Gas Exchange and Gas Laws

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

Gas Laws

• Gas exchange relies on the fundamental properties of gases, including their behavior in solution
• The general gas law states that the product of pressure and volume equals the number of moles times the gas constant times temperature, PV = nRT
• P = Pressure in mm Hg, V = Volume in Liters, n = Moles, R = Gas constant, T = Temperature in Kelvin
• Respiratory physiology uses BTPS (body temperature, ambient pressure, saturated with water vapor) for the gas phase and STPD (standard temperature, pressure, dry) for the liquid phase
• STPD includes standard temperature (0°C or 273 K), standard pressure (760 mm Hg), and dry gas
• Converting gas volume from BTPS to STPD involves 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
• Inspiration involves diaphragm contraction, increasing lung volume and decreasing gas pressure

Dalton's Law

• Dalton's Law of Partial Pressures states that the partial pressure of a gas in a mixture is the pressure that gas would exert if it occupied the total volume
• Partial pressure equals total pressure multiplied by the fractional concentration of dry gas, Px = PB x F
• PX = Partial pressure of gas in mm Hg, PB = Barometric pressure in mm Hg, PH2O = Water vapor pressure at 37°C which is 47mm Hg, F = Fractional concentration of gas
• Barometric pressure (PB) sums the partial pressures of O2, CO2, N2, and H2O
• Gases in dry air at 760 mm Hg: O2 is 21% (0.21), N2 is 79% (0.79), CO2 is 0% (0.0004)
• Airway humidification results in water vapor pressure of 47 mm Hg at 37°C
• CX = PX x Solubility, where CX is the dissolved gas concentration, PX is the partial gas pressure, and Solubility is the gas's solubility in blood

Diffusion of Gases (Fick's Law)

• Gas transfer across cell membranes or capillary walls occurs via 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 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

• Lung diffusing capacity (Dl) combines the diffusion coefficient, surface area, and membrane thickness
• Dl accounts for the time it takes for gas to combine with proteins in pulmonary capillary blood, like O2 binding to hemoglobin
• Dl is measured using carbon monoxide (CO), due to the CO transfer across the alveolar/pulmonary capillary barrier is diffusion-limited
• Dl decreases in emphysema due to decreased surface area, in fibrosis or pulmonary edema due to increased diffusion distance, and in anemia due to reduced hemoglobin
• Dl increases during exercise due to additional capillaries being perfused with blood

Gas Transport Forms

• In alveolar air, gas exists as partial pressure however in blood, gases are dissolved, bound to proteins, or chemically modified
• Total gas concentration in solution is the sum of dissolved, bound, and chemically modified gas
• Henry's Law states that at a given partial pressure, higher solubility means higher concentration
• Only dissolved gas molecules contribute to partial pressure
• Nitrogen (N2) is only carried in dissolved form, and because it is never bound or chemically modified, it is used for respiratory physiology measurements
• O2 and CO bind to hemoglobin inside red blood cells (O2 and CO) and in plasma proteins (CO2)
• Most CO2 is carried as bicarbonate (HCO3-) in red blood cells via carbonic anhydrase

Gas Transport in the Lungs

• Pulmonary capillaries are perfused with mixed venous blood from the right heart
• Gas exchange occurs: O2 diffuses from alveolar gas into pulmonary capillary blood, and CO2 diffuses from pulmonary capillary blood into alveolar gas
• Blood leaving the pulmonary capillary enters the left heart and becomes systemic arterial blood
• In dry inspired air, oxygen partial pressure (P_O2) is about 160 mm Hg (21% of 760 mm Hg)
• Airway humidification dilutes oxygen, resulting in P_O2 of about 150 mm Hg (after subtracting water vapor pressure)
• Alveolar air has lower P_O2 (100 mm Hg) and higher P_CO2 (40 mm Hg), due to gas exchange
• Blood entering pulmonary capillaries is mixed venous blood with lower oxygen and higher carbon dioxide (P_O2 ≈ 40 mm Hg, P_CO2 ≈ 46 mm Hg)
• Oxygenated blood leaving pulmonary capillaries is arterial blood with higher oxygen (P_O2 ≈ 100 mm Hg)
• A physiologic shunt creates a slight difference between oxygen levels in alveolar air and systemic arterial blood, which can include any bronchial and coronary venous blood.

Difference in Lung Diseases

• In lung diseases, the shunt increases, causing disparity in gas exchange
• The A-a gradient, the difference in oxygen between alveolar air and arterial blood, indicates inefficiency in oxygen transfer in the lungs

Explore gas exchange in the respiratory system, focusing on O2 and CO2 diffusion in the lungs and tissues. Understand the general gas law (PV=nRT) and its application in respiratory physiology. Learn about BTPS and STPD conditions and their significance in gas volume conversion.