ABSTRACT
As a component of the cardiovascular system, the heart is responsible for maintaining adequate blood flow to meet the metabolic needs of the body. This is accomplished by the integration of neural, metabolic, anatomic, and physiologic subsystems that combine to form the intact, functioning human heart. An understanding of cardiac function must consider each of these factors, because a knowledge of only one, or even several, without an appreciation of the others gives an incomplete picture of the physiologic mechanisms responsible for this function. In discussing cardiac physiology, it is appropriate to begin with the molecular events underlying contraction and relaxation, to provide the basis for understanding the performance of the intact organ.
The basis of cardiac function is the relationship between the contractile proteins, actin and myosin. The nature of this relationship determines to a large extent the characteristics of activation and relaxation in individual muscle cells and in the intact heart. As in skeletal muscle, the functional unit of cardiac muscle is the sarcomere. The sarcomere is composed principally of four proteins (1). These are the previously mentioned contractile proteins, actin and myosin and the regulatory complex consisting of tropomyosin and troponin. In electron micrographs, the sarcomere appears as an arrangement of thick and thin filaments. This arrangement is shown schematically in Figure 1. The thick filament exists as an aggregate of myosin molecules. Myosin consists of a pair of heavy, coiled polypeptide chains, each of which is attached to a globular head region.
