Dalton's Law and Atmospheric Gases

Dalton's Law

• Dalton's Law states that the total pressure exerted by a gas mixture equals the sum of the pressures exerted independently by each gas in the mixture.
• The pressure exerted by each gas (its partial pressure) is directly proportional to the percentage of that gas in the gas mixture.

Atmospheric Gases

• Earth's atmospheric gases include nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and trace gases like argon.
• Partial pressure of each gas is determined by multiplying the barometric pressure (PB) by the percentage of gas in the atmosphere.
• At sea level with a normal barometric pressure of 760 mmHg, the partial pressure of oxygen is approximately 159.6 mmHg, calculated as: Atmospheric P O2 = 0.21 × 760.
• The total atmospheric pressure is the collective sum of partial pressures of all atmospheric gases: P N2, P O2, and P C O2, and trace gases.
• As altitude increases, atmospheric pressure decreases, which causes the partial pressure exerted by each gas to decrease.
• The concentration of all atmospheric gases remains the same at both high and low elevations.
• The percentage of oxygen remains approximately 21 percent both at sea level and at the top of Mt. Everest (253 mmHg = Pb).

Pressure and Diffusion Gradients

• A pressure gradient involves the movement of gas from an area of high pressure/concentration to an area of low pressure/concentration.
• Pressure gradients are the primary mechanism responsible for moving air in and out of the lungs during ventilation.
• Each individual gas (e.g., N2, O2, CO2, trace gases) moves in the same direction, either in or out of the lungs.
• Gas diffusion involves the movement of individual gas molecules independently from an area of high pressure/concentration to an area of low pressure/concentration.

Partial Pressure of Gases

• Partial pressures of gases vary in dry air, alveolar gas, arterial blood, and venous blood.
• P O2 levels: Dry air at 159.0, alveolar gas at 100.0, arterial blood at 95.0, and venous blood at 40.0.
• P C O2 levels: Dry air at 0.2, alveolar gas at 40.0, arterial blood at 40.0, and venous blood at 46.0.
• PH2O (water vapor): 47.0 across alveolar gas, arterial blood, and venous blood.
• PN2 (and other gases in minute quantities): 573.0 across alveolar gas, arterial blood, and venous blood.
• Total pressure in the air and alveoli is 760.0, 755.0 in the Arterial Blood and 706.0 in Venous Blood.
• Water vapor pressure varies based on temperature, with 47.0 mm Hg at 37°C when absolute humidity is 44.0 mg/L.

Ideal Alveolar Gas Equation

• Alveolar oxygen tension (PAO2) can be computed using the ideal alveolar gas equation: PAO2 = [PB – PH2O] FIO2 – PaCO2 / 0.8

Fick's Law

• Fick's Law explains how gas diffusion happens and is expressed as: V gas = A.D.(P₁ - P₂)/ T
• The layers an oxygen molecule crosses in the alveolar capillary membrane are: liquid lining, alveolar epithelial cell, basement membrane of the alveolar epithelial cell, interstitial space, basement membrane of the capillary endothelium, capillary endothelium, plasma of the capillary blood, erythrocyte membrane, and intracellular fluid in the erythrocyte.
• The area (A) component of Fick's Law is verified when alveolar collapse or alveolar fluid decreases the ability of oxygen to enter pulmonary capillary blood.
• 'D' denotes the diffusion constant, determined by Henry and Graham's Law.
• P1 - P2 represents the difference in partial pressure of the gas between two points.
• Decreased alveolar oxygen pressure (PAO2 or P1) caused by high altitude or alveolar hypoventilation reduces diffusion of oxygen into pulmonary capillary blood.
• The thickness (T) factor is confirmed when increased alveolar tissue thickness caused by alveolar fibrosis or alveolar edema reduces movement of oxygen across alveolar-capillary membranes.
• The normal thickness of the alveolar capillary membrane is 0.36-2.5 microns.
• The normal pressure gradient for oxygen across the alveolar-capillary membrane is 60 mm Hg when breathing room air at sea level, and the normal carbon dioxide gradient is 6 mm Hg.

Henry's Law

• Henry's Law explains how gases dissolve in the alveoli and bloodstream during gas exchange.
• The amount of oxygen that dissolves into the bloodstream is directly proportional to the partial pressure of oxygen in alveolar air.
• Oxygen has a high tendency to dissolve into deoxygenated blood because the partial pressure of oxygen is higher in alveolar air than in deoxygenated blood.
• Carbon dioxide diffuses out of the solution and back into gaseous form, having a greater partial pressure in deoxygenated blood than in the alveolar air.

Graham's Law

• Graham's Law says that the diffusion of a gas through a liquid is directly proportional to the solubility coefficient and indirectly proportional to the square root of the weight of the gas.
• Carbon dioxide diffuses about 20 times faster than oxygen because C02 is highly soluble in water, and co2 has a faster diffusion rate
• The solubility of CO2 is 24 times more than oxygen regarding the AC membrane.
• The weight of oxygen is lighter than Co2.

Diffusion and Perfusion Limitation

• Perfusion-limited gas transfer across the alveolar wall depends on the amount of blood that flows past the alveoli.
• Diffusion-limited gas movement across the alveolar wall depends on the integrity of the alveolar-capillary membrane itself.