Des Jardins Chapter 4

The Diffusion of Pulmonary GassesDes Jardins Chapter 4IntroductionMechanics of ventilation only moves bulk amounts of air in and out of lungsNext step in process of respiration:Movement of gases across alveolar-capillary membrane (AC-membrane)Process occurs by gas diffusionIntroductionTo fully appreciate gas diffusion, must understand:Daltons lawPartial pressures of atmospheric gasesFundamental differences between:Pressure gradientsWhich move gas in and out of lungsDiffusion gradientsWhich move gas across AC membraneGas Diffusion: Pressure Gradients versus Diffusion GradientsPressure gradientMovement of gas from area of high pressure (high concentration) to area of low pressure (low concentration)Primary mechanism responsible for moving air in and out of lungs during ventilationEach individual gas (e.g., N2, O2, CO2, trace gases) moves in same directionEither in or out of lungsGas Diffusion: Pressure Gradients versus Diffusion GradientsGas diffusionMovement of individual gas molecules from area of high pressure (high concentration) to area of low pressure (low concentration)Each individual gas (e.g., N2, O2, CO2) can continue to move independently from other gases from high-pressure area to low-pressure areaGas Diffusion: Pressure Gradients versus Diffusion GradientsDiffusion gradientsIndividual gas partial pressure differencesKinetic energyDriving force responsible for diffusionGas Diffusion: Pressure Gradients versus Diffusion GradientsTwo different gases can move (diffuse) in opposite directions based on individual diffusion gradientsE.g., under normal circumstances, O2 diffuses from alveoli into pulmonary capillaries, while simultaneously CO2 diffuses from pulmonary capillaries into alveoliGas Diffusion: Pressure Gradients versus Diffusion GradientsDiffusion of O2 and CO2 continues until partial pressures of O2 and CO2 are in equilibriumPartial Pressure of Gases in the Air, Alveoli, and Blood

Table 4-1.

Partial Pressure of Gases in the Air, Alveoli, and Blood

Table 4-2.

43.8Partial Pressure of Oxygen and Carbon DioxideIn Table 4-1, why is PO2 in the atmosphere (159) so much higher than the PO2 in the alveoli (100)?Partial Pressure of Oxygen and Carbon DioxideIn Table 4-1, why is PO2 in the atmosphere (159) so much higher than the PO2 in the alveoli (100)?Answer:Alveolar oxygen must mixor compete, in terms of partial pressureswith alveolar CO2 pressure and alveolar water vapor pressurePCO2 = 40 torrPH2O = 47 torrIdeal Alveolar Gas EquationClinically, alveolar oxygen tension (PAO2) can be computed from ideal alveolar gas equation

or

PAO2 = [PB PH2O] FIO2 PaCO2 0.8Ideal Alveolar Gas EquationIf patient is receiving FIO2 of .40 on a day when barometric pressure is 755 mmHg and if PaCO2 is 55, then patients alveolar oxygen tension is:

Ideal Alveolar Gas EquationClinically, when PaCO2 is less than 60 mmHg and when patient is receiving oxygen, the following simplified equation may be used:

Oxygen and Carbon Dioxide Diffusion Across AC-MembraneNormal gas pressure for O2 and CO2 as blood moves through AC-membrane

Figure 4-4.Gas DiffusionFicks law

.Ficks Law of DiffusionThe rate of diffusion across a sheet of tissue (the alveolar-capillary membrane) is:Directly proportional to theSurface area of the tissueSolubility of the gasPartial pressure gradientInversely proportional to the Thickness of the tissue18The bulk movement of a gas through a biological membrane is described by Ficks first law of diffusionGiven that the area of and distance across the alveolar capillary membrane are relatively constant in healthy people, diffusion in the normal lung mainly depends on gas pressure gradientsFicks LawDiffusion is Directly Proportional to Surface AreaWhat is the surface area of the alveoli?

19About 70 square meters

This provides a large surface area, ideal for gas exchangeFicks LawDiffusion is Directly Proportional to Surface AreaA decreased alveolar surface areaAlveolar collapseFluid in the alveoliDecreases the diffusion of oxygen into the pulmonary capillary blood20Ficks LawDiffusion is Directly Proportional to the Concentration Gradient

21Ficks LawDiffusion is Directly Proportional to the Concentration Gradient Decreased alveolar oxygen pressure (PAO2)High altitudesAlveolar hypoventilationDecreases the diffusion of oxygen into the pulmonary capillary bloodFicks LawDiffusion is Inversely Proportional to Tissue Thickness

23The alveolar-capillary membrane is less than one millionth of a millimeter thick, providing an optimal thickness for diffusion to occurFicks LawDiffusion is Inversely Proportional to Tissue ThicknessAn increased alveolar tissue thicknessAlveolar fibrosisPulmonary edemaDecreases the diffusion of oxygen into the pulmonary capillary bloodMechanism of DiffusionFicks First Law of DiffusionThe rate of diffusion across a sheet of tissue (the alveolar-capillary membrane) is:Directly proportional to theSurface area of the tissueSolubility of the gasPartial pressure gradientInversely proportional to the Thickness of the tissue25The bulk movement of a gas through a biological membrane is described by Ficks first law of diffusionGiven that the area of and distance across the alveolar capillary membrane are relatively constant in healthy people, diffusion in the normal lung mainly depends on gas pressure gradientsFicks Law of DiffusionThe rate of diffusion across a sheet of tissue (the alveolar-capillary membrane) is:Directly proportional to theSurface area of the tissueSolubility of the gasPartial pressure gradientInversely proportional to the Thickness of the tissue26The bulk movement of a gas through a biological membrane is described by Ficks first law of diffusionGiven that the area of and distance across the alveolar capillary membrane are relatively constant in healthy people, diffusion in the normal lung mainly depends on gas pressure gradientsFicks Law

Figure 4-8.