Pulmonary/Respiration Responses During Exercise – Chapter 10

Overview & Key Purposes

• Pulmonary (external) respiration
  • Ventilation: bulk movement of air into/out of lungs
  • Gas exchange in the lungs ( \text{O}2\uparrow,\;\text{CO}2\downarrow )
• Cellular (internal) respiration
  • Tissue \text{O}2 utilization & \text{CO}2 production
• Primary duties of the system during exercise
  • Maintain gas exchange between body & environment
  • Regulate acid–base balance as metabolic rate rises

Four Sequential Processes of Respiration

1. Pulmonary ventilation (movement of ambient air → alveoli)
2. Alveolar gas exchange (alveolus ⇌ pulmonary capillary)
3. Gas transport (via blood circulation)
4. Systemic gas exchange (capillary ⇌ working cells)

Structural Organization

• Upper tract: nose, nasal cavity, pharynx, larynx
• Lower tract: trachea, bronchial tree, lungs
• Zones
  • Conducting zone
  • Trachea → terminal bronchioles (humidify, warm, filter)
  • Dead-space volume ( V_D )
  • Respiratory zone
  • Respiratory bronchioles, alveolar ducts/sacs
  • Site of diffusion; surfactant prevents collapse
• Pleural membranes
  • Visceral (lung surface) & parietal (thoracic wall)
  • Intrapleural pressure < atmospheric → keeps alveoli patent

Microscopic Anatomy

• ~8\times10^6 terminal bronchioles; 5\times10^5 respiratory bronchioles; 8\times10^6 alveolar sacs
• Alveolar cells
  • Type I: thin plates, 95 % surface → diffusion surface
  • Type II: progenitor “caretaker”, secretes surfactant, proliferates after injury
  • Resident macrophages for immune defense

Mechanics of Breathing (Bulk Flow)

• Inspiration (active)
  • Diaphragm descends, external intercostals lift ribs → thoracic volume ↑
  • Intrapulmonary P drops (~-2\;\text{mmHg}) ⇒ air in
• Expiration (passive at rest)
  • Diaphragm & ribs recoil, volume ↓, pressure ↑ ⇒ air out
• Accessory muscles
  • Inspiration: sternocleidomastoid, scalenes
  • Expiration (forced/exercise): internal intercostals, abdominals
• Exercise adaptations
  • Respiratory muscles DO fatigue when >120 min or 90–100\%\,\dot V!O_2^{max}
  • Endurance training ↑ oxidative capacity → ↓ work of breathing

Airflow & Resistance

• Poiseuille-like relationship \displaystyle \text{Airflow}=\frac{P1-P2}{\text{Resistance}}
• Resistance (\propto 1/\text{radius}^4); diameter ↓ in:
  • Asthma/Exercise-induced bronchospasm (transient)
  • Chronic obstructive pulmonary disease (COPD)
  • Chronic bronchitis: mucus hyper-secretion
  • Emphysema: airway collapse, alveolar destruction

Pulmonary Ventilation Variables

• Minute ventilation: V=\dot VE=VT\times f (tidal volume × frequency)
• Partitioning ⇒ \dot VE=\dot VA+\dot V_D
  • \dot V_A = alveolar ventilation (reaches respiratory zone)
  • \dot V_D = dead-space ventilation

Lung Volumes & Capacities (Typical 70 kg male)

• Volumes
  • V_T=500\;\text{ml}
  • IRV=3000\;\text{ml}
  • ERV=1200\;\text{ml}
  • RV=1300\;\text{ml}
• Capacities & formulae
  • Vital capacity VC=IRV+V_T+ERV=4700\;\text{ml}
  • Inspiratory capacity IC=V_T+IRV=3500\;\text{ml}
  • Functional residual capacity FRC=ERV+RV=2500\;\text{ml}
  • Total lung capacity TLC=VC+RV=6000\;\text{ml}
• Spirometry
  • FEV1: air exhaled in 1 s; FEV1/VC\ge80\% normal; COPD ~\le33\%

Gas Laws & Diffusion

• Dalton: P{air}=P{O2}+P{CO2}+P{N_2}
  • At 760\;\text{mmHg} → P{O2}=159\;\text{mmHg}
• Fick’s Law \displaystyle \dot V{gas}=\frac{A\,D\,(P1-P_2)}{T}
  • ↑ area, ↑ gradient, ↓ thickness ⇒ ↑ diffusion
• Altitude effect (selected values)
  • 0\,\text{m}: PB=760\;\text{mmHg}, P{iO_2}=149
  • 4000\,\text{m}: PB=440, P{iO_2}\approx92
• Hypoxic ventilatory response (>1200\,\text{m})
  • Hyperventilation ⇒ PCO_2 ↓ ⇒ respiratory alkalosis, curve left shift, saturation maintained
• High-altitude illness: AMS, HAPE, HACE

Pulmonary & Systemic Circulation

• Pulmonary circuit: same flow, ~1/5 systemic pressure
• Gravitational distribution at rest
  • Base: high perfusion (V/Q
• Exercise
  • Light → improves V/Q toward \approx1
  • Heavy → mismatch due to diffusion/perfusion limits

Ventilation–Perfusion (V/Q) Data (upright)

| Region | Ventilation (L·min⁻¹) | Blood flow (L·min⁻¹) | V/Q |
| Apex | 0.24 | 0.07 | 3.43 |
| Base | 0.82 | 1.29 | 0.64 |

