The heart functions as the motor of the circulatory system, which consists of the heart, blood vessels and blood. The heart pumps approximately 2,000 gallons of blood through the body each and every day, according to the American Heart Association. Because it must work continuously, the functions of the cells within the heart, known as cardiac cells, occur as part of the autonomic nervous system. This system controls organs, like the heart, that work involuntarily, which means without active control from the brain.
The heart is a large, strong muscle made up of myocytes—cardiac muscle cells. Most muscle cells contract due to neural stimulation, meaning that the brain sends signals to the muscles using the complex system of nerves. The heart muscle, however, contracts without neural stimulation, which is a property called automaticity, according to "The Fundamentals of Anatomy and Physiology".
Specialized cells found in the right atrium, which is the upper right chamber of the heart, function as the sinus node. The sinus node, also called a natural pacemaker, produces electrical impulses that regulate the contractions of all the cardiac cells.
The heart's main function is to pump oxygen-rich blood throughout the body to provide cells with vital nutrients. In order to do this, the cardiac cells must contract. In fact, the American Heart Association reports that the average adult heart contracts 100,000 times a day.
Individual myocytes can contract individually. In order for the heart to pump effectively, every cell must contract in unison. To do this, the cellular membranes of cardiac cells intertwine each other. These cells come together in a structural area known as intercalated discs. The formation of these discs provides an electrical connection between the cells, which allows them to contract in unison.
The ability of cardiac cells to contract is dependent on a complex series of ion exchanges. Every cardiac cellular membrane contains valves that open and close allowing ions, such as sodium, potassium and calcium, to flow into and out of the cells. This flow of ions regulates the strength and duration of the contraction.
A cardiac cell at rest contains a large number of potassium ions inside the cell while the number of sodium ions is larger outside the cells. As the cell beats in response to the electrical impulses, the ion channel valves open allowing sodium ions to rush in as described by the website Cells Alive. This increases the number of positive ions inside the cell, depolarizing the cell and creating an action potential that causes contraction. Potassium ions slowly release to the outside of the cell allowing the cell to repolarize and return to a resting state.
In addition to sodium and potassium, cardiac cells require the presence of calcium ions. As the cardiac cell contracts, calcium ions flow into the cell which increases the duration of the contraction.