The Heart

The heart muscle, called the myocardium, has a fibrous covering called the pericardium. The endocardium is a three-layered membrane that lines the interior of the heart chambers.

The heart has four valves—two atrioventricular (tricuspid and mitral) and two semilunar (pulmonic and aortic). Atrioventricular valves control bloodflow between the atria and ventricles. Semilunar valves control the bloodflow between the ventricles and the pulmonary artery and the aorta.

There are three major coronary arteries: the right, left, and circumflex. Each provides nutrients to the myocardium. Blockage in one or more of these arteries can result in a myocardial infarction ("heart attack.")

The myocardium is capable of generating and conducting its own electrical impulses. The impulse begins in the sinoatrial (SA) node and moves to the atrioventricular (AV) node. The heart beats about 60 to 80 beats a minute. The ventricle can beat independently at a rate of about 30 to 40 beats per minute.

Drugs can affect cardiac contraction by stimulating or inhibiting the heart. Contractions are also influenced by the autonomic nervous system (ANS). The sympathetic nervous system increases heart rate and the parasympathetic nervous system decreases heart rate. (See Chapter 15.)

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