Physiological Responses to Exercise

Energy Systems & Exercise Types - Fueling the Fire

• Energy Systems - ATP Production:
  • Phosphagen (ATP-PCr): Immediate fuel (Creatine Phosphate). For ~10-15s maximal power (sprints, heavy lifts). $PCr + ADP \leftrightarrow ATP + Cr$. Anaerobic.
  • Glycolytic (Anaerobic): Glucose breakdown. For 15s - 2min high-intensity efforts (e.g., 400m race). Produces lactate.
  • Oxidative (Aerobic): Primary source for >2min activities. Uses glucose, fats, amino acids (minor). Occurs in mitochondria; requires $O_2$.
• Exercise Types & Primary Fuel Use:
  • Aerobic (Endurance): E.g., marathon, cycling. Relies on oxidative system (fats at low intensity, CHO at high).
  • Anaerobic (Strength/Power): E.g., weightlifting, sprints. Relies on phosphagen & glycolytic systems (PCr, glucose).

⭐ The "crossover concept": As exercise intensity ↑, fuel shifts from predominantly fat to carbohydrate metabolism.

Cardiovascular Responses - Heart & Vessels Go!

• Heart Rate (HR): ↑ linearly with intensity. Max HR $\approx 220 - age$.
• Stroke Volume (SV): ↑, plateaus (untrained); keeps ↑ (trained). Due to ↑preload/contractility, ↓afterload.
• Cardiac Output (CO): $CO = HR \times SV$. ↑ 4-6x to meet $O_2$ demand.
• Blood Pressure (BP):
  • Systolic (SBP): ↑ linearly.
  • Diastolic (DBP): ↔ or slight ↓ (↓ Total Peripheral Resistance (TPR) in active muscle).
  • Mean Arterial Pressure (MAP): ↑.
• Blood Flow Redistribution:
  • ↑ To active muscles (local vasodilation: adenosine, $K^+$, $CO_2$, NO).
  • ↓ To splanchnic/kidneys (sympathetic vasoconstriction).
  • ↑ Coronary flow (metabolic autoregulation).
• Venous Return: ↑ via muscle/respiratory pumps, venoconstriction.

⭐ During maximal exercise, skeletal muscle blood flow can increase from ~1 L/min to >20 L/min, receiving 80-85% of cardiac output.

Respiratory Responses - Lungs at Full Tilt

• Ventilation ↑↑:
  • TV & RR ↑ → VE ↑ (100-200 L/min).
  • Alveolar ventilation ↑, Physiological dead space ↓.
• Gas Exchange Enhanced:
  • Pulmonary blood flow ↑, V/Q matching improves (initially).
  • Diffusion capacity ($D_{LCO}$) ↑ (↑ surface area, ↑ capillary volume).
  • $PaO_2$ maintained/↑; $PaCO_2$ ↓ (hyperventilation).
  • Arterial-venous $O_2$ difference ↑.
• Control:
  • Phase I (start): Central command, proprioceptors.
  • Phase II (build-up): Humoral factors (↑ $K^+$, temp; ↓ pH).
  • Phase III (steady): Chemoreceptor fine-tuning.

⭐ In healthy individuals, $PaO_2$ is well-maintained during most exercise intensities; $PaCO_2$ typically decreases due to hyperventilation.

Metabolic & Thermoregulatory Responses - Fuel & Heat Game

• Fuel Dynamics:
  • Initial burst (~10s): ATP-PCr system.
  • High intensity (~1-2 min): Anaerobic glycolysis ($Glucose \rightarrow Lactate$).
  • Sustained (>2 min): Aerobic metabolism (Glucose, FFAs).
    • Crossover concept: ↑Exercise intensity ⇒ ↑CHO reliance over fats.
• Key Hormonal Adaptations:
  • ↑Epinephrine, Norepinephrine, Glucagon, Cortisol, Growth Hormone.
  • ↓Insulin.
  • Net effect: Mobilizes glucose (glycogenolysis, gluconeogenesis) & FFAs.
• Thermoregulation - Balancing Heat:
  • Heat Production: Primarily from contracting muscles.
  • Heat Dissipation:
    • Evaporation (sweating): Main cooling route.
    • Cutaneous vasodilation: Shunts blood & heat to skin.
  • Challenges: Dehydration, hyperthermia if cooling fails.

⭐ During prolonged, submaximal exercise, there's a gradual shift from carbohydrate to fat as the primary fuel source (fat oxidation increases as glycogen depletes).

High‑Yield Points - ⚡ Biggest Takeaways

• Cardiac output significantly increases via elevated heart rate and stroke volume.
• Blood flow is redistributed towards active muscles and skin, away from splanchnic circulation.
• Pulmonary ventilation dramatically increases; V/Q ratio becomes more uniform.
• VO2 max represents maximal oxygen uptake, a key indicator of aerobic fitness.
• Endurance training induces bradycardia, cardiac hypertrophy, and increased mitochondrial density.
• Oxygen debt (EPOC) occurs post-exercise, restoring metabolic homeostasis.
• Lactate threshold signals significant anaerobic metabolism onset during intense exercise.