The heart--a fist-sized organ made of muscle--occupies the space under the rib cage between the lungs, just to the left of the breastbone. It's divided into four chambers, two on top known as the atria and two on bottom known as ventricles. The heart functions to pump over six quarts of blood every minute, providing cells with oxygen and nutrients, the Cleveland Clinic says. The structure of the heart and the three different types of cardiac tissue allow it to function continuously.
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Cells known as myocytes, specialized muscle cells, make up the thick muscular layer of the heart wall known as the myocardium. The myocytes have the ability to contract, even without stimulation from nerves. The sinus node, a group of specialized cells found in the right atrium, produces electrical impulses that trigger the myocytes to contract. Each individual cell can contract, but for the heart to work efficiently, they must all work together.
The cellular membranes of the myocytes connect together forming gap junctions, which allow the electrical impulses to travel between cells. These gap junctions, as described by Cells Alive, allow the cells of the myocardium to work together as one.
The endocardium is the name for the interior surface of the heart chambers. Endocardium also covers the heart valves, the one-way passages that allow blood to flow between chambers of the heart. The endocardium consists of endothelial cells, also known as skin cells.
Endothelial cells are flat cells that form overlapping regions to seal the heart and connect blood vessels. Although they function to prevent leaks, they also function as filters allowing gases, fluids and specific molecules to enter into and out of the cells. The most recent finding, according to the University of Western Australia, is that the endothelial cells of the endocardium function as receptors and interaction sites to cells such as white blood cells. This allows the body to heal damage to the heart tissue or blood vessels.
The pericardium is a thin, double-layered sac that surrounds the heart and the ends of the blood vessels where they attach to the heart. The space between the two thin membranes of the pericardium contains a small amount of fluid. The pericardium functions to protect the heart from outside contaminants and to contain the heart to prevent pathological distension--or enlarged heart, as described by Dr. Robert Matthews.