Cardiac cycle

The cardiac cycle is the period of time from beginning of one heart beat to beginning of the next. As such, it consists of two alternating phases:

• Systole, in which the chambers contract and eject the blood

• Diastole, in which the chambers relax allowing blood to fill them

Atria and ventricles undergo phases of systole and diastole; however, the duration of each phase in the chambers differs. In the atria, whose primary function is to receive blood returning to the heart from the veins, diastole is the predominant phase, lasting for almost 90% of each cardiac cycle at rest. In the ventricles, whose primary function is to develop enough force to eject blood into the pulmonary or systemic circulations, systole is much longer lasting and accounts for almost 40% of each cycle at rest.

A discussion of the cardiac cycle requires the correlation of pressure changes; ventricular volume changes; valve activity; and heart sounds. In this section, the focus will be on the left side of the heart (see Table 13.3). Identical events occur simultaneously on the right side of the heart; however, the pressures are lower.

Ventricular filling. This process occurs during ventricular diastole. When the ventricle has completely relaxed and pressure in the ventricle is lower than pressure in the atrium, the AV valve opens. The pressure in the atrium at this time is greater than that of the ventricle due to the continuous return of blood from the veins. The initial phase of filling is rapid because blood had accumulated in the atrium prior to the opening of the AV valve; once this valve opens, the accumulated blood rushes in. The second phase of filling is slower as blood continues to flow from the veins into the atrium and then into the ventricle. This phase of filling is referred to as diastasis. Up to this point, ventricular filling has occurred passively, and at rest approximately 75% of the blood entering the ventricle does so in this manner. The third phase of ventricular filling results from atrial contraction. At this time, the remaining 25% of the blood is forced into the ventricle by this active process. The volume of blood in the ventricles at the end of the filling period is referred to as the end-diastolic volume (EDV) and is approximately 120 to 130 ml at rest. Note that during the entire diastolic filling period, the aortic (semilunar) valve is closed. The ventricular pressure during the filling phase is very low (0 to 10 mmHg) and pressure in the aorta during diastole is approximately 80 mmHg. Therefore, the aortic valve remains closed to prevent the backward flow of blood from the aorta into the ventricle during ventricular diastole.

Ventricular contraction. This process occurs during ventricular systole. When the ventricular myocardium begins to contract and squeeze down on the blood within the chamber, the pressure increases rapidly. In fact, ven-

VI oo

Table 13.3 Summary of Events Occurring during Cardiac Cycle

Period Pressures

AV valve

Aortic valve

Ventricular volume

Heart sounds

Filling

Isovolumetric contraction

Ejection

Isovolumetric relaxation

Diastole

Open Closed

Increases from 60 ml (ESV) to 130 ml (EDV)

TP segment P wave PR segment

None

Systole

Pa < Pv Pv increases toward 80 mmHg

Closed

Closed

No change

QRS complex First heart sound

Systole

Closed

Open

Diastole toward 0 mmHg

Closed

Closed

Decreases from 130 ml No change (EDV) to 60 ml (ESV)

ST segment

None

T wave

Second heart sound tm cn cn TO S

Si oT

tricular pressure is almost instantly greater than atrial pressure. As a result, the AV valve closes to prevent the backward flow of blood from the ventricle into the atrium during ventricular systole. The closure of this valve results in the first heart sound ("lub"). The ventricle continues its contraction and its build-up of pressure; however, during a period of several milliseconds ventricular pressure is climbing toward that of the aorta (from less than 10 mmHg up toward 80 mmHg). Until ventricular pressure exceeds aortic pressure, the aortic valve remains closed. As a result, both valves leading into and out of the chamber are closed and this period is referred to as isovolumetric contraction. During this phase neither filling of the ventricle nor ejection of blood from the ventricle occurs, so blood volume does not change. Eventually, the build-up of ventricular pressure overtakes the aortic pressure and the aortic valve is pushed open. At this point, ejection, or ventricular emptying, takes place. It is important to note that the chamber does not eject all of the blood within it. Some blood remains in the ventricle following contraction and this volume, referred to as end-systolic volume (ESV), is approximately 50 to 60 ml at rest. Therefore, the volume of blood pumped out of each ventricle per beat, or the stroke volume (SV), is about 70 ml in a healthy adult heart at rest.

After systole, the ventricles abruptly relax and ventricular pressure decreases rapidly. Pressure in the aorta, which has peaked at 120 mmHg during systole, remains above 100 mmHg at this point. Therefore, the blood in the distended artery is immediately pushed back toward the ventricle down the pressure gradient. The backward movement of blood snaps the aortic valve shut, resulting in the second heart sound ("dub"). During this portion of ventricular diastole, there is a period of several milliseconds where ventricular pressure is dissipating and falling back toward zero. Because atrial pressure is close to zero, the AV valve remains closed. Therefore, during this phase of isovolumetric relaxation, both valves leading into and out of the chamber are closed. As with isovolumetric contraction, no change takes place in the blood volume of the ventricle during this phase of isovolumetric relaxation. When the ventricular pressure falls to a point at which it is once again exceeded by atrial pressure, the AV valve opens, ventricular filling occurs, and the cardiac cycle begins again.

Due to the alternating phases of systole and diastole, the heart pumps blood intermittently. It contracts to pump the blood into the arteries and then it relaxes so it can once again fill with blood. However, capillary blood flow is not interrupted by this cycle because blood flow to the tissues is continuous. This steady blood flow is due to the elastic properties of the arterial walls. When the stroke volume is ejected into the arterial system, some of the blood is pushed forward toward the tissues. The remainder of the stroke volume is retained in the arteries. These large blood vessels are characterized by an abundance of collagen fibers and elastin fibers. These connective tissue fibers allow the arteries to be quite strong, capable of withstanding high pressures but also reasonably distensible. The rapid addition of the stroke volume causes arterial distension, or stretch, resulting in

"storage" of a portion of this blood in these vessels. During diastole, when the heart relaxes, the arteries recoil and regain their original shape. This recoil squeezes down on the stored blood and pushes it forward toward the tissues. Therefore, blood flow is continuous during ventricular systole and diastole.

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