Study Notes on the Cardiac Cycle and Heart Function

The Heart and the Cardiac Cycle

Overview of the Heart

  • The heart is an organ with a pump function, vital for circulation.

  • Each heartbeat results in an increase in pressure within the circulatory system, leading to a pressure difference that guarantees blood flow to all organs.

The Cardiac Cycle

  • The cardiac cycle consists of a sequence of electrical and mechanical events that are repeated with each heartbeat.

  • Two main phases of the cardiac cycle:

    • Systole: Heart muscle contracts.

    • Diastole: Heart muscle relaxes.

  • Both phases occur similarly in the right and left chambers, but there are differences in pressures and volumes.

Focus on the Left Heart Cavities

  • The explanation will specifically address the left cavities of the heart.

  • In the left ventricle:

    • Residual volume of approximately 50 milliliters of blood remains from the previous cycle.

    • Pressure is around 2 to 3 millimeters of mercury (mmHg).

Atrial Filling Phase

  • Blood enters the left atrium from the pulmonary veins, filling the cavity until an increase in atrial pressure occurs, which exceeds the ventricular pressure.

  • Result: Opening of the atrioventricular valves, initiating the ventricular filling stage (also known as inflow stage).

Initiation of Atrial Systole

  • After about two-thirds of the filling time, the sinus node generates an electrical impulse, prompting atrial systole.

  • This is represented by the P wave on the electrocardiogram (ECG).

  • Atrial contraction leads to:

    • Increase in ventricular pressure.

    • Gradual decrease in atrial pressure as atrium empties.

  • When ventricular pressure exceeds atrial pressure, the atrioventricular valve closes, producing the first heart sound (S1).

  • End of ventricular filling stage:

    • End diastolic volume approximately 130 milliliters.

Isovolumetric Contraction Stage

  • The ventricular filling phase leads to ventricular systole, where pressure begins to increase without a change in volume, as the valves are closed.

  • This phase is called isovolumetric contraction.

  • Ventricular pressure must exceed aortic pressure (approximately 70 mmHg) to open the aortic valve.

Ejection Stage

  • Opening of the aortic valve initiates the ejection stage where blood flows from left ventricle to aorta, leading to:

    • Progressive decrease of ventricular volume.

    • Increase in ventricular pressure to around 120 mmHg during contraction.

  • As the ventricle finishes ejecting blood, contraction strength decreases:

    • This change is represented by the T wave on the electrocardiogram.

  • The aorta distends and pressure rises to approximately 120 mmHg, exceeding decreasing ventricular pressure.

  • This causes a retrograde flow that results in the closure of the aortic valve, producing the second heart sound (S2).

End of Ejection Phase

  • To remember events:

    • Isovolumetric contraction stage begins with closure of atrioventricular valves and ends with opening of semilunar valves.

    • Ejection stage begins with opening of semilunar valves and ends with closing.

Isovolumetric Relaxation Stage

  • Following ventricular contraction, there is a significant drop in pressure, returning to the initial 2 to 3 mmHg.

  • Approximately 50 milliliters of blood remains in the ventricle after each cycle.

  • This period is known as the isovolumetric relaxation stage:

    • Begins with the closing of the semilunar valves.

    • Ends with the opening of the atrioventricular valves.

Summary

  • Overall, the cardiac cycle consists of systematic phases that ensure efficient blood flow and pressure regulation in the cardiovascular system, vital for sustaining life and facilitating metabolic processes.