Gas Exchange
Energy and Temperature
Gases have weak molecular forces, constant random motion, and no fixed volume or shape. Internal energy is composed of Potential energy (position) and Kinetic energy (motion). Kinetic energy is the primary focus — gases exist in constant random kinetic movement. Temperature measures the energy released as heat.
Three temperature scales are used clinically: Fahrenheit (°F), Celsius (°C), and Kelvin (K). Kelvin is used in ALL gas laws. Conversion formulas:
- C = 5/9(F − 32)
- K = C + 273
- C = K − 273
- F = (9/5 × C) + 32
STPD (Standard Temperature Pressure Dry): 0°C, 760 mmHg, 1 mol = 22.4 L. BTPS (Body Temperature Pressure Saturated): 37°C, 760 mmHg, 1 mol = 27.11 L.
Gas Pressure
All gases exert pressure. Pascal's Principle: P = F/A — a gas or liquid exerts pressure equally in all directions within a container. "Tension" refers to the pressure of dissolved gases. Pressure depends on kinetic activity and gravity.
Units: Pa (Pascal), psi (pounds per square inch), mmHg (millimeters of mercury), atm (atmospheres), Torr (1 mmHg = 1 Torr). A Barometer measures atmospheric pressure. Standard atmospheric pressure = 760 mmHg.
Archimedes' Principle: Buoyancy applies in both liquids and gases (relevant to aerosol generators). Poiseuille's Law: ΔP = 8ηL/πr⁴ — the driving pressure gradient (ΔP) is required for gas flow. Halving the radius increases required pressure by 16×.
Viscosity is the force opposing flow. Laminar flow uses discrete streamlines and requires less pressure. Turbulent flow creates irregular eddy currents and requires more pressure. Flow type is determined by density, viscosity, velocity, and tube radius.
Volume and Weight
Avogadro's Law: Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. 1 mol = 6.023 × 10²³ molecules. At STPD, molar volume = 22.4 L.
Density = Mass / Volume. O₂ (GMW 32): density = 32/22.4 = 1.43 g/L. N₂ (GMW 28): density = 28/22.4 = 1.25 g/L.
Atmospheric air composition: N₂ 78%, O₂ 21%, Ar 0.9%, others 0.1%. Density of air = (1.43 × 0.21) + (1.25 × 0.78) = 0.30 + 0.99 = 1.29 g/L.
Clinical application — Heliox therapy: 30% O₂ + 70% He. O₂ density contribution = (32/22.4) × 0.30, He contribution = (4/22.4) × 0.70.
Ideal Gas Law
PV = nRT. R = 0.0821 L·atm/mol·K. Applies when n (number of moles) is constant.
Boyle's Law: P₁V₁ = P₂V₂ — inverse relationship, temperature constant. Clinical application: Compliance ΔV/ΔP, PFT lung volumes.
Charles' Law: V₁/T₁ = V₂/T₂ — direct relationship, pressure constant. Hot air balloon example. Converts ATPS → BTPS.
Gay-Lussac's Law: P₁/T₁ = P₂/T₂ — direct relationship, volume constant.
Combined Gas Law: P₁V₁/T₁ = P₂V₂/T₂.
Gas Transportation
Blood is a liquid connective tissue composed of plasma and cells. RBCs (erythrocytes): biconcave discs, 7–8 μm, no nucleus (more surface area), no mitochondria (anaerobic ATP — don't consume O₂ they carry).
Hemoglobin: Globin (4 chains: 2α, 2β) + Heme (iron Fe²⁺ at center). Each Hb binds 4 O₂ molecules.
O₂ transport: 98.5% as oxyhemoglobin, 1.5% dissolved. CO₂ transport: 23% as carbaminohemoglobin, 7% dissolved, 70% as bicarbonate (HCO₃⁻).
Hb also buffers H⁺ ions. Cooperative interaction: threshold binding and dissociation.
Hemoglobin species: Methemoglobin (metHB) — Fe³⁺ cannot bind O₂. Carboxyhemoglobin (HbCO) — CO has 200× greater affinity for Hb than O₂.
Gas Diffusion
Diffusion = movement from high → low concentration. Kinetic energy drives it. Graham's Law: diffusion rate ∝ 1/√(molecular weight) — lighter gases diffuse faster. Heating and agitation speed diffusion.
Dalton's Law: Total pressure = sum of partial pressures. Each gas exerts its own pressure independently. Pgas = Gas% × pB. Partial pressure determines gas movement across membranes.
Oxygen cascade: Movement from atmosphere → lungs → blood → tissues.
Henry's Law: Volume of gas dissolved = solubility coefficient × partial pressure. Solubility coefficient: O₂ in plasma = 0.023 mL/dL at 37°C/760 torr; CO₂ = 0.510 mL/dL. High temperature ↓ solubility, Low temperature ↑ solubility.
Gas Exchange
External respiration (pulmonary): O₂ moves from alveoli → blood, CO₂ from blood → alveoli. Internal respiration (systemic): O₂ from blood → tissues, CO₂ from tissues → blood.
O₂ exchange: Alveolar PO₂ ~100 mmHg > blood PO₂ ~40 mmHg in pulmonary capillaries; systemic capillaries PO₂ ~100 mmHg > tissue PO₂ ~40 mmHg.
CO₂ exchange: Pulmonary blood PCO₂ ~45 > alveolar PCO₂ ~40; tissue PCO₂ ~45 > arterial PCO₂ ~40.
Hypoxia (cellular low O₂) vs Hypoxemia (blood low O₂). Types:
- Hypoxic hypoxia — low PaO₂
- Anemic hypoxia — low hemoglobin
- Ischemic hypoxia — low flow (normal PaO₂ and Hb)
- Histotoxic hypoxia — cellular toxicity
Four factors affecting gas exchange rate: (1) Partial pressure difference, (2) Surface area, (3) Diffusion distance, (4) Molecular weight and solubility.