Cardiovascular Responses to Exercise

Cardiac Output Dynamics - Heart's Power Up!

• Cardiac Output ($CO$) is the volume of blood pumped by the heart per minute: $CO = HR \times SV$.
  • Resting $CO$ ≈ 5 L/min; Max exercise $CO$ ≈ 20-25 L/min (untrained), up to 35-40 L/min (trained).
• Heart Rate ($HR$):
  • Increases linearly with exercise intensity until $HR_{max}$ (Max $HR$ ≈ 220 - age).
  • Initial rapid ↑ due to vagal withdrawal, then slower ↑ from sympathetic stimulation.
• Stroke Volume ($SV$):
  • Increases with exercise intensity, plateaus at ~40-60% $VO_2$ max in untrained individuals.
  • In trained athletes, $SV$ may continue to rise until $VO_2$ max.
  • Mechanisms: ↑ Preload (Frank-Starling mechanism), ↑ myocardial contractility, ↓ afterload.
• Fick Principle: Relates $CO$, oxygen consumption ($VO_2$), and arteriovenous oxygen difference ($CaO_2 - CvO_2$): $VO_2 = CO \times (CaO_2 - CvO_2)$.

⭐ During upright exercise, stroke volume increases up to about 40-60% of maximal oxygen uptake ($VO_2$ max), after which it plateaus in untrained individuals; further increases in cardiac output are primarily mediated by heart rate. However, in elite endurance athletes, SV may continue to increase up to $VO_2$ max.

BP & Blood Flow - Pressure & Pipes

• Systolic BP (SBP): ↑ linearly with intensity (e.g., 8-12 mmHg/MET).

• Diastolic BP (DBP): Stable or slight ↓ (due to ↓ muscle vascular resistance).

• Mean Arterial Pressure (MAP): ↑; $MAP \approx DBP + 1/3(SBP-DBP)$.

• Pulse Pressure (PP): ↑ ($PP = SBP - DBP$).

• Total Peripheral Resistance (TPR): ↓ significantly; $TPR = MAP/CO$.

  • Caused by vasodilation in exercising muscles.

• Blood Flow Redistribution:

  • ↑ To active muscles (up to 80-85% CO), heart, skin.
  • ↓ To splanchnic organs (kidneys, gut), inactive areas.
  • Brain blood flow: Maintained.

• Local Muscle Vasodilation Mechanisms:

⭐ During dynamic exercise, SBP increases progressively with workload, while DBP typically remains unchanged or may slightly decrease.

Coronary Circulation - Heart's Fuel Line

• High aerobic demand. MVO2 (Myocardial $O_2$ consumption) determinants:
  • Heart Rate (HR), Contractility, Wall Tension (Preload & Afterload).
  • 📌 Mnemonic: "CHAWP" (Contractility, HR, Afterload, Wall tension, Preload).
• Rate Pressure Product (RPP = HR $\times$ SBP) estimates MVO2.
• Coronary $O_2$ extraction: ~70-80% at rest (highest organ extraction).
  • ↑MVO2 (e.g., during exercise) primarily met by ↑Coronary Blood Flow (CBF).
• CBF Regulation:
  • Primary: Local metabolic factors (adenosine, $NO$, $K^+$, hypoxia).
  • Secondary: Neural influences (sympathetic α-constriction, β2-dilation).
• Left Ventricular (LV) Flow: Predominantly diastolic due to systolic compression of vessels.

⭐ During strenuous exercise, CBF can increase 3-5 fold above resting levels, mainly through local metabolic vasodilation to match the heightened MVO2 demand of the myocardium.

Training Adaptations - Athlete's Super Heart

• Structural (Physiological LV Hypertrophy):
  • ↑ LV internal diameter & wall thickness (eccentric).
  • ↑ Myocardial contractility & compliance.
• Functional Enhancements:
  • At Rest:
    • ↓ Resting HR (bradycardia, 40-60 bpm) via ↑ vagal tone.
    • ↑ Resting SV (↑ preload, ↑ contractility).
    • CO (HR × SV) largely unchanged.
    • Slight ↓ resting BP.
  • During Exercise:
    • ↑ Max SV (key for ↑ VO2 max).
    • ↑ Max CO (up to 30-40 L/min).
    • ↓ HR at submaximal workloads.
    • Faster HR recovery.
    • ↑ a-vO2 difference (↑ O2 extraction).
• Peripheral Adaptations:
  • ↑ Blood volume, ↑ muscle capillary density.
  • ↑ Mitochondrial density & oxidative enzymes.

⭐ Athlete's heart: physiological LV hypertrophy with enhanced diastolic function, unlike pathological hypertrophy.

High‑Yield Points - ⚡ Biggest Takeaways

• Cardiac Output (CO) ↑ linearly with exercise intensity.
• Stroke Volume (SV) ↑, plateaus in untrained; athletes show continued ↑.
• Heart Rate (HR) ↑ linearly with workload up to HRmax.
• Systolic BP (SBP) ↑ progressively; Diastolic BP (DBP) is stable or slightly ↓.
• Total Peripheral Resistance (TPR) ↓ due to active muscle vasodilation.
• Blood flow redistributes to active muscles, heart, and skin.
• Arteriovenous O2 difference widens significantly.