1. As the ventricles begin their contraction, the intraventricular pressure rises, causing the AV valves to snap shut. At this time, the ventricles are neither being filled with blood (because the AV valves are closed) nor ejecting blood (because the intraventricular pressure has not risen sufficiently to open the semilunar valves). This is the phase of isovolumetric contraction.
2. When the pressure in the left ventricle becomes greater than the pressure in the aorta, the phase of ejection begins
■ Figure 13.13 The relationship between heart sounds and the intraventricular pressure and volume. The numbers refer to the events described in the text.
as the semilunar valves open. The pressure in the left ventricle and aorta rises to about 120 mmHg (fig. 13.13) when ejection begins and the ventricular volume decreases.
3. As the pressure in the left ventricle falls below the pressure in the aorta, the back pressure causes the semilunar valves to snap shut. The pressure in the aorta falls to 80 mmHg, while pressure in the left ventricle falls to 0 mmHg.
4. During isovolumetric relaxation, the AV and semilunar valves are closed. This phase lasts until the pressure in the ventricles falls below the pressure in the atria.
5. When the pressure in the ventricles falls below the pressure in the atria, the AV valves open and a phase of rapid filling of the ventricles occurs.
6. Atrial contraction (atrial systole) empties the final amount of blood into the ventricles immediately prior to the next phase of isovolumetric contraction of the ventricles.
Similar events occur in the right ventricle and pulmonary circulation, but the pressures are lower. The maximum pressure produced at systole in the right ventricle is 25 mmHg, which falls to a low of 8 mmHg at diastole.
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