Properties of Gases & Gas Transport - DSA

What is Boyle’s Law and how does it describe movement of air?

• Boyle’s Law: At a constant temperature, pressure is inversely proportional to volume.

• Formula: P × V = constant.

• In the lung: As thoracic volume increases during inspiration → alveolar pressure decreases → air flows in.

• As thoracic volume decreases during expiration → alveolar pressure increases → air flows out.

What is Dalton’s Law and how is it applied to respiratory physiology?

• Dalton’s Law: The total pressure of a gas mixture = sum of partial pressures of individual gases.

• Partial pressure (Pgas) = Fractional concentration (Fgas) × Barometric pressure (PB).

• Example: At sea level (PB = 760 mmHg), PO₂ = 0.21 × 760 ≈ 160 mmHg.

• Must subtract water vapor pressure (47 mmHg at body temp) when calculating inspired PO₂ (PIO₂).

What are the fractional concentrations and sea level partial pressures for O₂, CO₂, and N₂?

• Fractional concentrations (Fgas):

  • O₂ = 0.21

  • CO₂ = 0.0003

  • N₂ = 0.79

• Sea level partial pressures (PB = 760 mmHg):

  • PO₂ = 160 mmHg

  • PCO₂ = 0.23 mmHg

  • PN₂ = 600 mmHg

What is water vapor pressure and why is it important?

• Water vapor pressure (PH₂O): The partial pressure exerted by water molecules in humidified inspired air.

• At body temperature (37°C), PH₂O = 47 mmHg.

• Must subtract this when calculating PIO₂ because inspired air is fully saturated with water vapor.

What are the normal airway, alveolar, arterial, and mixed venous PO₂ and PCO₂ values?

• Airway (humidified inspired): PO₂ ≈ 150 mmHg, PCO₂ ≈ 0 mmHg

• Alveolar (PAO₂, PACO₂): PO₂ ≈ 100 mmHg, PCO₂ ≈ 40 mmHg

• Arterial (PaO₂, PaCO₂): PO₂ ≈ 95 mmHg, PCO₂ ≈ 40 mmHg

• Mixed venous (PvO₂, PvCO₂): PO₂ ≈ 40 mmHg, PCO₂ ≈ 46 mmHg

Using Dalton’s Law, what are the PIO₂ and PAO₂ values at different barometric pressures?

• Miami (Sea level, PB = 760 mmHg): PIO₂ ≈ 150 mmHg, PAO₂ ≈ 95 mmHg

• Cleveland (PB = 747 mmHg): PIO₂ ≈ 149 mmHg, PAO₂ ≈ 92 mmHg

• Denver (PB = 640 mmHg): PIO₂ ≈ 134 mmHg, PAO₂ ≈ 77 mmHg

What is Henry’s Law and why is it important for gas diffusion? 

• Henry’s Law: The concentration of a gas dissolved in a liquid is proportional to its partial pressure and its solubility coefficient.

• Formula: Cgas = Pgas × Solubility.

• Importance: Explains how O₂ and CO₂ dissolve in blood plasma and diffuse across alveolar-capillary membranes.

What are the four determinants of gas diffusion according to Fick’s Law?

1. Surface area (A) available for diffusion

2. Thickness (Δx) of the barrier

3. Diffusion coefficient (D) (depends on solubility and molecular weight)

4. Partial pressure gradient (ΔP) across the barrier

• Formula: Vgas = (D × A × ΔP) / Δx

In what forms is O₂ carried in blood?

• Dissolved O₂ in plasma (small amount, measured by PaO₂)

• Bound to hemoglobin (HbO₂) (major form, ~98%)

• Total O₂ content = dissolved O₂ + Hb-bound O₂

In what forms is CO₂ carried in blood?

• Dissolved CO₂ (small fraction)

• Carbaminohemoglobin (CO₂ bound to Hb)

• Bicarbonate (HCO₃⁻) (major form, ~90%) via carbonic anhydrase reaction

How do gas partial pressures change as gases move through lungs and circulation?

• Inspired air: PO₂ ≈ 150, PCO₂ ≈ 0

• Alveoli: PO₂ ≈ 100, PCO₂ ≈ 40

• Arterial blood: PO₂ ≈ 95, PCO₂ ≈ 40

• Tissues (systemic capillaries): PO₂ ≈ 40, PCO₂ ≈ 46

• Mixed venous blood: PO₂ ≈ 40, PCO₂ ≈ 46

• Gradient drives O₂ diffusion into tissues and CO₂ diffusion into blood.