Des Jardins Chapter 4

1. Des Jardins Chapter 4 The Diffusion of Pulmonary Gasses

2. Introduction • Mechanics of ventilation only moves bulk amounts of air in and out of lungs • Next step in process of respiration: • Movement of gases across alveolar-capillary membrane (AC-membrane) • Process occurs by gas diffusion

3. Introduction • To fully appreciate gas diffusion, must understand: • Dalton’s law • Partial pressures of atmospheric gases • Fundamental differences between: • Pressure gradients • Which move gas in and out of lungs • Diffusion gradients • Which move gas across AC membrane

4. Gas Diffusion: Pressure Gradients versus Diffusion Gradients • Pressure gradient • Movement 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 ventilation • Each individual gas (e.g., N2, O2, CO2, trace gases) moves in same direction • Either in or out of lungs

5. Gas Diffusion: Pressure Gradients versus Diffusion Gradients • Gas diffusion • Movement 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 area

6. Gas Diffusion: Pressure Gradients versus Diffusion Gradients • Diffusion gradients • Individual gas partial pressure differences • Kinetic energy • Driving force responsible for diffusion

7. Gas Diffusion: Pressure Gradients versus Diffusion Gradients • Two different gases can move (diffuse) in opposite directions based on individual diffusion gradients • E.g., under normal circumstances, O2 diffuses from alveoli into pulmonary capillaries, while simultaneously CO2 diffuses from pulmonary capillaries into alveoli

8. Gas Diffusion: Pressure Gradients versus Diffusion Gradients • Diffusion of O2 and CO2 continues until partial pressures of O2 and CO2 are in equilibrium

9. Partial Pressure of Gases in the Air, Alveoli, and Blood Table 4-1.

10. Partial Pressure of Gases in the Air, Alveoli, and Blood 43.8 Table 4-2.

11. Partial Pressure of Oxygen and Carbon Dioxide • In Table 4-1, why is PO2 in the atmosphere (159) so much higher than the PO2 in the alveoli (100)?

12. Partial Pressure of Oxygen and Carbon Dioxide • In Table 4-1, why is PO2 in the atmosphere (159) so much higher than the PO2 in the alveoli (100)? • Answer: • Alveolar oxygen must mix—or compete, in terms of partial pressures—with alveolar CO2 pressure and alveolar water vapor pressure • PCO2 = 40 torr • PH2O = 47 torr

13. Ideal Alveolar Gas Equation • Clinically, alveolar oxygen tension (PAO2) can be computed from ideal alveolar gas equation or PAO2 = [PB – PH2O] FIO2 – PaCO2 0.8

14. Ideal Alveolar Gas Equation • If patient is receiving FIO2 of .40 on a day when barometric pressure is 755 mmHg and if PaCO2 is 55, then patient’s alveolar oxygen tension is:

15. Ideal Alveolar Gas Equation • Clinically, when PaCO2 is less than 60 mmHg and when patient is receiving oxygen, the following simplified equation may be used:

16. Oxygen and Carbon Dioxide Diffusion Across AC-Membrane • Normal gas pressure for O2 and CO2 as blood moves through AC-membrane Figure 4-4.

17. Gas Diffusion • Fick’s law .

18. Fick’s Law of Diffusion • The rate of diffusion across a sheet of tissue (the alveolar-capillary membrane) is: • Directly proportional to the • Surface area of the tissue • Solubility of the gas • Partial pressure gradient • Inversely proportional to the • Thickness of the tissue

19. Fick’s LawDiffusion is Directly Proportional to Surface Area • What is the surface area of the alveoli?

20. Fick’s LawDiffusion is Directly Proportional to Surface Area • A decreased alveolar surface area • Alveolar collapse • Fluid in the alveoli • Decreases the diffusion of oxygen into the pulmonary capillary blood

21. Fick’s LawDiffusion is Directly Proportional to the Concentration Gradient

22. Fick’s LawDiffusion is Directly Proportional to the Concentration Gradient • Decreased alveolar oxygen pressure (PAO2) • High altitudes • Alveolar hypoventilation • Decreases the diffusion of oxygen into the pulmonary capillary blood

23. Fick’s LawDiffusion is Inversely Proportional to Tissue Thickness

24. Fick’s LawDiffusion is Inversely Proportional to Tissue Thickness • An increased alveolar tissue thickness • Alveolar fibrosis • Pulmonary edema • Decreases the diffusion of oxygen into the pulmonary capillary blood

25. Mechanism of DiffusionFick’s First Law of Diffusion • The rate of diffusion across a sheet of tissue (the alveolar-capillary membrane) is: • Directly proportional to the • Surface area of the tissue • Solubility of the gas • Partial pressure gradient • Inversely proportional to the • Thickness of the tissue

26. Fick’s Law of Diffusion • The rate of diffusion across a sheet of tissue (the alveolar-capillary membrane) is: • Directly proportional to the • Surface area of the tissue • Solubility of the gas • Partial pressure gradient • Inversely proportional to the • Thickness of the tissue

27. Fick’s Law Figure 4-8.