Muscle Mass of Cardiac Chambers

The atria pump blood over only very short distances (across the AV valves into the ventricles); therefore, they are very thin-walled structures with very little muscle mass. Conversely, the right ventricle must pump blood all the way through the lungs, and therefore has a thicker wall and more muscle mass. Finally, the left ventricle pumps blood out to the entire body, and thus has the thickest wall of all the chambers, three to four times thicker than the wall of the right ventricle (Figure 1.3).

Ventricular

Systole

Atrial

Systole

Left Ventricular

Pressure Curve

1

ECG

-^JpV-

Figure 1.2. Synchronous recording of left ventricular pressure curve and ECG. The time required for mechanical contraction during atrial and ventricular systole is shown by the left ventricular pressure curve. Note how these times exceed the times required for atrial and ventricular depolarization (P wave and QRS duration.)

Figure 1.2. Synchronous recording of left ventricular pressure curve and ECG. The time required for mechanical contraction during atrial and ventricular systole is shown by the left ventricular pressure curve. Note how these times exceed the times required for atrial and ventricular depolarization (P wave and QRS duration.)

Recording a Wave of Depolarization 3

Figure 1.3. Schematic drawing showing relative greater thickness of the septum and wall of the left ventricle as compared to the right ventricle. Arrows represent force vectors, illustrating that as a result of its greater thickness, the left ventricle generates greater voltages,

One determinant of the size of an electrical complex on the electrocardiogram is how much voltage is generated by depolarization of a given portion of the heart. Thus, the QRS complex is normally larger than the P wave because depolarization of the greater muscle mass of the ventricles generates more voltage than does depolarization of the thinner walls of the atria.

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