Chapter 6 – Physical Principles of Respiratory Care

Learning Objectives

• Comprehensive overview includes the ability to:

  • Describe properties of gases, liquids, and solids.

  • Convert among the three most common temperature scales (Fahrenheit, Celsius, Kelvin).

  • Explain four heat-transfer mechanisms (conduction, convection, radiation, evaporation/condensation).

  • Detail liquid properties (pressure, buoyancy, specific gravity, viscosity) and their impact on flow.

  • Predict pressure changes inside liquids and tubes under varying conditions.

  • Relate surface tension to the radius of curvature and discuss its clinical significance.

  • Outline vaporization, condensation, and the energy exchanges involved.

  • Define absolute and relative humidity, dew-point, and factors that limit air’s water-holding capacity.

  • Apply Dalton’s and Henry’s laws to respiratory physiology and device function.

  • Use Boyle’s, Charles’, Gay-Lussac’s, and the combined gas law to forecast temperature–pressure–volume interactions.

  • Define critical temperature/pressure and recognize their importance for medical gases (e.g., liquid O₂).

  • Distinguish laminar, turbulent, and transitional flow; cite conditions that precipitate transitions.

  • Calculate pressure drops with Poiseuille’s law, identify the Venturi effect, and discuss entrainment applications.

States of Matter

• Three primary states in clinical contexts:

  • Solids

  • High internal order, fixed volume/shape.

  • Strong cohesive (attractive) forces; atoms perform minimal “jiggle”.

  • Liquids

  • Fixed volume but adopt container shape.

  • Weaker molecular attraction; surface shape dictated by internal & external forces.

  • Gases

  • No fixed volume/shape; very weak cohesive forces.

  • Molecules in rapid, random motion with frequent collisions.

• Plasma (fourth state)

  • Mix of neutral atoms, free electrons, and nuclei.

  • Responds to electromagnetic fields; generally irrelevant to bedside respiratory care.

Internal Energy of Matter

• All atoms possess internal energy, manifested as motion.

• Two forms:

  • Potential energy – energy of position (inter-molecular attraction). Dominant in solids/liquids; weak in gases.

  • Kinetic energy – energy of motion. Dominant contributor in gases.

Thermodynamics & Heat Transfer

• First law (energy conservation): Heat moves spontaneously from hotter ➜ cooler objects until equilibrium.

• Heat-transfer mechanisms

  • Conduction: direct molecular contact (dominant in solids).

  • Convection: bulk movement of fluid (liquid/gas); e.g., forced-air heating, blood circulation.

  • Radiation: electromagnetic waves; no direct contact needed.

  • Evaporation/Condensation: phase change-mediated heat exchange.

• Transitioning to lower entropy (cooling) requires removal of heat; vice-versa for warming.

Temperature & Absolute Zero

• Temperature quantifies average kinetic energy (molecular collisions).

• Absolute zero

  • Hypothetical point where kinetic energy vanishes.

  • Defined as 0\,\text{K} = -273\,^{\circ}\text{C} = -459.4\,^{\circ}\text{F}.

  • Unattainable per 3rd law of thermodynamics; experimentally approached but never reached.

Temperature Scales & Conversions

• Water-based reference points:

  • Freezing: 0^{\circ}\text{C} = 32^{\circ}\text{F}.

  • Boiling: 100^{\circ}\text{C} = 212^{\circ}\text{F}.

• Kelvin (SI) anchored to absolute zero.

• Conversions:

  • ^{\circ}\text{K} = ^{\circ}\text{C} + 273

  • ^{\circ}\text{C} = \frac{5}{9}(^{\circ}\text{F} - 32)

  • ^{\circ}\text{F} = \frac{9}{5} \, ^{\circ}\text{C} + 32

Change of State

• Melting/Freezing occur at identical temperatures (melting point = freezing point).

• Sublimation – direct solid ➜ vapor (e.g., dry ice, solid CO₂) when vapor pressure remains low.

Properties of Liquids

• Pressure: proportional to column height and weight density.

• Buoyancy: upward force because pressure beneath object > above; explains aerosol suspension.

• Specific Gravity: density ratio to reference (water).

• Viscosity

  • Resistance to flow; proportional to cohesive forces.

  • Blood ≈ 5× viscosity of water; ↑viscosity (polycythemia) increases cardiac workload.

• Cohesion vs. Adhesion

  • Cohesion: attraction between like molecules.

  • Adhesion: attraction between unlike molecules (liquid ↔ tube wall).

• Surface Tension

  • Force at liquid-air interface causes droplets/bubbles to assume spherical form.

  • Clinically counteracted by pulmonary surfactant to stabilize alveoli: P = \frac{2\gamma}{r} (Law of Laplace; smaller radius ➜ higher collapsing pressure without surfactant).

• Capillary Action

  • Upward movement in narrow tubes via combined adhesion + surface tension; used in wicking humidifiers and capnography water traps.

