The cardiac cycle is the set of events that occur from the initiation of one heartbeat to the initiation of the next. Each cycle includes the electrical signal that initiates each contraction as well as the mechanical movement of the heart and the opening and closing of the valves. It is the coordination of these elements that allows blood to be pumped through the body. To understand the cardiac cycle, both the physical and electrical properties of the heart must be explored.
The mammalian heart has four chambers. The top two chambers are the Atria (sig. Atrium) and the bottom two are the Ventricles. Blood enters the heart via the Atria, is pumped into the Ventricles and then out of the heart with each beat and thus, is moved throughout the entire body. Like all other anatomical nomenclature, right and left are assigned from the perspective of the patient, not the observer. Thus, the Right Atrium and Ventricle, like the right arm and leg, are on the observer's left.
There are also four valves in the heart. These valves close off exits from the heart chambers and insure that blood flows in the correct direction when the heart contracts. If the valves do not close tightly, blood can leak backwards, creating a heart murmur. The Tricuspid Valve regulates blood flow between the Right Atria and Right Ventricle and opens during Atrial Contraction to allow blood to flow from the Right Atria to the Right Ventricle. This valve closes during Ventricle Contraction and prevents backward flow of blood from Right Ventricle to the Right Atria. On the left side of the heart, the Mitral Valve performs the same function between the Left Atria and Ventricle. The third valve, the Pulmonic Valve, located between the Right Ventricle and the Pulmonary Artery, closes during Atrial Contraction, allowing the Right Vertical to fill with blood. When the Ventricles contract, this valve opens, and allows blood to leave the heart through the pulmonary artery. Lastly, the Aortic Valve, between the Right Ventricle and the Aorta, also closes during Atrial Contraction and allows the Left Vertical to fill with blood. When the Ventricles contract, this valve opens with the Pulmonic Valve, and allows blood to enter the Aorta.
Unlike other muscle cells, Myocardial Cells (heart muscle cells) have a special attribute (automaticity) that allows the cell to contract without external electrical stimulation from the nervous system (spontaneously). As this electrical signal is transmitted from cell to cell each cell contracts. Because the signal travels so fast, the individual contractions seem to happen simultaneously and the heart beats in a uniform fashion, first the Atria then the Ventricles. The primary pacemaker in the heart (the source of the electrical signal) is the Sinoatrial (SA) Node. Located above the Right Atrium, this cluster of heart cells initiates each heartbeat by signaling the cells in the Atria to contract. At the same time, the electrical impulse travels to the Atrioventricular (AV) Node and is transmitted across to the Ventricles. In the Ventricles, the signal is received by a structure called the Bundle of His and is transmitted to the Purkinje Fibers, where the signal is disseminated throughout the Ventricles, causing the cells in the Ventricles to contract.
All mammals have a closed, double circuit circulatory system powered by the heart. One circuit moves blood from the heart to the lungs so it can be oxygenated and the second circuit moves the oxygenated blood through the body (and returns the spent blood to the oxygenation circuit). Just prior to the initial electrical signal from the AV Node, blood enters the Right Atrium via the Vena Cava. As the Atria contract, blood moves through the Tricuspid Valve, into the Right Ventricle. When the Ventricles contract blood is pumped through the Pulmonary Valve and exits the heart through the pulmonary artery (the only artery in the body that contains unoxygenated blood). Blood then flows to the lungs where it is oxygenated and returns to the heart via the Pulmonary Vein (the only vein that moves oxygenated blood). Re-entering the heart through the Left Atria (again just before the AV Node initiates a signal), blood flows through the Mitral Valve when the Atria contract, and enters the Left Ventricle. When the Ventricles contract, blood flows through the Aortic Valve and finally heads toward the body via the Aorta.
It is important to understand that both the pulmonary circuit (that oxygenates blood) and the systemic circuit (that moves blood through the body) operate simultaneously. As deoxygenated blood enter the Right Atrium from the body, oxygenated blood enters the Left Atrium from the lungs. When the Atria contract, blood from both Atria enters the recently emptied Ventricles. And when the Ventricles contracts, moving blood both toward the lungs (to be oxygenated) and the body (to supply oxygen to the cells in the rest of the body), the Atria are filling with blood.
The Cardiac Cycle can seem complex at first, but by carefully tracking the flow of blood through the heart, the elegance of the system can be easily understood.