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Section II Respiratory Gases Exchange

Section II Respiratory Gases Exchange. I Physical Principles of Gas Exchange. Partial pressure The pressure exerted by each type of gas in a mixture Diffusion of gases through liquids Concentration of a gas in a liquid is determined by its partial pressure and its solubility. P b.

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Section II Respiratory Gases Exchange

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  1. Section II Respiratory Gases Exchange

  2. I Physical Principles of Gas Exchange

  3. Partial pressure • The pressure exerted by each type of gas in a mixture • Diffusion of gases through liquids • Concentration of a gas in a liquid is determined by its partial pressure and its solubility

  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 PN = 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. Transport of gases between the alveoli and (pulmonary) capillaries and eventually from the capillaries to the tissues • diffusion dependent on perfusion and the partial pressure (pp) exerted by each gas • gases diffuse from area of  conc. (pp) to  conc. (pp)

  8. Diffusion • concentration pp of gas diffusion • CO2 more soluble than O2, therefore it diffuses faster

  9. 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

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

  11. Oxygen and Carbon Dioxide Diffusion Gradients • Oxygen • Moves from alveoli into blood. • Blood is almost completely saturated with oxygen when it leaves the capillary • P02 in blood decreases because of mixing with deoxygenated blood • Oxygen moves from tissue capillaries into the tissues • Carbon dioxide • Moves from tissues into tissue capillaries • Moves from pulmonary capillaries into the alveoli

  12. 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.

  13. PO2 and PCO2 in Blood

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

  15. 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

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

  17. 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

  18. Matching 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”

  19. 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

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

  21. 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

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

  23. IVFactors Affecting the Gas Diffusion in the Lung

  24. The Properties of the Gas • Molecular weight. Diffusion rate is inversely proportional to the square root of the molecular weight • Temperature • Solubility in water • Each gas has a specific solubility O2 Solubility coefficient = 0.003 ml 02/100 ml Blood • C02 = 0.06 ml/100 ml Blood (x 20 of 02) PO2 100 mm HG Saturated very quickly Reserve diffusive Capacity of the lung 45 40 PCO2 0 0.75 sec Time

  25. 2. Partial Pressure of the Gases • Alveoli ventilation • Blood perfusion in the lung capillary • Speed of the chemical reaction • The slow speed of the chemical reaction HCO3- + H+ ----- H2CO3 ---H2O + CO2 reduces the CO2 exchange in the lung. • So, during the gas exchange in the external respiration, the exchange of CO2 is a little lower than that of O2.

  26. 3. Properties of the Lung • Area of the respiratory membrane • Distance of the diffusion • VA/Qc

  27. 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.

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

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

  30. 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

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

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