Cardiac Cycle

Cardiac cycle: sequence of events

The cardiac cycle is the sequence of events that occurs in the human heart as it pumps blood throughout the body. The cycle begins with atrial systole and ends with the opening of the bicuspid and tricuspid valves. Let's look at each stage of the cycle in more detail.

1. Atrial systole.

During atrial systole, a contraction wave excited by the sinus node propagates through the atria, forcing blood into the ventricles. The ventricles are already filled with blood due to the fact that the pressure in them is lower than in the atria, and the tricuspid and bicuspid valves are open. The impulse passes through the atrioventricular node more slowly than through other parts of the nodal tissue; This explains the short pause that occurs after atrial systole, before the onset of ventricular systole.

2. Beginning of ventricular systole.

Under the influence of an impulse conducted by a bundle of nodular tissue from the atrioventricular node, the muscular walls of the ventricles begin to contract, causing a rapid increase in pressure in the ventricles. The bicuspid and tricuspid valves immediately close and the first heart sound occurs.

3. A period of increasing pressure.

The pressure in the ventricles increases rapidly, but until it equals the pressure in the arteries, the semilunar valves remain closed and blood does not flow into or out of the ventricles.

4. Opening of the semilunar valves.

When intraventricular pressure begins to exceed arterial pressure, the semilunar valves open and blood rushes into the pulmonary artery and aorta. When the contraction of the ventricles comes to an end, the blood is pushed out more slowly and finally stops. Ventricular diastole begins.

5. Beginning of ventricular diastole.

As the ventricles relax, the pressure in them drops until it becomes lower than the pressure in the arteries. Then the semilunar valves slam shut and a second heart sound occurs.

6. A period of decreasing pressure.

After the semilunar valves close, the ventricular walls continue to relax and intraventricular pressure continues to fall. The tricuspid and bicuspid valves, which closed during the preceding ventricular systole, remain closed to prevent blood from flowing back from the arteries into the ventricles.

7. End of diastole.

When the pressure in the ventricles reaches its lowest level, the sinus node generates a new impulse, which begins the next cycle of heart contractions. The atria begin to contract to force blood into the ventricles, and the cycle begins again.

This process repeats more than 70 times per minute in a healthy person at rest, and the heart rate can increase depending on physical activity or other factors. Knowledge of the cardiac cycle helps to understand the functioning of the cardiovascular system and diagnose certain heart diseases.

Cardiac cycle - (cyclus cardiacus), a complex of processes in the human heart associated with the preparation, development and completion of cardiac activity. Every 70-80 ms, blood is pushed out of the heart and into the lungs, and then moves through the systemic circulation back to the heart to be re-oxygenated. Time to fill the heart chambers from the total duration