Chapter 20 Lecture - Part 2: Cardiac Physiology

Review of Cardiac Conducting System

• SA Node: Pacemaker of the heart
• Pathway: SA Node → Atria → AV Node → AV Bundle → Bundle Branches → Moderator Band → Purkinje Fibers → Ventricles
• Electrocardiogram (ECG/EKG): Records electrical activity, not contraction
  • P Wave: Atrial depolarization
  • QRS Complex: Ventricular depolarization, atrial repolarization
  • T Wave: Ventricular repolarization

Contractile vs Conducting Cells

• Conducting Cells: 1% of myocardium, transmit action potentials
• Contractile Cells: 99% of myocardium, responsible for contraction
• Similarities with skeletal muscle: involve troponin, tropomyosin, myosin, actin, calcium

Differences Between Cardiac and Skeletal Muscle

• Cardiac Muscle:
  • Longer action potential
  • Longer refractory period
  • Prevents tetanic contractions
  • Calcium sourced from both sarcoplasmic reticulum and extracellular
• Skeletal Muscle:
  • Short action potential and refractory period
  • Allows tetanic contractions

Phases of Cardiac Muscle Action Potential

• Rapid Depolarization: Sodium channels open
• Plateau Phase: Calcium enters, potassium leaves
• Repolarization: Calcium channels close, potassium continues to leave

Cardiac Cycle

• Systole: Contraction phase (atrial and ventricular)
• Diastole: Relaxation phase, filling time
• Pressure Gradients: Blood flows from higher to lower pressure
• Valve Function: Open/close due to pressure differences

Heart Sounds

• S1 (Lubb): AV valves closing
• S2 (Dupp): Semilunar valves closing
• Additional sounds (S3, S4) are usually imperceptible

Cardiac Output

• Definition: Blood volume pumped by a ventricle in one minute
• Equation: Cardiac Output = Heart Rate x Stroke Volume
• Typical resting cardiac output is about 6 liters/minute

Factors Influencing Cardiac Output

• Heart Rate: Influenced by autonomic nervous system and hormones (epinephrine, norepinephrine, thyroid hormones)
• Stroke Volume: Influenced by end diastolic volume (EDV) and end systolic volume (ESV)

Autonomic Regulation of Heart Rate

• Sympathetic Stimulation: Norepinephrine, increases heart rate
• Parasympathetic Stimulation: Acetylcholine, decreases heart rate
• Medulla Oblongata: Contains cardiac centers for rate regulation

Stroke Volume Influences

• Filling Time: Longer time increases EDV
• Venous Return: More return increases EDV
• Preload: Stretch of ventricular walls at end of diastole
• Afterload: Resistance ventricles must overcome, affects ESV
• Contractility: Strength of contraction, affected by sympathetic stimulation

Key Takeaways

• Understand ECG components and what they represent
• Differentiate between cardiac and skeletal muscle contractions
• Recognize cardiac cycle phases and their physiological significance
• Grasp the concept of cardiac output and factors affecting it

Next Steps: Review questions, understand equations conceptually, prepare for Chapter 21.