Ch22 - Respiratory part2.pptx

Ch22 - Respiratory part2.pptx - Gas Exchange Physiology 1...

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Gas Exchange Physiology 1
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22.6 Gas Exchange Gas exchange occurs between lungs and blood as well as blood and tissues External respiration : diffusion of gases between blood and lungs Internal respiration : diffusion of gases between blood and tissues Both processes are subject to: Basic properties of gases Composition of alveolar gas 2
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Basic Properties of Gases Dalton’s law of partial pressures Total pressure exerted by mixture of gases is equal to sum of pressures exerted by each gas Partial pressure Pressure exerted by each gas in mixture Directly proportional to its percentage in mixture 3
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Basic Properties of Gases (cont.) Total atmospheric pressure equals 760 mm Hg Nitrogen makes up ~78.6% of air; therefore, partial pressure of nitrogen, P N 2 , can be calculated: 0.786 760 mm Hg 597 mm Hg due to N 2 Oxygen makes up 20.9% of air, so P O 2 equals: 0.209 760 mm Hg 159 mm Hg 4
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Partial Pressure Gradient (PPG) Gas exchange at pulmonary & systemic capillaries is via passive diffusion of O 2 & CO 2 due to PPG Partial pressure – individual pressure exerted by a particular gas within a mixture of gases (PO 2 & PCO 2 ) PPG – occurs when the PP of a gas differs across a membrane *****A gas will always diffuse from a higher PP to a lower PP** 5
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Basic Properties of Gases (cont.) Air also contains 0.04% CO 2 , 0.5% water vapor, and insignificant amounts of other gases At high altitudes, partial pressures declines, but at lower altitudes (under water), partial pressures increase significantly 6
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Basic Properties of Gases (cont.) Henry’s law For gas mixtures in contact with liquids: Each gas will dissolve in the liquid in proportion to its partial pressure At equilibrium, partial pressures in the two phases will be equal Amount of each gas that will dissolve depends on: Solubility : CO 2 is 20 more soluble in water than O 2 , and little N 2 will dissolve Temperature: as temperature of liquid rises, solubility decreases Example of Henry’s law: hyperbaric chambers 7
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External Respiration External respiration (pulmonary gas exchange) involves the exchange of O 2 and CO 2 across respiratory membranes Exchange is influenced by: Partial pressure gradients and gas solubilities Thickness and surface area of respiratory membrane Ventilation-perfusion coupling : matching of alveolar ventilation with pulmonary blood perfusion 8
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External Respiration (cont.) Partial pressure gradients and gas solubilities Steep partial pressure gradient for O 2 exists between blood and lungs Venous blood PO 2 = 40 mm Hg Alveolar PO 2 = 104 mm Hg Drives oxygen flow into blood Equilibrium is reached across respiratory membrane in ~0.25 seconds, but it takes red blood cell ~0.75 seconds to travel from start to end of pulmonary capillary Ensures adequate oxygenation even if blood flow increases 3 9
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External Respiration (cont.) Partial pressure gradient for CO 2 is less steep Venous blood P CO 2 = 45 mm Hg
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