Chapter 6 – Physical Principles of Respiratory Care

Learning Objectives

• Identify, describe, and be able to apply every principle below to clinical/respiratory-care scenarios.
  • Properties of gases, liquids, and solids; temperature measurement and conversion; mechanisms of heat transfer.
  • Surface tension & radius relationships; vaporization/condensation energetics; humidity concepts; gas diffusion, Dalton’s and Henry’s laws; combined gas laws; critical temperature/pressure.
  • Fluid-flow patterns, transitions, and pressure changes (Boyle’s, Bernoulli, Venturi, Coanda effects).

States of Matter

• Three primary states
  • Solids
  • High internal order; fixed volume & shape.
  • Strong intermolecular attractive (cohesive) forces.
  • Molecules vibrate (“jiggle”) over the shortest mean distance.
  • Liquids
  • Fixed volume but adopt shape of container.
  • Weaker mutual attraction vs. solids; shape governed by internal/external forces.
  • Gases
  • Neither fixed volume nor shape; very weak attractions; rapid, random motion with frequent collisions.
• Plasma (fourth state)
  • Mix of neutral atoms, free electrons, and nuclei; responds to electromagnetic fields; not clinically relevant to routine respiratory care.

Internal Energy of Matter

• All atoms/molecules possess internal energy (IE) → sum of potential + kinetic.
  • Potential energy (PE): “energy of position;” due to attractive forces. Dominant in solids/liquids, weak in gases.
  • Kinetic energy (KE): “energy of motion;” directly related to molecular velocity → proportional to temperature; dominant in gases.

Laws of Thermodynamics (overview)

• Science of heat/energy and their transformations.
  • Provide restrictions (e.g., forbid perpetual motion).
  • Respiratory equipment design (humidifiers, ventilators) follows these laws.

Heat Transfer

• First Law of Thermodynamics: Energy neither created nor destroyed—only converted.
• Heat moves from higher → lower temperature until equilibrium.
• Four transfer mechanisms
  1. Conduction: Direct molecule-to-molecule; major mode in solids.
  2. Convection: Bulk mixing of fluid layers (liquids/gases); e.g., forced-air furnace, heated aerosol delivery.
  3. Radiation: Electromagnetic waves, no contact required; heat lamps for infants.
  4. Evaporation/Condensation: Latent heat exchange accompanying phase change between liquid ↔ gas.

Temperature

• Operational definition: Measure of average KE (collision frequency/intensity).
• Absolute zero: No KE; molecules cease motion; theoretical, unattainable (Third Law).

Temperature Scales & Conversions

• Water-based scales
  • Freezing: 0\,^\circ C = 32\,^\circ F
  • Absolute zero: -273\,^\circ C = 0\,K
• Conversion formulas
  • K = C + 273
  • C = \tfrac{5}{9}(F - 32)
  • F = \tfrac{9}{5}C + 32

Change of State (Solid ↔ Liquid ↔ Gas)

• Melting point: Solid → liquid; temperature identical to freezing point (liquid → solid).
• Sublimation: Solid directly → vapor; requires low vapor pressure (e.g., dry ice CO_2).

Properties of Liquids

• Pressure inside a liquid column depends on height (h) & density (ρ): P = \rho g h.
• Buoyancy (Archimedes): Upward force = weight of displaced fluid; key for aerosol suspension.
• Specific gravity: Density ratio relative to water (≈1 g·cm^{-3}).
• Viscosity (η): Internal friction resisting flow; directly proportional to cohesive forces.
  • Blood ≈ 5× more viscous than water; ↑η → ↑cardiac work (polycythemia).
• Cohesion vs. Adhesion
  • Cohesion (like-like) vs. adhesion (like-unlike).
• Surface tension (γ): Cohesive force at liquid–air interface; drives droplets toward spheres.
• Capillary action: Adhesion + surface tension pull fluid up narrow tubes; mechanism for wicking humidifier cores & pulmonary surfactant spreading.

