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PART 4 Regulation and Maintenance

Complete heart block (P waves and QRS complexes are not coordinated)

Premature ventricular contraction (PVC) (no P waves precede PVCs)

Bundle branch block

Atrial fibrillation (no clear P waves and rapid QRS complexes)

Ventricular fibrillation (no P, QRS, or T waves)

FIGURE 20.17 Alterations in an Electrocardiogram

20.7 Cardiac Cycle

LEARNING OUTCOMES

After reading this section, you should be able to

A. Describe the cardiac cycle and the relationship amongthe contraction of each of the chambers, the opening andclosing of valves, the pressure in each of the chambers, thephases of the electrocardiogram, and the heart sounds.

B. Discuss the heart sounds and their significance.

The right and left halves of the heart can be viewed as two separatepumps that work together. Each pump consists of a “primer pump”(the atrium) and a “power pump” (the ventricle). Both atrial primerpumps complete the filling of the ventricles with blood, and bothventricular power pumps produce the major force that causes bloodto flow through the pulmonary and systemic arteries. The term cardiac cycle refers to the repetitive pumping process that beginswith the onset of cardiac muscle contraction and ends with the begin-ning of the next contraction (figure 20.18; table 20.2). Blood movesfrom an area of higher pressure to an area of lower pressure. Pressurechanges produced within the heart chambers as a result of cardiacmuscle contraction and relaxation move blood along the previouslydescribed routes of the pulmonary and systemic circulations.The duration of the cardiac cycle varies considerably amonghumans and during an individual’s lifetime. It can be as short as0.25–0.3 second in a newborn or as long as 1 or more seconds ina well-trained athlete. The normal cardiac cycle of 0.7–0.8 seconddepends on the capability of cardiac muscle to contract and on thefunctional integrity of the conducting system.The cardiac cycle involves a predictable pattern of contractionand relaxation of the heart chambers. As you study the events ofthe cardiac cycle listed in table 20.2, keep in mind that the term systole (sis ′ t ō -l ē ) means to contract, and diastole (d ī -as ′ t ō -l ē )means to dilate. Therefore, atrial systole is contraction of the atrialmyocardium, and atrial diastole is relaxation of the atrial myocar-dium. Similarly, ventricular systole is contraction of the ventricu-lar myocardium, and ventricular diastole is relaxation of theventricular myocardium. When the terms systole and diastole areused alone, they refer to ventricular systole and diastole.Before we begin our discussion of the cardiac cycle, it isimportant to have a clear image of the state of the heart. At thebeginning of the cardiac cycle, the atria and ventricles are relaxed,the AV valves are open, and the semilunar valves are closed. Dur-ing the cardiac cycle, changes in chamber pressure and the open-ing and closing of the heart valves determine the direction ofblood movement. As the cardiac cycle is described, it is importantto focus on these pressure changes and heart valve movements.At rest, most of the blood movement into the chambers is apassive process resulting from the greater blood pressure in theveins than in the heart chambers. As the blood moves intothe  atria, much of it flows into the ventricles for two reasons:(1)  The AV valves are open and (2) atrial pressure is slightlygreater than ventricular pressure. This time period when blood ispassively moving into the ventricles is called passive ventricularfilling .The SA node generates an action potential that stimulatesatrial contraction and begins the cardiac cycle. As the atria con-tract, they carry out the primer pump function by forcing moreblood into the ventricles. This period is referred to as active ven-tricular filling (figure 20.18, step 1 ).The action potential passes to the AV node, down the AVbundle, bundle branches, and Purkinje fibers, stimulating ven-tricular systole. As the ventricles contract, ventricular pressuresincrease, causing blood to flow toward the atria and close the AVvalves. Recall that the semilunar valves are closed at this point aswell. Ventricular contraction continues and ventricular pressures