The
atrium and ventricle on the right are separated from their counterparts on the
left by a wall of muscle, called a septum. Into the right atrium comes
"used" blood returning from coursing through the body, during which
trip it has given up its oxygen to body cells in exchange for cell wastes. It
now needs freshening and it flows from the atrium through a valve into the
right ventricle. The valve, the tricuspid, is there to prevent blood from being
pushed back into the atrium when the ventricle contracts. The contraction of
the ventricle pushes the bluish "used" blood into the pulmonary
artery toward the lungs.
Thus the right side of the heart is a pump devoted to
moving blood toward the lungs for oxygenation. When the blood, freshened in the
lungs, returns through the pulmonary veins to the heart it enters the left
atrium. From here it goes, through the mitral valve, to the left ventricle. And
it is the contraction of the left ventricle that sends a surge of fresh blood
into the aorta, the great artery which comes out of the heart and from which
branches run to all parts of the body. Valves to prevent backward flow of blood
are also located where the aorta and pulmonary artery emerge from the heart.
THE
HEARTBEAT
The beat of the heart-on the average, 72 times a minute--starts in a
knot of tissue called the sinoatrial node located in the atria. The node
contains nerve cells and fibers and muscle cells and is called the heart's
pacemaker because it gives rise to the impulse, or spark that starts a wave of
contraction. The wave spreads over the muscle of the atria and, upon reaching
another node near the junction of atria and ventricles, produces an impulse
which leads to contraction of the ventricles.
As already noted, the heart does
not lie entirely on the left side, de- spite a popular notion to that effect.
Rather it is near the midline with about one third of its bulk on the right and
two thirds on the left. The flatter base of the heart faces backward, and the
sharper apex faces out and downward. It is the apex that reaches to the left,
and because it pulses with each beat; the heart appears to be centered at that
spot rather than stretching toward it.
