Cardiac Muscle and Circulatory Physiology

Circulatory Pathways

• Left heart ➜ systemic circulation (oxygen-rich)
  • Left ventricle ejects into aorta ➜ arteries ➜ arterioles ➜ capillaries ➜ tissues (incl. coronary circulation)
• Right heart ➜ pulmonary circulation (oxygen-poor)
  • Right ventricle ➜ pulmonary arteries ➜ lungs (gas exchange) ➜ pulmonary veins ➜ left atrium
• Blood returns to right atrium via superior/inferior vena cavae

Pressure–Volume Relationships

• Blood flows down pressure gradients (higher ➜ lower)
• Container (chamber/vessel) volume vs. pressure: inverse
  • ↑ chamber size ➜ ↓ pressure
  • ↓ chamber size (contraction) ➜ ↑ pressure
• Fluid volume vs. pressure: direct
  • ↑ blood volume ➜ ↑ pressure
  • ↓ blood volume (e.g., hemorrhage) ➜ ↓ pressure
• Standard arterial pressure: 120/80\ \text{mmHg}
  • 120 = systolic (ventricular contraction)
  • 80 = diastolic (ventricular relaxation)

Cardiac Cycle Key Volumes

• End-systolic volume (ESV): blood left in ventricle after contraction
• End-diastolic volume (EDV): blood in ventricle just before contraction
• Atrial contraction completes ventricular filling when ventricular pressure ≈ atrial pressure
• Ejection begins when ventricular pressure > aortic/pulmonary arterial pressure

Cardiac Output

• Stroke volume: SV = EDV - ESV
• Cardiac output: CO = HR \times SV (L·min^{-1})
• Sample calculation:
  EDV = 100\ \text{ml},\ ESV = 20\ \text{ml} \Rightarrow SV = 80\ \text{ml}
  HR = 70\ \text{beats·min}^{-1} \Rightarrow CO = 70 \times 80\ \text{ml·min}^{-1} = 5600\ \text{ml·min}^{-1} = 5.6\ \text{L·min}^{-1}

Myocardial Cell Types & Contraction Mechanism

• Two cell types
  • Myocardial contractile cells (MCC): generate force
  • Myocardial autorhythmic cells (MAC): set rhythm (covered separately)
• Excitation–contraction coupling in MCCs
  • Action potential travels down T-tubule ➜ opens voltage-gated Ca^{2+} channel
  • Extracellular Ca^{2+} enters and binds ryanodine receptor on sarcoplasmic reticulum
  • Calcium-induced calcium release ➜ Ca^{2+} floods cytosol ➜ binds troponin ➜ sliding filament contraction
• Relaxation/ion reset
  • Ca^{2+}-ATPase pumps Ca^{2+} back into SR
  • Na^+/Ca^{2+} exchanger (NCX) extrudes Ca^{2+} (3 Na^+ in / 1 Ca^{2+} out)
  • Na^+/K^+-ATPase restores Na^+ gradient, preventing unwanted depolarization