The p h a se s o f th e cardiac cycle • Figure 11.7
Relaxation period (diastole)
a. Ventricles relax isovolumetrically (no change in
volume) and repolarize (relaxation of cardiac
muscle fibers)(T wave of EKG).
b. Blood from body and lungs fills atria.
c. When ventricular pressure falls below atrial
pressure, AV valves open and ventricles fill
with blood.
d. At end of this phase, ventricles are about 3/4
full and the atria begin to depolarize (P wave of
EKG).
Atrial contraction (atrial systole)
a. As P wave spreads over atria, they contract and
force the final 25% of blood into the ventricles
(ventricles are full ~130 mL).
b. AV valves are open and semilunar valves are closed.
c. End of atrial systole = end of ventricular relaxation
0
0.2
0.4
0.6
0.8
Seconds
Isovolumetric
contraction
Ventricular
ejection
0
0.2
0.4
Seconds
Ventricular contraction (ventricular systole)
a. Ventricular depolarization begins (QRS complex of EKG
begins).
b. Ventricular fibers contract, but do not shorten (isovolumetric
contraction). Ventricular pressure increases.
c. When ventricular pressure exceeds atrial pressure, the AV
valves close. Ventricular pressure continues to increase.
d. When ventricular pressure exceeds pressure in aorta and
pulmonary arteries, the semilunar valves open.
e. Ventricles pump about 70 mL of blood out (ventricular
ejection) into aorta and pulmonary arteries.
f. As blood leaves the ventricles, the ventricular pressure falls
below aortic and pulmonary artery pressures and the
semilunar valves close.
g. Ventricular systole ends with the onset of the T wave.
Which of the following correctly pairs events of the car-
diac cycle and the EKG?
a. QRS complex, ventricular systole, opening of the AV
valves
b. P-wave, atrial diastole, flow of blood into the ventricles
c. T wave, closing of the AV valves, flow of blood into the
ventricles
d. QRS complex, ventricular systole, opening of the
semilunar valves
e. P wave, atrial systole, closing of the AV valves
The Heart Pumps Blood Through Blood Vessels to All Tissues
317
PROCESS DIAGRAM
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