The cardiac cycle describes the sequence of events that occur with each heartbeat. This rhythmic process allows the heart to efficiently pump blood throughout the body. The coordinated actions within the heart chambers are fundamental for circulating oxygen and nutrients. Understanding this cycle provides insight into how the heart functions.
The Heart’s Pumping Action
The cardiac cycle consists of two primary phases: systole and diastole. Systole refers to the period when the heart muscle contracts, pushing blood out of its chambers. Diastole is the phase of relaxation, during which the heart chambers expand and refill with blood. These two phases work in a synchronized rhythm, ensuring a constant flow of blood.
During systole, the heart generates pressure to propel blood into the arteries. Diastole allows the heart to rest and prepare for the next contraction by collecting blood returning from the body and lungs. The alternation of contraction and relaxation maintains efficient blood circulation.
Inside the Cycle: Atrial and Ventricular Events
The cardiac cycle involves precise, sequential events within the heart’s upper chambers, the atria, and its lower chambers, the ventricles. The cycle begins with both the atria and ventricles relaxed (diastole), allowing blood to flow passively from the veins into the atria and then into the ventricles. As the ventricles continue to fill, pressure changes within the chambers guide the movement of blood.
Toward the end of ventricular filling, the atria contract (atrial systole). This contraction provides a final push, ejecting additional blood into the ventricles. This ensures ventricles are optimally filled before their own contraction. The atria then relax as the ventricles prepare for contraction.
The ventricles begin to contract, marking the start of ventricular systole. Initially, all heart valves close, causing a rapid increase in pressure within the ventricles without a change in blood volume. This period, known as isovolumic contraction, builds force to open the valves leading to the major arteries. Once ventricular pressure exceeds the pressure in these arteries, blood is ejected from the heart into the pulmonary artery and the aorta.
Following ejection, the ventricles begin to relax, entering ventricular diastole. As they relax, the pressure inside the ventricles rapidly decreases. This pressure drop causes the valves leading to the major arteries to close, preventing blood from flowing back into the heart. Relaxation continues, and once ventricular pressure falls below atrial pressure, the valves between the atria and ventricles open, allowing the ventricles to begin refilling, and the cycle restarts.
The Sounds of a Beating Heart
The familiar “lub-dub” sounds heard with each heartbeat result from the heart’s valves closing during the cardiac cycle. These sounds are produced by turbulence as blood flow abruptly stops against the closed valve leaflets. Listening to these sounds can provide information about heart function.
The first heart sound, “lub” (S1), occurs at the beginning of ventricular contraction. This sound is caused by the simultaneous closing of the valves between the atria and the ventricles. This closure prevents blood from flowing backward into the atria when the ventricles begin to pump.
The second heart sound, “dub” (S2), is heard as the ventricles finish ejecting blood and begin to relax. This sound results from the closing of the valves that lead out of the ventricles into the major arteries. The S2 sound marks the end of ventricular contraction and the start of relaxation, preparing the heart for the next cycle of filling.