Physiology of the Heart
The physiology of the heart pertains to its pumping action- that is, the heartbeat. It is estimated that the heart beats twoand- a-half billion times in a lifetime, continuously recycling some 5 liters (L) of blood to keep us alive. In this section, we will consider what causes the heartbeat, what it consists of, and its consequences.
Conduction System of the Heart
The conduction system of the heart is a route of specialized cardiac muscle fibers that initiate and stimulate contraction of the atria and ventricles. The conduction system is said to be intrinsic, meaning that the heart beats automatically without the need for external nervous stimulation. The conduction system coordinates the contraction of the atria and ventricles so that the heart is an effective pump. Without this conduction system, the atria and ventricles would contract at different rates.
The heartbeat is controlled by nodal tissue, which has both muscular and nervous characteristics. This unique type of cardiac muscle is located in two regions of the heart:
The SA (sinoatrial) node is located in the upper posterior wall of the right atrium; the AV (atrioventricular) node is located in the base of the right atrium very near the interatrial septum (Fig. 12.7).
The SA node initiates the heartbeat and automatically sends out an excitation impulse every 0.85 second. The SA node normally functions as the pacemaker because its intrinsic rate is the fastest in the system. From the SA node, impulses spread out over the atria, causing them to contract.
When the impulses reach the AV node, there is a slight delay that allows the atria to finish their contraction before the ventricles begin their contraction. The signal for the ventricles to contract travels from the AV node through the two branches of the atrioventricular bundle (AV bundle) before reaching the numerous and smaller Purkinje fibers. The AV bundle, its branches, and the Purkinje fibers consist of specialized cardiac muscle fibers that efficiently cause the ventricles to contract.
The SA node is called the pacemaker because it usually keeps the heartbeat regular. If the SA node fails to work properly, the ventricles still beat due to impulses generated by the AV node. But the beat is slower (40 to 60 beats per minute). To correct this condition, it is possible to implant an artificial pacemaker, which automatically gives an electrical stimulus to the heart every 0.85 second. Should the AV node be damaged, the ventricles still beat because all cardiac muscle cells can contract on their own. However, the beat is so slow that the condition is called a heart block. An area other than the SA node can become the pacemaker when it develops a rate of contraction that is faster than the SA node. This site, called an ectopic pacemaker, may cause an extra beat, if it operates only occasionally, or it can even pace the heart for a while. Caffeine and nicotine are two substances that can stimulate an ectopic pacemaker.
Figure 12.7 Conduction system of the heart. (1) The SA node sends out a stimulus, which causes the atria to contract. (2) When this stimulus reaches the AV node, it signals the ventricles to contract. (3) Impulses pass down the two branches of the atrioventricular bundle to the Purkinje fibers, and (4) thereafter, the ventricles contract.
A graph that records the electrical activity of the myocardium during a cardiac cycle is called an electrocardiogram, or ECG. An ECG is obtained by placing on the patient’s skin several electrodes that are wired to a voltmeter (an instrument for measuring voltage). As the heart’s chambers contract and then relax, the change in polarity is measured in millivolts.
An ECG consists of a set of waves: the P wave, a QRS complex, and a T wave (Fig. 12A). The P wave represents depolarization of the atria as an impulse started by the SA node travels throughout the atria. The P wave signals that the atria are going to be in systole and that the atrial myocardium is about to contract. The QRS complex represents depolarization of the ventricles following excitation of the Purkinje fibers. It signals that the ventricles are going to be in systole and that the ventricular myocardium is about to contract. The QRS complex shows greater voltage changes than the P wave because the ventricles have more muscle mass than the atria. The T wave represents repolarization of the ventricles. It signals that the ventricles are going to be in diastole and that the ventricular myocardium is about to relax. Atrial diastole does not show up on an ECG as an independent event because the voltage changes are masked by the QRS complex.
An ECG records the duration of electrical activity and therefore can be used to detect arrhythmia, an irregular or abnormal heartbeat. A rate of fewer than 60 heartbeats per minute is called bradycardia, and more than 100 heartbeats per minute is called tachycardia. Another type of arrhythmia is fibrillation, in which the heart beats rapidly but the contractions are uncoordinated. The heart can sometimes be defibrillated by briefly applying a strong electrical current to the chest.
It is important to understand that an ECG only supplies information about the heart’s electrical activity. To be used in diagnosis, an ECG must be coupled with other information, including X rays, studies of blood flow, and a detailed history from the patient.
Figure 12A Electrocardiogram. a. A portion of an electrocardiogram. b. An enlarged normal cycle.