Anatomy and Chemistry Basics
The Cardiovascular System
Electrical currents generated by the heart during the cardiac cycle can be detected on the surface of the body by the electrodes of an electrocardiograph. A recording of these currents, called an electrocardiogram (ECG or EKG), represents a sum of all the concurrent action potentials produced by the heart as detected by the 12 electrodes of the electrocardiograph. A single cardiac cycle produces a distinctive wave pattern, where peaks and valleys are indicated by the letters P, Q, R, S, and T (see Figure 1 ). An interpretation of the major characteristics of the ECG follows:
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The P wave is a small wave that represents the depolarization of the atria. During this wave, the muscles of the atria are contracting.
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The QRS complex is a rapid down-up-down movement. The upward movement produces a tall peak, indicated by R. The QRS complex represents the depolarization of the ventricles.
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The T wave represents the repolarization of the ventricles. Electrical activity generated by the repolarization of the atria is concealed by the QRS complex.
Figure 1 Different periods of the cardiac cycle: (a) electrocardiogram, (b) heart sounds, (c) valves, (d) pressure, and (e) volume of left ventricle.
