Table of Contents
Definition of Cardiac Cycle and Phases
Each cardiac cycle represents two paired actions that begin when the sinoatrial (sa) node, a cluster of specialized nerve cells located at the apex of the right atrium, emits an electrical pacing impulse.
The impulse causes the right and left atria to contract simultaneously, sending blood to the respective ventricles.
The right atrium sends to the right ventricle deoxygenated blood returning to the heart from the body; the left atrium sends to the left ventricle oxygenated blood returning to the heart from the lungs.
The atrioventricular (av) node, a second cluster of specialized nerve cells located at the base of the right atrium, initiates the second phase of the cardiac cycle. The AV node picks up, amplifies, and focuses the electrical impulse that has passed through the atria, sending it along the bundle of His and the bundle branches in wavelike fashion.
The impulse causes the ventricles to contract simultaneously. The right ventricle pumps blood to the lungs for oxygenation, and the left ventricle pumps oxygenated blood into the aorta for the arterial network to carry through the body.
The pulse represents a completed cardiac cycle. The heart of an adult at rest completes about 80 cardiac cycles each minute.
Arrhythmia, valvular heart disease, coronary artery disease (cad), ischemic heart disease (ihd), congenital defects of the heart, and damage to the heart such as occurs with heart failure or heart attack are among the conditions that can disrupt the cardiac cycle.