Heart Muscle

Basic physiology: William Harvey recognized at the beginning of the 17th century that the heart pumps the blood through the whole body. The heart consists of four chambers, two atria that receive the blood and the two ventricles that pump the blood. The venous blood enters the heart via the right atrium, flows into the right ventricle and is pumped by the right ventricle through the lungs to pick up O2 and lose CO2. The rejuvenated blood enters the left atrium and delivers O2 to the tissues by the pumping action of the left ventricle (Fig. H1).

Alternating contractions and relaxations of the heart muscle, called myocardium, causes the pumping of the heart. There is a pacemaker in the right atrium that generates electrical impulses causing the atria to contract and thereby forcing blood into the ventricles. Following contraction (systole) the ventricles relax (diastole); the entire process is called the cardiac cycle. Normal human heart beats about 70-times per minute at rest. The rate of heart beat increases during exercise, emotional excitement and fever, and decreases during sleep.

Ultrastructure: Cardiac muscle is composed of interconnected mono-nucleated cells. The cells are imbedded in a weave of collagen. The ultrastructure of the heart contains a large number of myofibrils, striated like in skeletal muscle. A large fraction of the cell volume is occupied by mitochondria, which synthesize ATP to supply energy for the constantly working heart muscle. Myofibrils and mitochondria occupy about 85% of the heart cell volume, the rest contains the sarcolemma, T-tubules, sarcoplasmic reticulum, and specialized structures such as the intercalated disk, which connects adjacent heart cells (cardiomyocytes), and gap junction or nexus which makes contact between the plasma membranes of adjacent heart cells (Fig. H2).

Fig. H1. Scheme of the heart compartments and the direction of the blood flow during diastole. Note the pulmonary artery on the top of the right ventricle and the aorta on the top of the left ventricle. The valves separating the atria from the ventricles are not shown.

Electron micrograph of cardiac muscle. (Courtesy of Dr. Helen Rarick)

Electron micrograph of cardiac muscle. (Courtesy of Dr. Helen Rarick)

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