The cardiac cycle is the series of events comprising a complete contraction and relaxation of the heart’s four chambers. A cardiac cycle lasts about 0.22 seconds in a normal, healthy adult heart. Contraction of the heart depends on a network of nodes and fibers that carry electrical impulses through the heart. These impulses are called action potentials. The cardiac cycle can be visualized with an electrocardiogram, or ECG. A ECG breaks down the events of the cardiac cycle into three waves identified by the letters P, QRS and T. Each wave shown on an ECG represents an action potential carried out by the specialized cells of the heart.
The heart is divided into four chambers. A simple way of visualizing this is to imagine a square equally divided into four boxes. The two boxes in the top portion of the square are the right and left atria. The two boxes in the lower portion of the square are the right and left ventricles.
The atria receive blood as it is returned to the heart. The right atrium receives blood from the body through a large vein called the vena cava. The left atrium receives blood from the lungs where it picks up oxygen. Two vessels called the right pulmonary vein and the left pulmonary vein are responsible for delivering the blood from the lungs into the left atrium.
The ventricles are responsible for pumping blood out of the heart. The right ventricle pumps blood to the lungs via the pulmonary artery. The left ventricle is the largest and most muscular chamber of the heart. This is because the left ventricle is responsible for pumping all the blood into circulation through the body. Blood is pumped through the aorta, which branches into smaller and smaller vessels until all the tissues of the body receive the oxygen carried in the blood.
The heart generates and conducts its own electrical impulses, independent from the nervous system. A heart can continue to beat even after ties to the nervous system have been severed. This is evident in the pulsating motion of a heart during transplantation. This unique capability of the heart is called automaticity. Automaticity is coordinated by special cells in the heart called autorhythmic cells. The autorhythmic cells make up a network of nodes and fibers that send an impulse to the cardiac muscle cells to contract in a specific sequence. An impulse is transmitted when the autorhythmic cell membrane depolarizes. Normally, one side of the membrane has a more negative charge and the other a more positive charge. This means the membrane is polarized. When an impulse is propagated the ions cross the membrane and the polarized state is changed. The membrane is then depolarized. The process of depolarization moves through the heart and can be visualized by an ECG.
Contraction of the heart muscle starts with depolarization at the sinoatrial node. The sinoatrial, or SA node, is a group of autorhytmic cells located in the right atrial wall. The P wave shows SA node depolarization. The P wave lasts about 0.08 seconds. The atria contract about 0.1 second after the P wave starts. At the end of the P wave, depolarization is slowed at the atrioventricular, or AV, node. The AV node is located in the septum separating the two atria, just above the tricuspid valve that leads into the right ventricle. This delay allows the atria to finish contracting and clear the blood completely before ventricle contraction.
The SA node has the fastest depolarization rate of all the cardiac muscle cells. It is the pacemaker of the heart; it sets the heart rate at an inherent rate of about 100 impulses per minute. In actuality, the SA node produces about 75 impulses a minute due to external controls by the endocrine and nervous systems.
The QRS complex depicts the movement of the depolarization wave through the ventricles. This wave is the most recognizable because of the large upward spike it makes. Depolarization spreads from the AV node down to the AV bundle, also called the bundle of His. The bundle of His splits into left and right bundle branches. The bundle branches further divide into Perkinje fibers which penetrate through the ventricles. Ventricular contraction occurs almost simultaneously with ventricular depolarization. When the impulse reaches the apex of the heart, the muscle cells start to contract. The ventricles contract in a wringing motion from bottom to top. This pushes all the blood out of the ventricles and into circulation. The QRS complex lasts about 0.08 seconds.
The T wave depicts ventricular repolarization. Repolarization is the movement of ions back across the cardiac muscle cell membrane to their initial, polarized state. Repolarization takes longer than depolarization so the T wave is somewhat flattened in appearance. Repolarization of the atria occurs during ventricular depolarization and is not seen on an ECG because of the QRS complex.