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PULMONARY MECHANICS - Complete 10-Mark Answer

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DEFINITION

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1. MUSCLES OF VENTILATION

• Diaphragm - principal muscle; contracts and moves downward, increasing vertical chest diameter (accounts for ~75% of quiet breathing)
• External intercostals - elevate the ribs, increasing anteroposterior (AP) diameter by ~20%
• Accessory muscles (recruited during heavy breathing): sternocleidomastoids (lift sternum), anterior serrati (lift ribs), scaleni (lift first two ribs)

• Quiet expiration is passive - driven by elastic recoil of lungs and chest wall
• Forced expiration: internal intercostals + abdominal recti (compress abdominal contents upward, push diaphragm up)

Exam tip: "Inspiration is always active; quiet expiration is passive; forced expiration is active."

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2. PRESSURES GOVERNING AIR FLOW

A. Pleural (Intrapleural) Pressure

• Normally slightly negative (-5 cm H₂O at rest) - acts as suction holding lungs against the chest wall
• During inspiration: becomes more negative, reaching -7.5 cm H₂O
• During expiration: returns toward -5 cm H₂O
• The negativity is maintained by continuous lymphatic drainage of pleural fluid

B. Alveolar Pressure

• At rest (no airflow): equals atmospheric = 0 cm H₂O
• During inspiration: drops to -1 cm H₂O (below atmospheric) → drives air in
• During expiration: rises to +1 cm H₂O → drives air out
• Governed by Boyle's Law: as lung volume increases, alveolar pressure falls

C. Transpulmonary Pressure

• = Alveolar pressure - Pleural pressure
• Represents the elastic recoil force (recoil pressure) tending to collapse the lung
• Normal at FRC: approximately +5 cm H₂O
• Greater transpulmonary pressure = larger lung volume

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3. LUNG COMPLIANCE

• The inflation and deflation curves do not coincide - this is called hysteresis
• During deflation the lung recoils at a higher pressure for a given volume
• Hysteresis is partly due to surfactant behavior
• When lungs are inflated with saline (eliminating surface tension), hysteresis disappears - confirming that surface tension contributes significantly

• Pulmonary fibrosis, pulmonary edema, atelectasis, ARDS
• Higher lung volumes (reduced compliance at extremes)

• Emphysema (destruction of elastic tissue)
• Normal aging

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4. SURFACE TENSION AND SURFACTANT

Law of Laplace

Role of Surfactant

• Secreted by Type II pneumocytes (alveolar epithelial cells)
• Composition: primarily dipalmitoylphosphatidylcholine (DPPC) + other lipids + proteins (SP-A, SP-B, SP-C, SP-D)
• Reduces alveolar surface tension from 50 dynes/cm (no surfactant) to 5-30 dynes/cm
• Prevents alveolar collapse on expiration (when radius is small, surfactant concentrates and lowers T further)
• Prevents pulmonary edema (without surfactant, ~20 mmHg unopposed surface tension would pull fluid into alveoli)
• SP-A and SP-D are collectins involved in innate immunity

Clinical significance - Respiratory Distress Syndrome (RDS)

• Surfactant production begins at 26-28 weeks gestation; adequate amounts by 34-36 weeks
• Premature infants lack surfactant → Infant Respiratory Distress Syndrome (IRDS) / Hyaline Membrane Disease
• Alveolar surface tension can be 6-8× normal
• Treatment: exogenous surfactant replacement + CPAP

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5. ELASTIC RECOIL

• Lungs have a natural tendency to collapse (recoil inward) and chest wall tends to spring outward
• At FRC (Functional Residual Capacity), inward recoil of lungs = outward recoil of chest wall → equilibrium
• Elastic recoil has two components:
  1. Tissue elasticity (~33%): from elastic and collagen fibers in lung parenchyma
  2. Surface tension forces (~67%): from alveolar fluid interface

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6. AIRWAY RESISTANCE

Poiseuille's Law for Airway Resistance

• If radius halves → resistance increases 16-fold (2⁴)
• If radius doubles → resistance decreases 16-fold

Site of Greatest Resistance

• Medium-sized bronchi have the highest individual airway resistance
• Although terminal bronchioles are narrow, they are in parallel → total peripheral resistance is low
• Applying the parallel resistance rule: adding airways in parallel reduces total resistance

Laminar vs. Turbulent Flow

• Laminar flow (low velocity, smooth): obeys Poiseuille's law; R depends on viscosity
• Turbulent flow (high velocity, branching points): R depends on gas density (not viscosity)
  • Relevant in: upper airway obstruction, exercise, helium use (low density → reduces turbulent resistance)
• Reynolds number (Re): turbulence when Re >2000
  • Re = (velocity × diameter × density) / viscosity

Factors Altering Airway Resistance

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7. WORK OF BREATHING

• In emphysema: compliance is high but airway resistance increases (dynamic collapse) → increased airway resistance work
• In fibrosis/ARDS: compliance is low → increased elastic work
• In asthma: bronchoconstriction → increased airway resistance work; patients breathe at higher lung volumes to exploit radial traction

• Normal: <5% of total VO₂
• Severe lung disease: up to 30-40% → respiratory muscle fatigue → ventilatory failure (pump failure)

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8. DYNAMIC AIRWAY COMPRESSION (Equal Pressure Point)

• During forced expiration, as pleural pressure becomes highly positive, it can compress intrathoracic airways
• At the Equal Pressure Point (EPP), airway pressure = pleural pressure → dynamic compression begins
• Downstream from EPP, airways are compressed → flow-limited ("effort-independent" zone of MEFV curve)
• In emphysema: loss of elastic recoil moves EPP proximally → earlier collapse → air trapping

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9. COMPLIANCE OF THORAX AND LUNGS TOGETHER

• Combined lung-thorax compliance = 110 mL/cm H₂O (approximately half of lung alone)
• At extremes of lung volume, combined compliance can fall to <20% of lungs alone
• In a paralyzed patient, the effort to inflate the combined system requires almost twice the pressure compared to excised lungs alone

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10. CLINICAL APPLICATIONS (High-Yield Exam Points)

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SUMMARY TABLE FOR QUICK REVISION

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EXAM STRATEGY - HOW TO SCORE FULL MARKS

1. Start with a definition and classification (static vs. dynamic mechanics)
2. Muscles section - name specific muscles with their actions
3. Pressures section - give exact values with directions of change; draw the pressure diagram
4. Compliance - define with formula, give numbers, explain hysteresis, clinical conditions
5. Surface tension/surfactant - Law of Laplace formula, DPPC composition, IRDS
6. Airway resistance - Poiseuille's law formula, 4th-power relationship, sites, laminar/turbulent
7. Work of breathing - three components with percentages
8. Clinical correlations - table format maximizes marks per line
9. Draw diagrams - P-V loop (compliance), P-time/Volume-time curves for pressures

Sources: Guyton and Hall Textbook of Medical Physiology | Ganong's Review of Medical Physiology | Costanzo Physiology 7th Ed. | Murray & Nadel's Textbook of Respiratory Medicine