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Part 3 Respiratory Gases Exchange

Part 3 Respiratory Gases Exchange. I Physical Principles of Gas Exchange. 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. P b.

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Part 3 Respiratory Gases Exchange

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  1. Part 3 Respiratory Gases Exchange

  2. I Physical Principles of Gas Exchange

  3. 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

  4. Pb In a mixture of gases, each gas exerts a partial pressure proportional to its mole fraction. Total Pressure = sum of the partial pressures of each gas Total Pressure (at sea level) Pbarometric = 760 mm Hg 760 mmHg Pb Partial Pressures of Gases • Basic Composition of Air • 79% Nitrogen • 21% Oxygen • ~ 0% Carbon Dioxide Pgas = Pb x Fgas PN2 = 760 x 0.79 = 600.4 mm Hg P02 = 760 x 0.21 = 159.6 mm Hg

  5. Consider a container of fluid in a vacuum That is opened to the air Molecules of gas begin to enter the fluid Partial Pressure of Gases in Fluids Each gas has a specific solubilityO2 Solubility coefficient = 0.003 ml/100 ml Blood C02 = 0.06 ml/100 ml Blood (x 20 of 02) Gases dissolve in fluids by moving down aPartial Pressure gradient rather than a concentration gradient

  6. After a short time, the number of molecules the number of molecules ENTERING = LEAVING Partial Pressure of Gases in Fluids At equilibrium, if the gas phase has a PO2 = 100 mm Hg, the liquid phase also has a PO2 = 100 mm Hg An easy way to talk about gases in fluids.

  7. Alveolus Blood capillary Time for exchange PO2 100 mm Hg Saturated very quickly Reserve diffusive Capacity of the lung 45 40 PCO2 0 0.75 sec Time Diffusion: Blood Transit Time in the Alveolus

  8. II Gas exchange in the lung and in the tissue

  9. Partial pressure (mmHg) % in Dry Alveolar Venous DiffusionGas dry air air air blood gradient 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 Diffusion Gradients of Respiratory Gases at Sea Level NB. CO2 is ~20x more soluble than O2 in blood => large amounts move into & out of the blood down a relatively small diffusion gradient.

  10. PO2 and PCO2 in Blood

  11. III. A-a gradient, the efficiency of the gas exchange in alveoli

  12. What is an A - a gradient ? The DIFFERENCE between: Oxygen Content in Alveolus Gas (measured during exhalation) Oxygen Content in arterial blood (equivalent to that leaving lungs) In a healthy person, what would you expect the A - a to be? No difference, greater than 0, or less than 0 Normal: A – a, up to ~ 10 mm Hg, varies with age

  13. Factors contributing to A - a Gradient • Blood Shunts • Blood Mixing • Matching

  14. AIR FLOW Alveolar SPACE CO2 O2 Blood BLOOD FLOW arterial vessel Mixing Lowered O2/l00 ml SIMPLE CONCEPT OF A SHUNT No Gas Exchange = SHUNT

  15. Matching Important On the test Blood to Air Flow Matching What? Total Ventilation Oxygen Exchange Total Perfusion, Q NEXT NEW CONCEPT If the volumes used for exchange are aligned – We might consider the system to be “ideally matched”

  16. Dead Air Space (Airways) Alveolar Ventilation (VA) Oxygen Exchange Arterial Perfusion (Qc) Slide or Misalign the distribution volumes Shunt (Qs) (Bronchial Artery) Some Volumes are wasted, Matching Ratio = VA/Qc = 0.8 Normal Case; Small Shunt, low volume Dead Space Matching

  17. Matching ventilation & perfusion • Ventilation and perfusion (blood flow) are both better at the bottom (base) of the lung than that at the top (apex). • the change in blood flow is more steep than in ventilation. • the ventilation/perfusion ratio rises sharply from the base to the apex.

  18. Matching ventilation & perfusion (cont) Result: V/Q is greater or less than 0.8 in different regions If V/Q <0.8 = shunt like, If V/Q > 0.8 little benefit, Increases A - a gradient

  19. Dead Air Space Alveolar Ventilation VA Oxygen Exchange blood mixing Arterial Perfusion Q Shunt Severe Mismatch = Lung Disease with a Large A – a gradient

  20. IVFactors Affecting the Gas Diffusion in the Lung • Area of the respiratory membrane • Distance of the diffusion • VA/Q

  21. V Pulmonary Diffusion Capacity Concept: The ability of the respiratory membrane to exchange a gas between the alveoli and the pulmonary blood defined as the volume of a gas that diffuses through the membrane each minute for a pressure of 1 mmHg. DL = V/(PA – PC) V is a gas that diffuses through the membrane each minute, PA is the average partial pressure of a gas in the air of alveoli, PC is the average partial pressure of a gas in the blood of pulmonary capillary.

  22. Factors Affecting the DL • Body posture • Body height and weight • Exercise • Pulmonary diseases

  23. VI Internal Respiration • All cells require oxygen for metabolism • All cells require means to remove carbon dioxide • Gas exchange at cellular level

  24. Concept: Gas exchange between the capillary and the tissues throughout the body • Process: • Factors affecting the internal respiration: • Distance between the cells and the capillary • Rate of metabolic rate • Speed of the blood flow in capillary

  25. EXTERNAL AND INTERNAL RESPIRATION TISSUE CELL O2 + FOOD ATMOSPHERE SYSTEMIC CIRCULATION HEART PULMONARY CIRULATION LUNGS CO2 + H2O + ATP

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