The cardiac cycle refers to the sequence of electrical and mechanical events occurring in the heart during a single beat and the resulting changes in pressure, flow, and volume in the various cardiac chambers. The functional interrelationships of the cardiac cycle described below are represented in Figure 14.1.
Sequential Contractions of the Atria and Ventricles Pump Blood Through the Heart
The cycle of events described here occurs almost simultaneously in the right and left heart; the main difference is that the pressures are higher on the left side. The focus is on the left side of the heart, beginning with electrical activation of the atria.
Atrial Systole and Diastole. The P wave of the electrocardiogram (ECG) reflects atrial depolarization, which initiates atrial systole. Contraction of the atria "tops off" ventricular filling with a final, small volume of blood from the atria, producing the a wave. Under resting conditions, atrial systole is not essential for ventricular filling and, in its absence, ventricular filling is only slightly reduced. However, when increased cardiac output is required, as during exercise, the absence of atrial systole can limit ventricular filling and stroke volume. This happens in patients with atrial fibrillation, whose atria do not contract synchronously.
The P wave is followed by an electrically quiet period, during which atrioventricular (AV) node transmission occurs (the PR segment). During this electrical pause, the mechanical events of atrial systole and ventricular filling are concluded before excitation and contraction of the ventricles begin.
Atrial diastole follows atrial systole and occurs during ventricular systole. As the left atrium relaxes, blood enters the atrium from the pulmonary veins. Simultane-
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.