Mechanics of Contraction

Imagine that the heart is made up of a series of small barrels or cells (Figure 2-1). Each of these barrels is made up of two halves that slide over each other and are held together by interlocking pieces (actin and myosin proteins). The actin molecules are attached to the outside edges of the barrel wall, and the myosin molecules are interspersed between the actin molecules.

Figure 2-1 Myocardial cell (myocyte).

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Description

The outsides of the barrels (cells) are fused together to form long bands, or myofibrils (Figure 2-2). These bands, in turn, are held together side-to-side by wire (connective tissue) to form sheets, which are covered with fluid (extracellular fluid). The main function of the bands is to contract and expand. When one of the barrels contracts, the whole sheet shortens by a small amount. When all of the barrels contract, the whole sheet shortens significantly. The sheet returns to its starting size as all of the barrels relax. The sheets are arranged to form the four sacs that constitute the heart: two small, thin ones on top (the atria) and two large, thick ones on bottom (the ventricles).

Figure 2-2 Barrels are held together, forming myofibrils and sheets.

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