Liquid–Vapor Phase Changes & Humidity

• Boiling: vapor pressure equals/exceeds ambient pressure; liquid O₂ boils at -183^{\circ}\text{C}.

• Evaporation: sub-boiling vapor formation; consumes heat ➜ cools surrounding air.

• Absolute Humidity (AH)

  • Mass of water vapor per liter of air; units mg·L⁻¹.

  • Fully saturated air at 37^{\circ}\text{C} & 760\,\text{mmHg} holds 43.8\,\text{mg·L}^{-1} with water-vapor pressure 47\,\text{mmHg}.

• Relative Humidity (RH)

  • \%RH = \frac{\text{Absolute Humidity}}{\text{Saturated Capacity}} \times 100

  • Dew-point: temperature where gas becomes saturated (100 % RH) and condensation begins.

• Greater temperature or surface area ➜ greater evaporative rate (principle behind heated humidifiers & nebulizers).

Properties of Gases

• Kinetic Activity: random, high-speed collisions; velocity ∝ absolute temperature.

• Molar Volume @ STPD: One mole of an ideal gas occupies 22.4\,\text{L}.

• Density: mass ÷ volume; heliox lower density than air, useful for turbulent-flow airways.

• Diffusion: net movement high ➜ low concentration.

• Pressure Terminology

  • Total atmospheric pressure measured barometrically.

  • In liquids, gas pressure termed “tension.”

• Dalton’s Law

  • Partial pressure Pi = Fi \times P{\text{total}} where Fi is fractional concentration.

• Henry’s Law

  • Gas dissolved ∝ partial pressure above liquid at constant temperature; basis for oxygen loading in blood and hyperbaric therapy.

Gas Behavior Under Changing Conditions

• Boyle’s Law: P1 V1 = P2 V2 (T constant) – underpinning of negative-pressure ventilation & syringe mechanics.

• Charles’ Law: \frac{V1}{T1} = \frac{V2}{T2} (P constant).

• Gay-Lussac’s Law: \frac{P1}{T1} = \frac{P2}{T2} (V constant).

• Combined Gas Law: \frac{P1 V1}{T1} = \frac{P2 V2}{T2}.

• Assumptions (Ideal Gas Model):

  • Elastic collisions (no energy loss).

  • Molecular volume negligible relative to container.

  • No inter-molecular attractions.

• Water vapor must be subtracted (47 mm Hg @ 37 °C) when computing alveolar gas equations.

• Volume conversions routinely needed:

  • ATPD, ATPS, BTPS, STPD ➜ use standardized correction factors in pulmonary diagnostics.

Critical Temperature & Pressure

• Critical Temperature (T_c): highest T at which substance can remain liquid regardless of pressure.

• Critical Pressure (Pc): pressure needed to liquefy gas at Tc.

• Together define critical point; e.g., CO₂ has T_c = 31^{\circ}\text{C} enabling storage in pressurized cylinders as liquid.

Fluid Dynamics

Pressure & Resistance in Tubes

• Flowing fluid loses pressure due to resistance; \Delta P = R \times \dot{V} (Ohm-like relationship).

Flow Patterns

• Laminar: concentric streamlines; Poiseuille’s Law gives pressure requirement: \Delta P = \frac{8 \eta l \dot{V}}{\pi r^4} where \eta=viscosity, l=length, r=radius.

• Turbulent: chaotic, eddy currents; pressure relates to square of flow, predicted by Reynolds number

  • Re = \frac{\rho v d_R}{\mu}.

  • Transition near Re \approx 2000 in smooth tubes.

• Transitional: mix of laminar & turbulent (common at airway bifurcations).

Flow, Velocity, Cross-Sectional Area

• Flow (L·min⁻¹) = Velocity (cm·s⁻¹) × Cross-sectional area (cm²).

• Law of Continuity: As total cross-sectional area increases (bronchial tree), linear velocity decreases; vital for gas exchange time.

Bernoulli Principle & Venturi Effect

• Increased velocity through constriction ➜ decreased lateral (static) pressure.

• Venturi: pressure drop can draw (entrain) secondary fluid if opening present; forms basis of air-entrainment masks and nebulizers.

Fluid Entrainment & Coanda Effect

• High-velocity jet + low pressure area forms suction: utilized in jet nebulizers and aspirators.

• Coanda Effect: jet attaches to nearby wall, enabling flip-flop valves in fluidic ventilator circuits.

Clinical & Practical Implications

• Understanding gas laws underpins ventilator settings, cylinder storage, hyperbaric therapy, and aerosol generation.

• Viscosity & surface tension principles explain increased work of breathing in diseases (e.g., ARDS with surfactant deficiency).

• Humidity management (37 °C, 44 mg L⁻¹, 100 % RH) is crucial to maintain mucociliary function and prevent airway drying.

• Flow dynamics guide selection of devices (heliox to reduce turbulence, Venturi masks for precise FiO₂, heated humidifiers to augment vapor capacity).