevent backward flow into the
atria. Chordae tendineae are fibrous cords attached to the ventricular side of the cusps.
Since these cords of dense FCT have a fixed length, they cannot be stretched or
shortened. The other ends of these cords are attached to the papillary muscles. The
papillary muscles are special extensions of the muscular walls of the ventricles. As the
ventricles contract and become smaller, these muscles take up the slack in the cords.