Oxygen Transport

• \approx98\% bound to hemoglobin (Hb)
  • Capacity: 1.34\;\text{ml O}_2\cdot\text{g}^{-1}\,Hb
  • Content (100 % sat):
  • Males: [Hb]=150\;\text{g·L}^{-1} \Rightarrow 200\;\text{ml O}_2·L^{-1}
  • Females: [Hb]=130 \Rightarrow 174\;\text{ml O}_2·L^{-1}
• Hb–O2 reaction: \text{Hb}+O2\rightleftharpoons\text{HbO}2
  • Direction depends on P{O2} & affinity
  • Lung (high P{O2}): loading; Tissues (low P{O2}): unloading
• Oxy-Hb dissociation curve
  • Sigmoidal; large unloading between 60\to20\;\text{mmHg}
  • Bohr effect (right shift)
  • ↓pH, ↑temperature, ↑2-3 DPG ⇒ ↓affinity ⇒ enhanced off-loading
• Sex difference: women’s smaller airway diameter → ↑ work of breathing at high intensities

Myoglobin

• Higher affinity than Hb; hyperbolic curve
• Acts as O₂ reservoir & shuttle within muscle, smoothing transition at exercise onset

Carbon Dioxide Transport

• \sim70\% as bicarbonate; \sim20\% carbamino-Hb; \sim10\% dissolved
• Key reaction (catalyzed by carbonic anhydrase):
  \text{CO}2+\text{H}2\text{O}\rightleftharpoons\text{H}2\text{CO}3\rightleftharpoons\text{H}^++\text{HCO}_3^-
• Tissue: \text{HCO}3^- exits RBC (Cl⁻ shift) ; Lung: reaction reverses when O2 binds Hb
• RER > 1 during heavy exercise arises from non-metabolic \text{CO}_2 (buffering lactic acid)

Acid–Base Balance via Ventilation

• ↑Ventilation ⇒ PCO_2 ↓ ⇒ H⁺ ↓ ⇒ pH ↑ (respiratory alkalosis)
• ↓Ventilation ⇒ PCO_2 ↑ ⇒ H⁺ ↑ ⇒ pH ↓ (respiratory acidosis)

Dynamic Responses to Exercise

1. Rest→steady submaximal
   • VE rises sharply, then plateaus; P{O2} slight ↓, P{CO2} slight ↑
2. Incremental (untrained)
   • VE linear to \approx50–75\%\,\dot V!O2^{max}, then exponential rise (ventilatory threshold (T{vent}))
   • P{O2} maintained (±10 mmHg)
3. Elite endurance athletes
   • (T{vent}) occurs at higher %; but arterial P{O_2} may fall 30–40 mmHg (exercise-induced hypoxemia) due to Q̇ ↑ & diffusion limits
4. Prolonged exercise in heat
   • “Ventilatory drift” upward; not driven by PCO_2 changes (thermal, catecholamine influences)
5. Breathing pattern shifts with intensity
   • Early: ↑tidal volume; High: ↑frequency predominates

Ventilatory Control Mechanisms

• Respiratory Control Center (medulla & pons)
  • Pre-Bötzinger & RTN (medulla): rhythm generation & CO₂ sensitivity
  • Pneumotaxic & caudal pons: fine-tune pattern
• Efferents: phrenic nerve (diaphragm) + spinal motor neurons

Afferent Inputs

1. Humoral chemoreceptors
   • Central (medullary): respond to CSF H^+,\,PCO_2
   • Peripheral (carotid, aortic bodies): sense arterial PO2,\,PCO2,\,pH,\,K^+
2. Neural
   • Higher brain centers (central command)
   • Muscle mechanoreceptors & metaboreceptors (pH, K⁺)

Exercise Control Scheme (Submaximal)

• Central command initiates ↑VE
• Muscle & peripheral feedback + blood gases fine-tune to metabolic needs
• Heavy exercise: lactic acidosis, ↑K⁺, ↑temp stimulate carotid bodies => additional VE rise

Training Adaptations & Performance Limitation

• Pulmonary structure unchanged by endurance training (lungs already over-built)
• Training ↓ VE at given submaximal workload (≈20–30 %)
  • Due to ↑ oxidative capacity & ↓ afferent feedback from locomotor muscles ⇒ ↓ H⁺ production
• Pulmonary system as limiter
  • Low–moderate intensity: not limiting
  • Healthy fit individuals at sea level: lungs usually adequate
  • Elite athletes at >95\%\,\dot V!O_2^{max}: respiratory muscle fatigue & arterial hypoxemia may limit performance (~40–50 % of elite endurance athletes)

Special Topics

• Exercise-induced asthma: bronchospasm during/after exercise → dyspnea, ↓ performance
• "Stitch" (exercise-related transient abdominal pain): likely irritation of parietal peritoneum; avoid large meals/fluids pre-exercise
• Sex differences: smaller female airways → ↑ resistance, ↑ work of breathing during severe efforts

Numerical / Formula Summary

• Airflow equation \dot V=\frac{P1-P2}{R}
• Minute ventilation \dot VE=VT\,f
• Partitioning \dot VE=\dot VA+\dot V_D
• Capacities
  • VC=IRV+ERV+V_T
  • TLC=VC+RV
• Fick’s diffusion \dot V{gas}=\frac{A\,D\,(P1-P_2)}{T}
• Hb carrying capacity 1.34\;\text{ml O}_2/\text{g Hb}
• Oxygen content examples
  • [Hb]{male}\times1.34=200\;\text{ml O}2·L^{-1}
• Spirometric indices
  • Normal FEV_1/VC\ge80\%; COPD \approx33\%

Ethical / Practical Implications

• Accurate spirometry essential for COPD/asthma diagnosis & exercise clearance
• Understanding altitude effects critical for mountaineers & military; gradual ascent prevents AMS/HAPE/HACE
• Sex-specific research required due to differing ventilatory mechanics
• Training respiratory muscles (inspiratory muscle training) may aid patients & select athletes