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Respiratory System Part 6

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Biomedical Physio & Kines
BPK 306
Mike Walsh

Respiratory Section #6 VI. ALVEOLAR VENTILATION-PERFUSION RELATIONS A. Regional Lung Ventilation Differences a. Can use radioactive xenon to detect how much ventilation is received by various parts of lung. Bottom of lung gets more ventilation than top b. Weight of lung causes greater pressures at lower lung. So intrapleural pressure is more positive at bottom of lung. c. Alveolar pressure equal in all parts of lung. Causes transpulmonary pressure to be less at bottom of lung. Lower transpulmonary pressure  alveoli less expanded (inflated) in lower lung regions at rest d. Not sure of these reasons for increased intrapleural pressure going down lung. Absolute values constant across mammalian species varying 10^4 in body size e. Lower resting volume and lower transpulmonary pressure puts bottom portions of lung lower and to the left. More compliance in this portion  for any inspirational pressure change, there will be greater volume change in lower lung regions B. Extreme Vrate Qrate Effects on Alveolar Gas Composition a. Conc’n of a substance in a mixing chamber is dependent on 2 factors: i. Rate of entry into chamber (gm/min) ii. Rate of removal from chamber (L/min) b. Entry of the substance into mixing chamber is analogous to ventilation in lungs c. Removal of substance from chamber is analogous to pulmonary BF d. Thus, conc’n of substance in lungs is related to VArate/Qrate (ventilation-perfusion relation: (gm/min)/(L/min) = gm/L) e. Pressure of gas (O2) in lung is dependent on gas entry rate into gas exchange area (VArate), and rate of gas entry out of lung (VO2rate) 1. No Ventilation a. Gas within alveoli (and also blood leaving alveolar cap bed) will have composition of mixed venous blood?? 2. No Perfusion a. No BF to alveolar space, then alveolar gas composition will be just like humidified room air. b. *Hyper ventilation always brings ur alveolar gas composition values closer to that of inspired air (important during exercise and at altitude) 3. Normal Alveolar Ventilation-Perfusion Relation a. Resultant alveolar gas composition (POw = 100 and PCO2 = 40 mm Hg) is an average of all the lung C. Normal VrateQrate Effects On Alveolar Gas Composition a. Body designed to enable decent VrateQrate relation throughout lung b. But it’s not perfect in all areas of lung i. Because percentage change of Vrate from top to bottom is much less than percentage change in BF
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