Cardiovascular Cycles

Cardiac Cycle

  • Cycle consists of heart contraction (systole) and relaxation (diastole).

  • Contraction phase: Systole

  • Relaxation phase: Diastole

  • Four phases of the cardiac cycle:

    • Ventricular Filling

    • Isovolumetric Contraction

    • Ventricular Ejection

    • Isovolumetric Relaxation

Key Concepts of the Cardiac Cycle

  • Blood flow occurs from areas of high pressure to low pressure.

  • During Ventricular Filling:

    • Heart is relaxed (diastole).

    • AV valves open, semilunar valves closed.

  • Isovolumetric Contraction:

    • All valves are closed; pressure increases without volume change.

    • First heart sound caused by closure of AV valves.

  • Ventricular Ejection:

    • Pressure in the ventricle exceeds aortic pressure; blood is expelled.

    • AV valves closed, semilunar valves open.

  • Isovolumetric Relaxation:

    • All valves closed; pressure drops with no volume change.

    • Second heart sound caused by closure of semilunar valves.

Cardiodynamics

  • Cardiac Output (CO) is the volume of blood pumped by a ventricle per minute:

    • CO = Heart Rate (HR) x Stroke Volume (SV)

  • Heart Rate: Number of cardiac cycles per minute.

  • Stroke Volume: Amount of blood pumped by a ventricle with each contraction (approximately 70 ml).

Factors Affecting Stroke Volume

  1. Preload: Volume of blood returning to the heart; greater volume causes more stretch and forceful contraction.

  2. Afterload: Resistance the heart must overcome to eject blood; increased pressure requires more force for contraction.

  3. Contractility: Force of contraction independent of preload and afterload; increased by calcium availability in muscle cells.