Cardiac cycle

Cardiac Cycle Overview

  • The lecture discusses the cardiac cycle, including anatomy, ECG, blood volumes, and blood pressure.

  • Importance of understanding student questions in refining lecture material.

SA Node Action Potential

  • Location: SA node is located in the right atrium.

  • Anatomy: Composed of branched nodal cells in the right atrium.

  • Function: Generates action potentials through voltage-gated channels.

  • Phases:

    • Repolarization: Potassium voltage-gated channels open.

    • Resting Membrane Potential: Restarts to trigger the next heartbeat.

  • Key Concept: Action potential occurs first, followed by contraction.

ECG and Cardiac Events

  • P Wave: Represents the depolarization of the atrium caused by the SA node action potential. Appears as a small bump in the ECG.

  • QRS Wave: Represents ventricular depolarization, occurring after atrial depolarization.

  • T Wave: Represents repolarization of the ventricles, occurs after ventricular contraction.

  • Importance of Timing: Depolarization leads to contraction, and repolarization leads to relaxation—this sequence is crucial for understanding cardiac events.

Cardiac Cycle Phases

  • The cardiac cycle includes both the contraction and relaxation phases of the heart, occurring repeatedly.

  • It can be divided into specific phases:

    1. Atrial Systole

    2. Isovolumetric Ventricular Systole

    3. Ventricular Systole

    4. Isovolumetric Ventricular Diastole

    5. Ventricular Diastole

Key Concepts of Blood Volume and Pressure

  • Blood Movement: Follows the pressure gradient; flows from high pressure to low pressure.

  • Valves: Ensure unidirectional blood flow, preventing backflow between chambers.

Phases Detailed

  1. Atrial Systole

    • Definition: Contraction of the atria, which fills the ventricles.

    • Blood Pressure Dynamics: Atrial pressure > ventricular pressure → AV valve opens, aortic valve is closed.

    • Trigger: Initiated by the P wave of the ECG, which precedes contraction.

  2. Isovolumetric Ventricular Systole

    • Definition: Ventricles begin to contract but no blood volume change occurs as all valves are closed.

    • Pressures: Ventricular pressure rises, but remains lower than aortic pressure until sufficient to open aortic valve.

    • ECG Trigger: QRS complex precedes this phase, indicating the commencement of ventricular contraction.

  3. Ventricular Systole

    • Definition: Significant contraction of ventricles, pumping blood into the aorta.

    • Pressures: Ventricular pressure exceeds aortic pressure, opening the aortic valve and allowing ejection of blood.

    • Characteristics: Marked drop in ventricular volume as blood is expelled.

    • ECG Phase: This phase begins post-QRS peak.

  4. Isovolumetric Ventricular Diastole

    • Definition: A brief interval where ventricles relax without any change in blood volume as all valves are closed.

    • Pressure Dynamics: Ventricular pressure drops below atrial pressure, preparing for filling but not yet allowing it since the AV valve is still closed.

    • ECG Characteristics: No new ECG event triggers this phase; it follows the T wave.

  5. Ventricular Diastole

    • Definition: Period when ventricles fill with blood.

    • Filling Mechanism: Initially created by the relaxation of ventricles (suction effect) and then assisted by atrial contraction (atrial systole).

    • Pressure Change: Atrial pressure becomes greater than ventricular pressure, allowing AV valve to open—blood flows into the ventricles.

    • Filling Process: Most ventricular filling occurs in late diastole, with a smaller contribution from atrial contraction.

Continuous Cardiac Activity

  • The heart beats approximately 60-75 times per minute, resulting in approximately 100,000 heartbeats daily.

  • Understanding cardiac cycle phases crucial for clinical assessments of heart health, including cardiac output and volume ejected per beat.

Cardiac Output and Regulation

  • Definition: Cardiac output represents how much blood the ventricle pumps out each minute. Calculated as:
    extCardiacOutput=extHeartRateimesextStrokeVolumeext{Cardiac Output} = ext{Heart Rate} imes ext{Stroke Volume}

  • Autonomic Nervous System Influence:

    • Parasympathetic System: Rest and digest (decreases heart rate and strength of contraction).

    • Mechanism: Acetylcholine binds to muscarinic receptors on ventricular muscle, decreasing calcium release, leading to weaker contractions.

    • Sympathetic System: Fight or flight (increases heart rate and strength of contraction).

    • Mechanism: Norepinephrine and epinephrine increase calcium release, facilitating stronger contractions, more cross-bridges formed, enhancing overall cardiac output.

Conclusion

  • The understanding of cardiac cycle dynamics is crucial for interpreting heart behavior under different physiological and pathological conditions.