Electrical Activity of the Heart and the Electrocardiogram

The pacemaker region of the heart (SA node) exhibits a spontaneous depolarization that causes action potentials, resulting in the automatic beating of the heart. Electrical impulses are conducted by myocardial cells in the atria and are transmitted to the ventricles by specialized conducting tissue. Electrocardiogram waves correspond to the electrical events in the heart as follows: P wave (depolarization of the atria); QRS wave (depolarization of the ventricles); and T wave (repolarization of the ventricles).

The lungs of a fetus are collapsed, and blood is routed away from the pulmonary circulation by an opening in the interatrial septum called the foramen ovale (fig. 13.15) and by a connection between the pulmonary trunk and aorta called the ductus arteriosus (fig. 13.16). These shunts normally close after birth, but when they remain open (are patent), murmurs can result. Since blood usually goes from left to right through these shunts, the left ventricle still pumps blood that is high in oxygen. When other defects are present that increase the pressure in the right pump (as in the tetralogy of Fallot), however, a significant amount of oxygen-depleted blood from the right side of the heart may enter the left side. The mixture of oxygen-poor blood from the right side with oxygen-rich blood in the left side of the heart lowers the oxygen concentration of the blood ejected into the systemic circulation. Since blood low in oxygen imparts a bluish tinge to the skin, the baby may be born cyanotic (blue).

Test Yourself Before You Continue

1. Using a drawing or flow chart, describe the sequence of events that occurs during the cardiac cycle. Indicate when atrial and ventricular filling occur and when atrial and ventricular contraction occur.

2. Describe how the pressure in the left ventricle and in the systemic arteries varies during the cardiac cycle.

3. Draw a figure to illustrate the pressure variations described in question no. 2, and indicate in your figure when the AV and semilunar valves close. Discuss the origin of the heart sounds.

4. Explain why blood usually flows from left to right through a septal defect. Under what conditions would a septal defect produce cyanosis?

Electricity Human Heart

Septal defect in atria

Blood Flow Pattern The Human Heart

Septal defect in ventricles

Septal defect in atria

Septal defect in ventricles

Figure 13.15 Abnormal patterns of blood flow due to septal defects. Left-to-right shunting of blood is shown (circled areas) because the left pump is at a higher pressure than the right pump. Under certain conditions, however, the pressure in the right atrium may exceed that of the left, causing right-to-left shunting of blood through a septal defect in the atria (patent foramen ovale). (RA = right atrium; RV = right ventricle; LV = left ventricle; AO = aorta; PA = pulmonary arteries.)

Ductus Arteriosus
Ductus arteriosus

■ Figure 13.16 The flow of blood through a patent (open) ductus arteriosus. The ductus is normally open in a fetus but closes after birth, eventually becoming the ligamentum arteriosum. (AO = aorta; PA = pulmonary arteries.)

As described in chapter 12, myocardial cells are short, branched, and interconnected by gap junctions. Gap junctions function as electrical synapses, and have been described in chapter 7 (see fig. 7.19) and chapter 12 (see fig. 12.31). The entire mass of cells interconnected by gap junctions is known as a myocardium. A myocardium is a single functioning unit, or functional syncitium, since action potentials that originate in any cell in the mass can be transmitted to all the other cells. The my-ocardia of the atria and ventricles are separated from each other by the fibrous skeleton of the heart, as previously described. Since impulses normally originate in the atria, the atrial myocardium is excited before that of the ventricles.

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