1. At the onset of
systole, ventricular pressure rapidly exceeds atrial pressure, and the
atrioventricular valves close. The aortic and the pulmonary valves are not yet
open, however, and so no ejection occurs during this isovolumetric ventricular
contraction(constant volume of blood in ventricle).
2. When ventricular
pressures exceed aortic and pulmonary trunk pressures, the aortic and pulmonary
valves open, and ventricular ejection of blood occurs. The volume of blood
ejected from each ventricle during systole is termed the Stroke Volume.
3. When the ventricles
relax at the beginning of diastole, the ventricular pressures fall
significantly below those in the aorta and pulmonary trunk , and the aortic and
pulmonary valves close. Because the atrioventricular valves are also still
closed, no change in ventricular volume occurs during this isovolumetic
ventricular relaxation.
4. When ventricular
pressures fall below the pressures in the right and the left atria, the
Atrioventricular valves open, and the ventricular fillinf phase of diastole
begins.
5. Filling occurs very
rapidly at first so that atrial contraction , which occurs at the very end of
diastole, adds only a small amount of additional blood to the ventricles.
6. The amount of blood in the
ventricles just before systole is the end-diastolic volume. The volume
remaining after ejection is the end-systolic volume, and the volume ejected is
the stroke volume
