Page 79 - The Miracle of Electricity in the Body

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D Diastole: During the first
phase of the cycle, oxy- At left, the phases of the cardiac
genated blood flows into the cycle. The three successive phas-
left ventricle and deoxy- es of the heart’s rhythmic beating
are made possible by very sensi-
genated blood simultane-
tive timing and adjustment. The
ously into the right ventricle.
contraction and compression
This blood then flows into
stages follow the heart chambers
the atria. filling with blood as the heart mus-
cle relaxes. The heart performs its
Ventricles relax
cycle in as little as a quarter of a
second, though this speed doubles
Atrial systole: Impulses
during exercise.
from the sinoatrial node initi-
ate the second phase of the
cycle, during which both atria
contract, squeezing any
blood that remains in the atria
into the ventricles.
Atria contract
Heart valve
open
Ventricular systole: The
ventricles contract during the
third phase of the cardiac cy-
cle. The valves at the exits of
both ventricles open and the
blood is forced into the aorta Heart valve closed
Ventricles contract and pulmonary artery. As this To the right, the
phase ends, the cycle starts heart valves in open
and closed positions.
again.

A Matchless Organization of Pumps and Valves

The pumps in the heart consist of two different sets of pumps, an
upper and a lower. The smaller and uppermost are known as the atria
and the larger, lower ones as the ventricles. For example, when clean
blood reaches the left side of the heart it first fills the small, upper atri-
um. From here it is pumped to the larger ventricle beneath. The large
ventricle sends blood to the organs of the body. The same process is car-
ried out in the pumps on the right-hand side of the heart. Between atria
and ventricles are one-directional valves that open to let the blood flow
pass. When the small atria contracts, these valves open and blood fills