Cardiac Cycle and Heart Physiology

Overview of the Cardiac Cycle

• Cardiac Cycle: One heartbeat, divided into two phases based on ventricular actions:
  • Systole: Ventricular contraction and ejection (~0.3 seconds)
  • Diastole: Ventricular relaxation and filling (~0.5 seconds)
• Each phase further divided:
  • Systole:
    • Isovolumetric ventricular contraction: Ventricles contract, pressure increases, all four valves closed.
    • Ventricular ejection: Ventricular pressure exceeds pulmonary trunk/aorta, semilunar valves open.
  • Diastole:
    • Isovolumetric relaxation: Ventricles relax, pressure below atria, all valves closed.
    • Ventricular filling: Atrial pressure exceeds ventricular, atrioventricular valves open, blood flows into ventricles.

Detailed Cardiac Cycle Phases

• Systole:
  • Ventricular Ejection: Stroke volume is the blood volume ejected, not all blood is ejected.
  • Valve status during phases:
    • Isovolumetric contraction: All valves closed.
    • Ventricular ejection: Atrioventricular valves closed, semilunar valves open.
• Diastole:
  • Ventricular Filling: 80% passive filling, 20% during atrial contraction (atrial kick).
  • Valve status:
    • Initial relaxation: All valves closed, semilunar valves prevent backflow.
    • Filling: Atrioventricular valves open.

Graphical Representation

• Pressure Tracking:
  • Atrial pressure increases during diastole, peaks with atrial contraction.
  • Ventricular pressure surges during contraction, then declines post-relaxation.
  • Aortic pressure follows ventricular pressure but maintains a higher baseline.
  • Dicrotic Notch: Closure of aortic semilunar valve, elastic recoil of aorta increases pressure temporarily.
• Volume Tracking:
  • Shows end-diastolic and end-systolic volumes, tracks filling and ejection.
  • P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization) align with cardiac events.

Heart Sounds and Murmurs

• Heart Sounds: "Lub-dub" caused by closing of atrioventricular and semilunar valves.
• Murmurs: Abnormal sounds indicating valve issues (e.g., stenosis or regurgitation).

Cardiac Output

• Definition: Blood volume pumped per minute (heart rate x stroke volume).
• Typical Output: 5 liters per minute at rest, can increase with exercise.
• Regulation Factors:
  • Heart Rate: Influenced by sympathetic and parasympathetic activity.
  • Stroke Volume: Affected by end-diastolic volume, sympathetic stimulation, afterload.

Mechanisms Affecting Heart Rate and Stroke Volume

• Heart Rate Control:
  • Parasympathetic Influence: Lowers heart rate, more parasympathetic than sympathetic at rest.
  • Sympathetic Influence: Increases heart rate via norepinephrine and epinephrine.
  • SA Node Activity: Depolarization rate affected by ion channel activity.
• Stroke Volume Control:
  • Frank-Starling Mechanism: Increased end-diastolic volume increases stroke volume.
  • Sympathetic Regulation: Increases contractility and stroke volume.
  • Afterload: Minimal influence under normal conditions.

Summary of Cardiac Regulation

• Factors Increasing Cardiac Output:
  • Increased end-diastolic volume.
  • Increased sympathetic activity.
  • Increased plasma epinephrine.
  • Decreased parasympathetic activity.
• Heart and Blood Flow Coordination:
  • Frank-Starling mechanism ensures coordinated function between heart sides.
  • Sympathetic influences modify both rate and force of contraction.