Exercise Physiology: How Physical Activity Modulates Breathing and Heart Rate

Exercise triggers integrated cardiorespiratory responses to meet increased metabolic demand. The magnitude depends on intensity, duration, fitness, and environment.

Ventilatory Response

• Neural drive (Phase 1): Immediate increase in respiratory rate (RR) and tidal volume (Vt) via motor cortex → respiratory centers (feedforward)
• Humoral drive (Phase 2): Rising PaCO₂, falling PaO₂, decreasing pH → peripheral/central chemoreceptors → further increase in ventilation
• Steady state (Phase 3): Ventilation matches CO₂ production (isocapnic buffering)
• Recovery: Rapid drop (neural) then slow (humoral)

Cardiovascular Response

• Heart rate (HR): Parasympathetic withdrawal (0–100 bpm) → sympathetic activation (>100 bpm). Linear with workload until maximum.
• Stroke volume (SV): Increases via Frank-Starling (preload), contractility, decreased afterload. Plateaus at ~40–50% VO₂max.
• Cardiac output (Q = HR × SV): Increases 4–6x resting (25 L/min → 15–25 L/min)
• Blood pressure: Systolic increases proportionally; diastolic stable or slightly decreases (vasodilation in muscle)
• Blood flow redistribution: Muscle 20% → 80–85% of cardiac output; splanchnic/renal flow decreases

Ventilatory-Cardiovascular Coupling

• Entrainment: Respiratory sinus arrhythmia — HR increases during inspiration, decreases during expiration
• Ventilatory equivalent: V̇E/V̇O₂ and V̇E/V̇CO₂ reflect efficiency; increased in disease
• Anaerobic threshold: V̇E/V̇CO₂ nadir; disproportionate ventilation for CO₂ buffering

Modulating Factors

Factor	Effect on HR/V̇E
Fitness (VO₂max)	Lower resting HR, lower submax HR, higher max HR, more efficient V̇E
Age	Max HR decreases (~220-age); reduced β-responsiveness, compliance
Heat/humidity	Increased HR (cutaneous flow), increased V̇E (thermoregulatory drive)
Altitude	Increased V̇E (hypoxic drive), increased HR (sympathetic), decreased max performance
Medications (β-blockers, Ca²⁺ blockers)	Blunted HR response; use RPE, not HR targets

Monitoring Applications

• Target HR zones (e.g., 64–76% HRmax moderate, 77–95% vigorous)
• Talk test (moderate = can talk, not sing; vigorous = few words)
• CPET (gold standard for unexplained dyspnea, exercise prescription)

Understanding these responses optimizes training, ensures safety, and aids clinical assessment.