Gas Exchange and Gas Laws in Respiration

• Gas exchange in the respiratory system involves the diffusion of O2 and CO2 within the lungs and peripheral tissues
• O2 moves from alveolar gas to pulmonary capillary blood and then to tissues, diffusing from systemic capillary blood into cells
• CO2 is transported from tissues to venous blood, then to pulmonary capillary blood, and finally to alveolar gas for expiration

Gas Laws

• The mechanisms of gas exchange rely on the fundamental properties of gases and their behavior in solution
• The general gas law states that the product of pressure and volume of a gas equals the number of moles times the gas constant times the temperature: PV = nRT
• P = Pressure (mm Hg), V = Volume (L), n = Moles (mol), R = Gas constant, T = Temperature (K)
• Respiratory physiology uses BTPS conditions for the gas phase and STPD conditions for the liquid phase
• BTPS includes body temperature (37°C or 310 K), ambient pressure, and gas saturation with water vapor
• STPD includes standard temperature (0°C or 273 K), standard pressure (760 mm Hg), and dry gas, and applies to gases dissolved in blood
• Gas volume at BTPS can be converted to STPD by multiplying the volume at BTPS by 273/310 x (Pb – 47)/760, where Pb is barometric pressure and 47 mm Hg is water vapor pressure at 37°C
• Boyle's Law states that at a given temperature, the product of pressure and volume for a gas remains constant: P1V1 = P2V2
• During inspiration, when the diaphragm contracts and lung volume increases, gas pressure in the lungs decreases to maintain a constant pressure-volume product, which drives airflow into the lungs

Dalton's Law of Partial Pressures

• The partial pressure of a gas in a mixture is the pressure it would exert if it occupied the total volume alone
• Partial pressure equals total pressure multiplied by the fractional concentration of dry gas: Px = PB x F
• For humidified gas, the relationship is determined by correcting the barometric pressure for water vapor pressure: PX=(PB-PH2O)×F
• PX = Partial pressure of gas (mm Hg), PB = Barometric pressure (mm Hg), PH2O = Water vapor pressure at 37°C (47 mm Hg), F = Fractional concentration of gas (no units)
• The sum of partial pressures of all gases in a mixture equals the total pressure of the mixture
• Percentages of gases in dry air at 760 mm Hg are: O2, 21% (0.21); N2, 79% (0.79); CO2, 0% (0.0004), with water vapor pressure being 47 mm Hg at 37°C
• CX=PX×Solubility, where CX is the concentration of dissolved gas (ml gas per 100 ml of blood), PX is the partial pressure of the gas (mm Hg), and Solubility is the solubility of the gas in blood (ml gas/100 ml blood per mm Hg)
• This formula calculates the volume percent of gas per 100 ml of blood, only considering the free dissolved gas

Fick's Law

• The transfer of gases across cell membranes or capillary walls occurs by simple diffusion
• The rate of transfer by diffusion (Vx) is directly proportional to the driving force, diffusion coefficient, and surface area, and inversely proportional to the membrane thickness
• Vx = (D x A x ΔP) / Δx, where Vx is the volume of gas transferred per unit time, D is the diffusion coefficient, A is the surface area, ΔP is the partial pressure difference, and Δx is the membrane thickness
• Lung diffusing capacity (Dl) combines the diffusion coefficient, surface area, and membrane thickness
• Dl also accounts for the time required for gas to combine with proteins in pulmonary capillary blood
• Dl can be measured using carbon monoxide (CO), where CO transfer is limited by the diffusion process
• During a single breath method the subject breathes a gas mixture containing a low concentration of CO; the rate of disappearance of CO from the gas mixture is proportional to Dl
• In emphysema, Dl decreases due to decreased surface area for gas exchange
• In fibrosis or pulmonary edema, Dl decreases due to increased diffusion distance
• In anemia, Dl decreases because of reduced hemoglobin in red blood cells
• During exercise, Dl increases because additional capillaries are perfused with blood

Overview of gas exchange mechanisms in the respiratory system, focusing on O2 and CO2 diffusion. Includes the general gas law (PV = nRT) and its application in respiratory physiology. Covers BTPS and STPD conditions.