Heart muscle wrings out blood like someone twisting a damp cloth
Layers of muscle around the heart's left ventricle run at right angles to one another. When they contract together, the chamber shortens, narrows and twists all at once, much like wringing water from a towel, squeezing out as much blood as possible with every beat and repeating the trick for a lifetime.
Cardiac muscle, or myocardium, is one of three kinds of vertebrate muscle, alongside skeletal and smooth muscle. It is striated like the muscles of the limbs but works involuntarily, forming a thick middle layer of the heart wall between the inner endocardium and the outer epicardium. Because it burns so much energy, it needs a constant blood supply, delivered by coronary arteries that branch from the base of the aorta and run over the heart's outer surface.
Under a microscope each cell is roughly a rectangle about 100 to 150 micrometres long and 30 to 40 wide. Unlike skeletal muscle cells, which carry many nuclei, most heart cells have just one. They are packed with mitochondria supplying the fuel molecule ATP, which makes them highly resistant to fatigue. Inside, filaments of actin and myosin slide past each other when an electrical signal releases calcium from an internal store, and their orderly arrangement produces the striped look. Tubes called T-tubules, fewer but wider than in skeletal muscle, carry the signal deep into each cell.
Coordination is everything. Cells join end to end at structures called intercalated discs, which both transmit mechanical force and let charged particles pass freely, so an impulse sweeps from cell to cell with little resistance. The network behaves as a single unit. If that teamwork collapses, as in ventricular fibrillation, individual cells may still twitch while the heart as a whole fails to pump. Specialised pacemaker cells in the sinoatrial node, near where the superior vena cava enters, set the rhythm; they barely contract themselves.
Smaller but more numerous supporting cells called fibroblasts build the surrounding scaffolding of collagen and elastin. After an injury such as a heart attack they can transform into myofibroblasts, laying down collagen while gently contracting to draw the damaged edges together.
Source: Cardiac muscle