Liquid ↔ Vapor Phase Changes & Humidity

• Vaporization: Liquid → gas via two pathways
  1. Boiling: Occurs when vapor pressure > ambient; O_2 boils at -183\,^\circ C.
  2. Evaporation: Molecules escape below boiling point; rate ↑ with temperature & surface area.
• Absolute humidity (AH): Water mass per gas volume; units mg·L^{-1}.
  • Fully saturated air at 37\,^\circ C & 760 mmHg holds 43.8 mg·L^{-1} (vapor pressure 47 mmHg).
• Relative humidity (RH): \%RH = \frac{AH}{Saturated\;Capacity} \times 100.
  • Dew point = temperature where gas becomes 100 % RH and condensation begins.
• Clinical Relevance: Heated humidifiers target BTPS (body temp/pressure saturated) to prevent mucosal drying.

Properties of Gases

• Kinetic theory: Gas molecules move fast, collide, exert pressure; velocity ∝ absolute temperature.
• Ideal molar volume: Any ideal gas occupies 22.4 L at STP (0 °C, 1 atm).
• Density (ρ): Mass/volume; lighter gases (He) diffuse faster.
• Diffusion: Net movement high → low concentration; governs O2/CO2 exchange.
• Pressure terminology
  • Partial pressure (Pi): Portion of total exerted by each gas (Dalton’s Law): Pi = Fi \times P{total}.
  • In liquids, gas pressure = tension (e.g., arterial O_2 tension).
• Henry’s Law: Gas dissolved ∝ partial pressure over liquid at constant T.

Gas Behavior Under Changing Conditions

• Boyle’s Law: P1 V1 = P2 V2 (T constant).
• Charles’ Law: \frac{V}{T} = k (P constant).
• Gay-Lussac’s Law: \frac{P}{T} = k (V constant).
• Combined Gas Law: \frac{P1 V1}{T1} = \frac{P2 V2}{T2}.
• Ideal-gas assumptions
  1. Elastic collisions (no energy loss).
  2. Negligible molecular volume.
  3. No intermolecular attractions.
• Effect of water vapor: Must subtract saturated vapor pressure (47 mmHg @ 37 °C) before applying laws to physiologic gases.

Common Clinical Conversions

• ATPD → BTPS, ATPS → BTPS, ATPS → STPD, STPD → BTPS — essential for spirometry & metabolic calculations.

Critical Temperature & Pressure

• Critical temperature (T_c): Highest T at which substance can remain liquid.
• Critical pressure (Pc): Pressure needed to maintain liquid at Tc.
• Pair (Tc, Pc) defines critical point → above which substance exists as supercritical fluid (relevant to O_2 storage & transfill operations).

Fluid Dynamics

• Hydrodynamics: Behavior of fluids in motion.
• Pressure drop along tube results from resistance (R): \Delta P = R \times \dot V (assuming linear laminar regime).

Flow Patterns

1. Laminar flow: Parallel streamlines; predicted by Poiseuille’s Law \Delta P = \frac{8 \eta l \dot V}{\pi r^4}
   • Minor radius changes dramatically affect ΔP (important in airway obstruction).
2. Turbulent flow: Chaotic eddies; governed by Reynolds number Re = \frac{\rho v d}{\mu}
   • Transition near Re \approx 2000 in smooth tubes.
3. Transitional flow: Mix of laminar & turbulent (common in upper airways).
4. Velocity vs. Cross-section (Continuity): In constant flow system, v \propto 1/A; as airway narrows, velocity ↑ (e.g., tracheal stenosis).

Bernoulli & Derived Effects

• Bernoulli Principle: ↑Velocity → ↓Static pressure + potential/internal energy (energy conservation in streamline flow).
• Venturi effect: When fluid passes through narrowing, pressure drop + velocity ↑; if downstream side open, surrounding fluid entrained (basis of Venturi masks, jet nebulizers).
  • Fluid entrainment: Low-pressure region draws in secondary gas/liquid.
• Fluidics / Coanda effect: Fluid jet attaches to adjacent surface; exploited in ventilator exhalation valves & neonatal CPAP generators.

Clinical & Ethical/Practical Implications

• Understanding gas laws prevents barotrauma & volutrauma when ventilator settings altered.
• Humidity/temperature control safeguards mucociliary function and patient comfort.
• Awareness of viscosity & Poiseuille emphasizes early treatment of airway narrowing (e.g., asthma) to avert steep pressure increases.
• Safe storage of medical gases respects critical points to avoid cylinder rupture.
• Venturi-based oxygen delivery provides precise FiO_2 ensuring equitable care for COPD patients (ethical obligation to avoid hyperoxia).