The cardiac cycle refers to the sequence of events that occur in the heart from the beginning of one heartbeat to the next. It's characterized by alternating periods of contraction (systole) and relaxation (diastole) of the heart muscles.
During the cardiac cycle, blood flow through the heart is regulated entirely by changing pressure gradients. This sequence of events begins with the heart in a state of total relaxation, known as mid-to-late diastole, during which blood passively flows from the atria into the ventricles.
The contraction of the atria marks the onset of atrial systole, pushing the remaining blood into the ventricles. Following this, the ventricles initiate their contraction process, a phase referred to as isovolumetric contraction, as both the atrioventricular and the semilunar valves are closed, maintaining a constant volume of blood inside the ventricles. Shortly after this, the pressure inside the ventricles leads to the opening of the semilunar valves, facilitating the ejection of blood into the aorta and pulmonary trunk.
After the ejection phase, the ventricles return to a relaxed state, a phase known as early diastole or isovolumetric relaxation, as all four valves are closed. The closing of the aortic valve briefly increases pressure inside the aorta, an event marked by the dicrotic notch on the pressure graph.
Meanwhile, the atria have been filling up with blood throughout this ventricular systole, thereby preparing for the next cycle.
The cardiac cycle lasts approximately 0.8 seconds in an average heart that beats around 75 times per minute. Atrial systole accounts for 0.1 seconds, ventricular systole for 0.3 seconds, and total heart relaxation for the remaining 0.4 seconds.
The right and left sides of the heart function in synchrony but differ in pressure due to the less demanding pulmonary circulation. As a result, the right ventricular walls are thinner than the left. Regardless of this difference in pressure, both sides of the heart eject an equal volume of blood with each heartbeat.
