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Physical principles of gas exchange
- 1. Physical Principles of Gas Exchange Dr. Sara Sadiq (MBBS, M.Phil)
- 4. • Partial pressure – The pressure exerted by each type of gas in a mixture • Concentration of a gas in a liquid – determined by its partial pressure and its solubility coefficient
- 5. Physics of Gas Diffusion and Gas Partial Pressure • Gas Pressures in a Mixture of Gases- a. Pressure is directly proportional to the concentration of all the gas molecules b. Rate of diffusion of each gas is directly proportional to the pressure caused by that gas alone (called the partial pressure)
- 6. Net Diffusion • Net diffusion = Difference between two partial pressures • Partial pressure greater in gas phase in alveoli more diffusion into blood (O2) • Partial pressure greater in dissolved state in blood more diffusion into alveoli (CO2) • Net diffusion from high-pressure area towards low pressure area
- 7. Partial Pressures of Gases • Basic Composition of Air • 79% Nitrogen • 21% Oxygen • ~ 0% Carbon Dioxide
- 8. Solubility of Gases in Fluids • Gases dissolve in fluids by moving down a Partial Pressure gradient rather than a concentration gradient
- 9. Partial Pressure of Gases in Fluids After a short time, the number of molecules the number of molecules ENTERING = LEAVING At equilibrium, if the gas phase has a PO2 = 100 mm Hg, the liquid phase also has a PO2 = 100 mm Hg
- 10. Henry’s Law Partial pressure = Concentration of dissolved gasses Solubility coefficient Solubility Coefficient 0.024Oxygen 0.57Carbon dioxide 0.018Carbon monoxide 0.012Nitrogen 0.008Helium
- 11. Factors Affecting Rate of Diffusion • Solubility • Cross-sectional area • Distance through which gas must diffuse • Molecular weight • Temperature
- 12. Diffusion Gradients of Respiratory Gases at Sea Level Total 100.00 760.0 760 760 0 H2O 0.00 0.0 47 47 0 O2 20.93 159.1 105 40 65 CO2 0.03 0.2 40 46 6 N2 79.04 600.7 569 573 0 Partial pressure (mmHg) % in Dry Alveolar Venous Diffusion Gas dry air air air blood gradient NB. CO2 is ~20x more soluble than O2 in blood => large amounts move into & out of the blood down a relatively small diffusion gradient.
- 13. 1. Alveolar air is partially replaced by atmospheric air 2. Oxygen is being absorbed into pulmonary blood 3. CO2 is constantly diffused 4. Dry atmospheric air is humidified before it reaches alveoli
- 14. Rate at Which Alveolar Air is Renewed by Atmospheric Air
- 15. Importance of Slow Replacement of Alveolar Air • Preventing sudden change in gas concentration in blood • Maintain respiratory control mechanism • Prevent excessive increase or decrease in tissue O2, CO2, or pH when respiration is temporarily interrupted
- 17. Oxygen concentration in the alveoli, as well as partial pressure, is controlled by 1. The rate of absorption of oxygen into the blood 2. The rate of entry of new oxygen into the lungs by ventilation Oxygen Concentration and Partial Pressure in the Alveoli
- 18. Effect of alveolar ventilation on the alveolar PO2 at two rates of oxygen absorption from the alveoli
- 20. Expired Air is a Combination of Dead Space Air and Alveolar Air
- 21. Oxygen Content in Alveolus Gas (measured during exhalation) Oxygen Content in arterial blood (equivalent to that leaving lungs) What is an A - a gradient ? The DIFFERENCE between: Normal: A – a, up to ~ 10 mm Hg, varies with age
Editor's Notes
- kinetic motion of the molecules Effect of a Concentration Gradient-
- Consider a container of fluid in a vacuum, That is opened to the air. Molecules of gas begin to enter the fluid
- CO2 20 times more soluble so exerted partial pressure is one-twentieth time less than O2
- 760 mmHg N2 79% (600) and O2 21% (159)
- Atmospheric air mainly contain N2 and O2 but when enters into resp passage, get humidified and PO2 decreases from 159 to 149 mmHg
- FRC = 23OO ml yet only 350 ml of new air is brought into alveoli with each normal inspiration 1st breath excess gas is present, even at the end of 16th breath excess gas still has not been completely removed from alveoli
- In normal alveolar ventilation ½ gas is removed in 17 sec Slow rate of alveolar ventilation ½ gas is removed in 34 sec Twice the rate of alveolar ventilation ½ gas is removed in 8 sec
- At normal ventilator rate of 4.2L/min and an O2 consumption of 250ml/min, the normal operating point is A During exercise when 1000ml of O2 is absorbed per min , the rate of alveolar ventilation increases four-fold to maintain alveolar PO2 at 104 mmHg Another effect shown in Figure 40-4 is that even an extreme increase in alveolar ventilation can never increase the alveolar PO2 above 149 mm Hg as long as the person is breathing normal atmospheric air at sea level pressure, because 149 mm Hg is the maximum PO2 in humidified air at this pressure.
- The first portion of this air, the dead space air from the respiratory passageways, is typical humidified air, as shown in Table 40-1. Then, progressively more and more alveolar air becomes mixed with the dead space air until all the dead space air has finally been washed out and nothing but alveolar air is expired at the end of expiration. Therefore, the method of collecting alveolar air for study is simply to collect a sample of the last portion of the expired air after forceful expiration has removed all the dead space air.