Describe the Cardiac cycle
The cardiac cycle refers to the sequence of events that occur during one complete heartbeat of the heart. It consists of two main phases: diastole and systole.
During diastole, the heart is in a relaxed state and fills with blood. The atria, which are the upper chambers of the heart, receive blood from the body (right atrium) and the lungs (left atrium) and contract to push the blood into the ventricles, the lower chambers of the heart. At this stage, the atrioventricular (AV) valves, located between the atria and ventricles, are open, allowing blood to flow into the ventricles. The semilunar valves, which are located between the ventricles and the major blood vessels leaving the heart, are closed.
Following diastole, the heart enters systole, which consists of two phases: atrial systole and ventricular systole. During atrial systole, the atria contract to push the remaining blood into the ventricles. This increases the amount of blood in the ventricles. Simultaneously, the semilunar valves remain closed to prevent blood from flowing back into the atria.
Next, ventricular systole occurs in two stages: isovolumetric contraction and ejection. In isovolumetric contraction, the ventricles contract, increasing the pressure and forcing the AV valves to close. This prevents blood from flowing back into the atria and causes the first heart sound, commonly known as "lub." The semilunar valves remain closed during this phase. In ejection, the pressure in the ventricles exceeds the pressure in the major blood vessels, causing the semilunar valves to open. This allows the blood to be ejected into the aorta from the left ventricle and into the pulmonary artery from the right ventricle. This phase is accompanied by the second heart sound, known as "dub."
After ejection, the ventricles relax, entering diastole again. The ventricles fill with blood from the atria, and the cardiac cycle repeats.
Overall, the cardiac cycle is a coordinated series of events that ensure the efficient pumping of blood by the heart to deliver oxygen and nutrients to tissues and organs throughout the body.
The cardiac cycle refers to the sequence of events that occur during one complete heartbeat. It involves both contraction (systole) and relaxation (diastole) of the heart muscles, allowing for the pumping of blood to the lungs and the rest of the body. Here is a step-by-step breakdown of the cardiac cycle:
1. Atria Contraction (Atrial Systole): The cardiac cycle begins with the contraction of the atria, which forces blood into the ventricles. This phase is known as atrial systole.
2. Atrial Relaxation (Diastole): After the atria contract and push blood into the ventricles, they relax and fill up with blood from the veins. This phase is known as atrial diastole.
3. Ventricular Contraction (Ventricular Systole): The next phase involves the contraction of the ventricles, which causes blood to be pumped out of the heart. The right ventricle pumps blood to the lungs, while the left ventricle pumps blood to the rest of the body. This phase is known as ventricular systole.
4. Ventricular Relaxation (Diastole): After the ventricles contract and expel blood, they relax and fill up with blood from the atria. This phase is known as ventricular diastole.
5. Closure of Valves: During certain phases of the cardiac cycle, specific valves within the heart close to prevent the backflow of blood. The atrioventricular (AV) valves, located between the atria and ventricles, close during ventricular systole to prevent blood from flowing back into the atria. The semilunar valves, located between the ventricles and major arteries (pulmonary artery and aorta), close during ventricular diastole to prevent blood from flowing back into the ventricles.
6. Opening of Valves: As the cardiac cycle continues, the valves open at the appropriate times to allow blood to flow in the desired direction. The AV valves open during atrial diastole to allow blood to flow from the atria into the ventricles. The semilunar valves open during ventricular systole to allow blood to be ejected from the ventricles into the arteries.
This sequence of events repeats with each heartbeat, continuously circulating blood throughout the body.