Describe cardiac cycle
The cardiac cycle refers to the sequence of events that occur during one complete heartbeat. It involves the contraction and relaxation of the heart muscles, specifically the atria and ventricles, which allows for the pumping of blood throughout the body.
The cardiac cycle consists of two main phases: systole and diastole.
1. Systole: During systole, the heart muscles contract and the chambers of the heart push blood forward. This phase can be further divided into two stages:
- Atrial Systole: The atria contract and push blood into the ventricles. This contributes to the filling of the ventricles.
- Ventricular Systole: The ventricles contract and pump blood out of the heart. The aortic and pulmonary valves open, allowing blood to be ejected into the aorta and the pulmonary artery, respectively.
2. Diastole: During diastole, the heart muscles relax and the chambers refill with blood. Similar to systole, diastole can be divided into two stages:
- Early Diastole: The ventricles relax and begin to fill with blood from the atria. The aortic and pulmonary valves close to prevent blood from flowing back into the ventricles.
- Late Diastole: The atria continue to contract and push any remaining blood into the ventricles, completing the filling process.
Overall, the cardiac cycle ensures the continuous flow of blood through the heart and arteries, delivering oxygen and nutrients to tissues and organs, while also removing waste products.
The cardiac cycle is a series of events that occur during one complete heartbeat, which involves the contraction and relaxation of the heart chambers to pump blood throughout the body. Here is a step-by-step description of the cardiac cycle:
1. Diastole: The cardiac cycle begins with diastole, which is the relaxation phase of the heart. During this phase, the atria and ventricles are relaxed, and blood fills the atria from the body and lungs via the superior and inferior vena cava and pulmonary veins, respectively.
2. Atrial Contraction: Atrial contraction, also known as atrial systole, occurs next. The electrical signal generated by the heart's natural pacemaker, the sinoatrial (SA) node, causes the atria to contract, pushing the remaining blood into the ventricles.
3. Ventricular Contraction: Ventricular contraction, or ventricular systole, follows atrial contraction. The electrical signal from the SA node reaches the atrioventricular (AV) node, which delays the impulse before transmitting it to the ventricles. The ventricles contract, and the pressure increases, closing the atrioventricular valves (mitral or bicuspid valve on the left side, and tricuspid valve on the right side). This prevents blood from flowing back into the atria and allows the blood to be forced into the arteries.
4. Isovolumetric Contraction: During the early phase of ventricular contraction, there is an isovolumetric contraction, where the ventricular pressure rapidly increases, but the volume of blood remains constant. All heart valves are closed at this time.
5. Ventricular Ejection: As the ventricular pressure exceeds the pressure in the arteries, the semilunar valves (aortic valve on the left side, and pulmonary valve on the right side) open, allowing blood to be ejected from the ventricles into the pulmonary artery and aorta. This is known as ventricular ejection, and it continues until the ventricular pressure falls below the arterial pressure.
6. Diastasis: After ventricular ejection, a brief period known as diastasis follows, during which the ventricles relax. The blood flow slows down, and the ventricles start to refill.
7. Isovolumetric Relaxation: During isovolumetric relaxation, all heart valves are closed again, and the ventricles continue to relax. The pressure in the ventricles decreases, causing the semilunar valves to close.
8. Atrial Filling: Finally, as the ventricular pressure falls below the atrial pressure, the atrioventricular valves open, and the ventricles passively fill with blood from the atria. This marks the beginning of another cardiac cycle, and the process starts again with diastole.
The entire cardiac cycle takes less than a second, with each phase having a specific role in ensuring the efficient circulation of blood in the body.