Cardiac Cycle Fundamentals and Regulation
The cardiac cycle is divided into systole (ventricular contraction and blood ejection) and diastole (ventricular relaxation and blood filling). The SA node initiates the heart rate, while the AV node ensures proper timing. Cardiac output, a key measure of heart function, is determined by stroke volume and heart rate. Important regulatory factors include preload, the end-diastolic ventricular volume, and afterload, the resistance the heart pumps against. The Frank-Starling law highlights the direct relationship between increased preload and enhanced stroke volume, demonstrating the heart's intrinsic ability to adapt.
The cardiac cycle consists of two primary phases: systole and diastole, which describe the heart's pumping actions.
Systole is the phase of ventricular contraction, during which blood is actively ejected from the heart into the circulation.
Diastole is the phase of ventricular relaxation, allowing the ventricles to fill with blood in preparation for the next contraction.
The SA node, often referred to as the heart's natural pacemaker, is responsible for setting the intrinsic heart rate.
The AV node plays a crucial role by delaying the electrical signal, ensuring that the atria contract and fill the ventricles before the ventricles begin their contraction.
Cardiac output, a measure of pumping efficiency, is calculated by multiplying the stroke volume (blood ejected per beat) by the heart rate (beats per minute).
Preload refers to the volume of blood that fills the ventricles at the end of diastole, influencing the stretch of cardiac muscle fibers.
Afterload represents the resistance or pressure the heart must overcome to eject blood from the ventricles during systole.
The Frank-Starling law of the heart states that a greater preload, resulting in increased ventricular stretch, leads to a stronger contraction and thus an increased stroke volume.
Key points
- The cardiac cycle involves systole (contraction) and diastole (relaxation).
- The SA node sets heart rate, and the AV node delays signals to ventricles.
- Cardiac output is calculated as stroke volume multiplied by heart rate.
- Preload is end-diastolic volume, and afterload is resistance to ejection.
- The Frank-Starling law links increased preload to increased stroke volume.