The cardiac cycle refers to the series of events that occur during one complete heartbeat. It involves the contraction and relaxation of the heart muscle, resulting in the pumping of blood through the circulatory system. The cardiac cycle is crucial for maintaining blood flow and delivering oxygen and nutrients to the body's tissues.
The cardiac cycle can be divided into two main phases: diastole and systole.
- Diastole: During diastole, the heart is in a relaxed state, allowing it to fill with blood. The following events take place:
- Atrial diastole: The atria (the upper chambers of the heart) relax, and blood from the body (in the right atrium) and the lungs (in the left atrium) flows into the atria.
- Atrial systole: The atria contract, forcing the remaining blood into the ventricles (the lower chambers of the heart).
- Ventricular diastole: The ventricles relax, and the semilunar valves close, preventing blood from flowing back into the atria. The atria continue to fill with blood.
- Systole: During systole, the heart contracts to pump blood out of the ventricles and into the arteries. The following events occur:
- Isovolumetric contraction: The ventricles contract, building pressure. Initially, the atrioventricular (AV) valves (tricuspid and mitral) are closed, preventing blood from flowing back into the atria. This phase is called isovolumetric because the volume of blood in the ventricles remains constant.
- Ventricular ejection: The pressure in the ventricles exceeds that in the arteries, causing the semilunar valves (aortic and pulmonary) to open. Blood is forcefully ejected from the ventricles into the aorta (from the left ventricle) and pulmonary artery (from the right ventricle).
- Isovolumetric relaxation: After ejection, the ventricles relax, and the semilunar valves close. The AV valves remain closed during this phase. The ventricles refill with blood from the atria, preparing for the next cardiac cycle.
The duration of the cardiac cycle is measured by the heart rate, which is the number of times the heart beats per minute. In a healthy adult, the average heart rate is around 60 to 100 beats per minute.
The cardiac cycle is controlled by electrical impulses generated by the heart's own conduction system, primarily the sinoatrial (SA) node and the atrioventricular (AV) node. These electrical signals coordinate the contraction and relaxation of the heart chambers, ensuring efficient blood flow throughout the body.
Understanding the cardiac cycle is crucial for diagnosing and treating various cardiovascular conditions. Abnormalities in the duration or sequence of events in the cardiac cycle can indicate cardiac disorders, such as arrhythmias, heart failure, or valve abnormalities. Medical professionals use techniques like electrocardiograms (ECGs), echocardiography, and auscultation to assess the different phases of the cardiac cycle and identify any potential issues.
