Phases of the Cardiac Cycle

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We now examine the phases of the cardiac cycle, the pressure changes that occur, and how the pressure changes and valves govern the flow of blood. A substantial amount of information about these events is summarized in figure 19.19, which is divided into colored bars numbered to correspond to the phases described here. Closely follow the figure as you study the following text. Where to begin when describing a circular chain of events is somewhat arbitrary. However, in this presentation we begin with the filling of the ventricles. Remember that all these events are completed in less than 1 second.

1. Ventricular filling. During diastole, the ventricles expand and their pressure drops below that of the atria. As a result, the AV valves open and blood flows into the ventricles, causing ventricular pressure to rise and atrial pressure to fall. Ventricular filling occurs in three phases: (a) The first one-third is rapid ventricular filling, when blood enters especially quickly. (fc) The second one-third, called diastasis (di-ASS-tuh-sis), is marked by slower filling. The P wave of the electrocardiogram occurs at the end of diastasis, marking the depolarization of the atria.

(c) In the last one-third, atrial systole completes the filling process. The right atrium contracts slightly before the left because it is the first to receive the signal from the SA node. As the ventricles fill, the flaccid cusps of the AV valves float up toward the closed position. At the end of ventricular filling, each ventricle contains an end-diastolic volume (EDV) of about 130 mL of blood. Only 40 mL (31%) of this is contributed by atrial systole.

2. Isovolumetric contraction. The atria repolarize, relax, and remain in diastole for the rest of the cardiac cycle. The ventricles depolarize, generate the QRS complex, and begin to contract. Pressure in the ventricles rises sharply and reverses the pressure gradient between atria and ventricles. The AV valves close as ventricular blood surges back against the cusps. Heart sound S1 occurs at the beginning of this phase and is produced mainly by the left ventricle; the right ventricle is thought to make little contribution. Causes of the sound are thought to include the tensing of ventricular tissues, acceleration of the ventricular wall, turbulence in the blood as it surges against the closed AV valves, and impact of the heart against the chest wall.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

19. The Circulatory System: The Heart

Text

© The McGraw-H Companies, 2003

Chapter 19 The Circulatory System: The Heart 735

Diastole

Systole

Diastole

Heart sounds

Phase of cardiac cycle

Cardiac Cycle PhasesPhases The Cardiac Cycle
S

1c 2

S2 S3

Time (sec)

Phases of cardiac cycle:

1. Ventricular filling 1a. Rapid filling 1b. Diastasis 1c. Atrial systole

2. Isovolumetric contraction

3. Ventricular ejection

4. Isovolumetric relaxation

Phases And Events Cardiac Cycle

Figure 19.19 Major Events of the Cardiac Cycle. Two complete cycles are shown. The phases are numbered across the bottom to correspond to the text description.

Explain why the aortic pressure curve begins to rise abruptly at about 0.5 second.

Figure 19.19 Major Events of the Cardiac Cycle. Two complete cycles are shown. The phases are numbered across the bottom to correspond to the text description.

Explain why the aortic pressure curve begins to rise abruptly at about 0.5 second.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

736 Part Four Regulation and Maintenance

This phase is called isovolumetric27 because even though the ventricles contract, they do not eject blood yet, and there is no change in their volume. This is because pressures in the aorta (80 mmHg) and pulmonary trunk (10 mmHg) are still greater than the pressures in the respective ventricles and thus oppose the opening of the semilunar valves. The myocytes exert force, but with all four valves closed, the blood cannot go anywhere.

3. Ventricular ejection. The ejection of blood begins when ventricular pressure exceeds arterial pressure and forces the semilunar valves open. The pressure peaks at 120 mmHg in the left ventricle and 25 mmHg in the right. Blood spurts out of each ventricle rapidly at first (rapid ejection) and then flows out more slowly under less pressure (reduced ejection). By analogy, suppose you were to shake up a bottle of soda pop and remove the cap. The soda would spurt out rapidly at high pressure and then more would dribble out at lower pressure, much like the blood leaving the ventricles. Ventricular ejection lasts about 200 to 250 msec, which corresponds to the plateau of the myocardial action potentials but lags somewhat behind it (review the tension curve in fig. 19.14).

The ventricles do not expel all their blood. In an average resting heart, each ventricle contains an EDV of 130 mL. The amount ejected, about 70 mL, is called the stroke volume (SV). The percentage of the EDV ejected, about 54%, is the ejection fraction. The blood remaining behind, about 60 mL in this case, is called the end-systolic volume (ESV). Note that EDV - SV = ESV. In vigorous exercise, the ejection fraction may be as high as 90%. Ejection fraction is an important measure of cardiac health. A diseased heart may eject much less than 50% of the blood it contains.

4. Isovolumetric relaxation. This is early ventricular diastole, when the T wave appears and the ventricles repolarize and begin to expand. There are competing theories as to how they expand. One is that the blood flowing into the ventricles "inflates" them. Another is that contraction of the ventricles deforms the fibrous skeleton, which subsequently springs back like a rubber ball that has been squeezed and released. This elastic recoil and expansion would cause pressure to drop rapidly and suck blood into the ventricles.

At the beginning of ventricular diastole, blood from the aorta and pulmonary trunk briefly flows backward through the semilunar valves. The backflow, however, quickly fills the cusps and closes them, creating a slight pressure rebound that appears as the dicrotic notch of the aortic pressure curve (fig. 19.19). Heart sound S2 occurs as blood rebounds from the closed semilunar valves and the ventricles expand. This phase is called isovolumetric because the semilunar valves are closed, the AV valves have not yet opened, and the ventricles are therefore not taking in blood.

When the valves open, ventricular filling (phase 1) begins again. Heart sound S3, if it occurs, is thought to result from the transition from expansion of the empty ventricles to their sudden filling with blood.

In a resting person, atrial systole lasts about 0.1 second; ventricular systole, 0.3 second; and the quiescent period (when all four chambers are in diastole), 0.4 second. Total duration of the cardiac cycle is therefore 0.8 second (800 msec) in a heart beating at 75 bpm.

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Responses

  • lyyli j
    Which phases of cardiac cycle is S2?
    8 years ago
  • ASMARA
    Why pressure changes in the aorta dont corrospond with changes in the ventricles?
    7 years ago
  • giuliana
    What are the various phases of the cardiac cycle and the ballots close during each face?
    6 years ago
  • peter
    Why does arotic pressure curve begin to rise abruptly at about 0.5second?
    6 years ago
  • max
    When do atria repolarize?
    2 months ago

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