The timing of various events in the cardiac cycle.

ously, blood enters the right atrium from the superior and inferior vena cavae. The gradual rise in left atrial pressure during atrial diastole produces the v wave and reflects its filling. The small pressure oscillation early in atrial diastole, called the c wave, is caused by bulging of the mitral valve and movements of the heart associated with ventricular contraction.

Ventricular Systole. The QRS complex reflects excitation of ventricular muscle and the beginning of ventricular systole (see Fig. 14.1). As ventricular pressure rises above atrial pressure, the left atrioventricular (mitral) valve closes. Contraction of the papillary muscles prevents the mitral valve from everting into the left atrium and enables the valve to prevent the regurgitation of blood into the atrium as ventricular pressure rises. The aortic valve does not open until left ventricular pressure exceeds aortic pressure. During the interval when both mitral and aortic valves are closed, the ventricle contracts isovolumetrically (i.e., the ventricular volume does not change). The contraction causes ventricular pressure to rise, and when ventricular pressure exceeds aortic pressure (at approximately 80 mm Hg), the aortic valve opens and allows blood to flow from the ventricle into the aorta. At this point, ventricular muscle begins to shorten, reducing the volume of the ventricle.

When the rate of ejection begins to fall (see the aortic blood flow record in Fig. 14.1), the aortic and ventricular pressures decline. Ventricular pressure actually decreases slightly below aortic pressure prior to closure of the aortic valve, but flow continues through the aortic valve because of the inertia imparted to the blood by ventricular contraction. (Think of a rubber ball connected to a paddle by a rubber band. The ball continues to travel away from the paddle after you pull back because the inertial force on the ball exceeds the force generated by the rubber band.)

Ventricular Diastole. Ventricular repolarization (producing the T wave) initiates ventricular relaxation or ventricular diastole. When the ventricular pressure drops below the atrial pressure, the mitral valve opens, allowing the blood accumulated in the atrium during systole to flow rapidly into the ventricle,- this is the rapid phase of ventricular filling. Both pressures continue to decrease—the atrial pressure because of emptying into the ventricle and the ventricular pressure because of continued ventricular relaxation (which, in turn, draws more blood from the atrium). About midway through ventricular diastole, filling slows as ventricular and atrial pressures converge. Finally, atrial systole tops off ventricular volume.

Pressures, Flows, and Volumes in the Cardiac Chambers, Aorta, and Great Veins Can Be Matched With the ECG and Heart Sounds

The pressures, flows, and volumes in the cardiac chambers, aorta, and great veins can be studied in conjunction with the ECG and heart sounds to yield an understanding of the coordinated activity of the heart. Ventricular diastole and systole can be defined in terms of both electrical and mechanical events. In electrical terms, ventricular systole is defined as the period between the QRS complex and the end of the T wave. In mechanical terms, it is the period between the closure of the mitral valve and the subsequent closure of the aortic valve. In either case, ventricular diastole comprises the remainder of the cycle.

The first (S]) and second (S2) heart sounds signal the beginning and end of mechanical systole. The first heart sound (usually described as a "lub") occurs as the ventricle contracts and ventricular pressure rises above atrial pressure, causing the atrioventricular valves to close. The relatively low-pitched sound associated with their closure is caused by vibrations of the valves and walls of the heart that occur as a result of their elastic properties when the flow of blood through the valves is suddenly stopped. In contrast, the aortic and pulmonic valves close at the end of ventricular systole, when the ventricles relax and pressures in the ventricles fall below those in the arteries. The elastic properties of the aortic and pulmonic valves produce the second heart sound, which is relatively high-pitched (typically described as a "dup"). Mechanical events other than vibrations of the valves and nearby structures contribute to these two sounds, especially Si,- these factors include movement of the great vessels and turbulence of the rapidly moving blood. The second heart sound often has two components—the first corresponds to aortic valve closure and the second to pulmonic valve closure. In normal individuals, splitting widens with inspiration and narrows or disappears with expiration.

A third heart sound (S3) results from vibrations during the rapid phase of ventricular filling and is associated with ventricular filling that is too rapid. Although it may be heard in normal children and adolescents, its appearance in a patient older than age 35 usually signals the presence of a cardiac abnormality. A fourth heart sound (S4) may be heard during atrial systole. It is caused by blood movement resulting from atrial contraction and, like S3, is more common in patients with abnormal hearts.

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Essentials of Human Physiology

Essentials of Human Physiology

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