The heart functions as a muscular pump, and its operation is defined by the cardiac cycle, which is the complete sequence of events that occurs during one single heartbeat. This mechanical and electrical process starts with the beginning of one beat and continues until the start of the next one. The fundamental purpose of this continuous cycle is to circulate blood effectively throughout the body, ensuring that every cell receives the necessary oxygen and nutrients. The cycle involves an alternating pattern of contraction and relaxation, generating the pressure gradients that drive blood flow.
The Electrical Initiation of the Heartbeat
The mechanical pumping action of the heart depends upon a precisely timed electrical signal that originates within the heart muscle. This impulse is generated by the sinoatrial (SA) node, a small cluster of specialized cells in the upper wall of the right atrium, which serves as the heart’s natural pacemaker. The SA node initiates a signal that spreads rapidly across both atria, causing them to contract simultaneously.
The electrical impulse then travels to the atrioventricular (AV) node, situated near the center of the heart. At this point, the signal is briefly delayed for approximately 0.09 seconds. This pause allows the atria to complete their contraction and fully empty their blood into the ventricles before the lower chambers begin their contraction.
After this delay, the signal continues down the bundle of His and through the Purkinje fibers, rapidly distributing the impulse across the ventricular muscle tissue. This distribution ensures that the two ventricles contract almost simultaneously and with the necessary force to push blood out of the heart. This coordinated electrical event triggers the heart’s physical movement.
The Two Mechanical Phases of the Cycle
The physical work of the cardiac cycle is divided into two main mechanical phases: systole, which is the contraction and ejection phase, and diastole, the relaxation and filling phase. These two phases involve a carefully orchestrated sequence of pressure changes and valve movements that direct blood flow.
Systole
Systole begins when the electrical signal reaches the ventricles and they contract forcefully. As pressure inside the ventricles rises, it exceeds the pressure in the atria, causing the atrioventricular (AV) valves (tricuspid and mitral) to snap shut. This closure produces the first characteristic heart sound.
Ventricular pressure continues to build rapidly during isovolumic contraction, a period where all four heart valves are briefly closed. Once ventricular pressure surpasses the pressure in the major arteries (aorta and pulmonary artery), the semilunar valves open. This marks the ejection phase, where blood is pumped out of the right ventricle to the lungs and out of the left ventricle to the rest of the body.
Diastole
Following ejection, the ventricles enter diastole, characterized by muscle relaxation and chamber filling. As the ventricular muscle relaxes, pressure within the chambers drops dramatically, causing blood in the aorta and pulmonary artery to attempt to flow backward. This backflow forces the semilunar valves (aortic and pulmonary) to close, producing the second heart sound.
The ventricles continue to relax in isovolumic relaxation, where pressure falls quickly while all valves remain closed. Once ventricular pressure drops below the pressure in the atria, the AV valves open. This allows blood passively collecting in the atria to rush into the ventricles, accounting for the majority of ventricular filling before the cycle begins again.
Quantifying the Cardiac Cycle
The measurable outcomes of the cardiac cycle are used to assess the health and function of the circulatory system. The most straightforward measure is heart rate, defined as the number of complete cardiac cycles (heartbeats) that occur within one minute. A normal resting heart rate for an adult falls between 60 and 100 beats per minute, representing the frequency of these events.
Blood pressure is another direct result of the pressure changes generated throughout the cycle, measured as the force of blood against the artery walls. This measurement is expressed as two numbers, reflecting the high and low pressures achieved during the cycle.
The higher number is the systolic pressure, which corresponds to the maximum pressure generated during ventricular systole, when the heart is actively contracting and ejecting blood. The lower number is the diastolic pressure, which represents the minimum pressure remaining in the arteries during ventricular diastole, when the heart is relaxed and refilling. The blood pressure reading provides a practical snapshot of the force dynamics created by the rhythmic contraction and relaxation of the heart muscle.