Pulmonary Ventilation

Pressures & Mechanics - Boyle's Beatbox

• Boyle's Law: $P_1V_1 = P_2V_2$. Air flows from high to low pressure. 📌 Boyle's Beatbox: ↑Volume, ↓Pressure.
• Key Pressures (relative to Atmospheric $0 \text{ cm H}_2\text{O}$):
  • Intra-alveolar (Palv):
    • Inspiration: $-1 \text{ cm H}_2\text{O}$
    • Expiration: $+1 \text{ cm H}_2\text{O}$
  • Intrapleural (Pip): Normally $-5 \text{ cm H}_2\text{O}$.
    • Inspiration: Becomes more negative (e.g., $-7.5 \text{ cm H}_2\text{O}$).

    ⭐ Intrapleural pressure is always negative during quiet breathing, becoming more negative during inspiration.

  • Transpulmonary (Ptp): $P_{tp} = P_{alv} - P_{ip}$. Keeps alveoli open.
• Mechanics of Breathing:
  • Inspiration (Active): Diaphragm & external intercostals contract → ↑ Thoracic volume → ↓Palv → Air in.
  • Expiration (Passive): Muscle relaxation, elastic recoil → ↓ Thoracic volume → ↑Palv → Air out.

Lung Volumes & Capacities - Spiro Squad

Spirometry measures lung air movement, key for diagnosis.

Lung Volumes:

• Tidal Volume (TV): Air/quiet breath. ~500 mL.
• Inspiratory Reserve Volume (IRV): Max air inhaled beyond TV. ~3000 mL.
• Expiratory Reserve Volume (ERV): Max air exhaled beyond TV. ~1100 mL.
• Residual Volume (RV): Air in lungs after max exhale. ~1200 mL.

Lung Capacities:

• Inspiratory Capacity (IC): Max air inhaled from FRC start. $IC = TV + IRV$. ~3500 mL.
• Functional Residual Capacity (FRC): Air in lungs after normal exhale. $FRC = ERV + RV$. ~2300 mL.
• Vital Capacity (VC): Max air exhaled after max inhale. $VC = IRV + TV + ERV$. ~4600 mL.
• Total Lung Capacity (TLC): Total air in lungs after max inhale. $TLC = VC + RV$. ~5800 mL.

📌 Residual Volume & capacities with it (FRC, TLC) Can't Measure Simply (via spirometry).

⭐ Functional Residual Capacity (FRC) cannot be measured by simple spirometry as it includes Residual Volume (RV).

Compliance & Resistance - Elastic Airflow

• Compliance (C): Lung distensibility. $C = \Delta V / \Delta P$.
  • ↑C: Emphysema, aging.
  • ↓C: Fibrosis, edema, ARDS, ↓surfactant.
  • Specific Compliance: $C / \text{FRC}$.
• Surface Tension & Surfactant:
  • Alveolar surface tension (Laplace: $P = 2T/r$) promotes collapse.
  • Surfactant (Type II pneumocytes; DPPC): ↓Surface tension, ↑C, prevents atelectasis.

    ⭐ Surfactant, produced by Type II pneumocytes, reduces surface tension and increases lung compliance, preventing alveolar collapse.

• Airway Resistance (R): Opposition to airflow. $R = \Delta P / \dot{V}$.
  • Poiseuille's Law: $R \propto \eta L / r^4$. Radius (r) is key (📌 Radius Rules Resistance).
  • Major site: Medium-sized bronchi.
  • ↑R: Bronchoconstriction (asthma, parasymp.), mucus, ↓lung volume.
  • ↓R: Bronchodilation (symp.), ↑lung volume (radial traction).

Work & Dead Space - Effort & Empties

• Work of Breathing (WOB): Energy for lung inflation (overcomes elastic recoil, airway resistance).
  • ↑ WOB: Restrictive (fibrosis), Obstructive (asthma, COPD).
  • O2 cost: 1-3% of total O2 consumption; ↑ to 25-30% (disease/exercise).
• Dead Space (DS): Air not participating in gas exchange.
  • Anatomical DS (ADS): Volume of conducting airways; ~150 \text{ mL} (or ~2 \text{ mL/kg}).
  • Alveolar DS: Volume of alveoli ventilated but not perfused; ↑ in V/Q mismatch.
  • Physiological DS (PDS): Sum of ADS + Alveolar DS. 📌 PADS. Bohr's equation: $V_D/V_T = (P_aCO_2 - P_ECO_2) / P_aCO_2$.
• Alveolar Ventilation ($V_A$): Volume of fresh air for gas exchange. $V_A = (\text{Tidal Volume } V_T - \text{Dead Space } V_D) \times \text{Respiratory Rate (RR)}$.

⭐ Physiological dead space approximates anatomical dead space in healthy individuals but increases significantly in lung diseases with V/Q mismatch (e.g., pulmonary embolism, emphysema).

High‑Yield Points - ⚡ Biggest Takeaways

• Normal quiet inspiration: driven by diaphragm contraction and external intercostals.
• Forced inspiration: uses accessory muscles like scalenes and sternocleidomastoids.
• Forced expiration: an active process using abdominal and internal intercostal muscles.
• Intrapleural pressure: always negative relative to atmosphere, preventing lung collapse.
• Surfactant (Type II pneumocytes): reduces alveolar surface tension, increasing compliance.
• Airway resistance: highest in medium-sized bronchi, not the smallest airways.
• Lung compliance: lung's distensibility; increased in emphysema, decreased in fibrosis.