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Respiratory System Part 8

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Simon Fraser University
Biomedical Physio & Kines
BPK 306
Mike Walsh

Respiratory System # 8 VIII. GAS DIFFUSION IN THE PULMONARY SYSTEM A. Introduction a. Review Fick’s Law of Diffusion (can be for passive diffusion or transfer of gas through a tissue) i. Vgas proportional to AD(P1-P2)/T ii. Body capable of changing surface area and/or driving pressure differential iii. Due to difficulty measuring A and T, we simplify it to: 1. Vgasrate = DL*(P1-P2) a. DL is the diffusing capacity of lung in mL of gas transferred per min per mm Hg alveolar-arterial partial pressure gradient (mL/min/mmHg) B. The Pressure Gradient a. A gas diffuses down a pressure gradient. Not a conc’n gradient. It goes against a conc’n gradient C. Gas in Solution a. Total volume of gas in blood depends on its solubility in accordance with Henry’s Law : Gas Volume = Pressure*Solubility Coefficient D. O2 and CO2 Diffusion a. Concerned mainly with O2 and CO2 diffusion (examine CO, N2O and He as well) b. CO2 molecular weight is 44, O2 is 32 c. Solubility of CO2 is 24x that of O2 at 37 C in saline d. CO2 diffuses at 20x the rate of O2 E. Thickness or Barrier to Difstsion a. Already discussed in 1 lecture F. Interaction of O2 and Hemoglobin a. w/o Hb, blood can only carry 0.3 mL of O2 per 100 mL blood (at PO2 = 100 mm Hg) b. 1 gram of Hb carries betw 1.34 – 1.39 mL O2 c. W/ [Hb] = 15 gm/100 mL blood, blood increases O2 content to 20 mL O2 per 100 mL blood (67x increase). Due to: i. One molec Hb binds 4 O2 molecs ii. Bound O2 doesn’t exert a partial pressure (reduces PO2 in blood so there’s a bigger O2 gradient for blood to get more O2 from alveoli) G. O2 Dissociation Curve a. At normal PaO2 = 95 mm Hg, large variation of PO2 (due to VArate/Qrate differences for example) causes very little change in blood O2 content b. Sigmoidal shape c. During resting conditions, tissues extract about 5 mL O2 per 100 mL blood (19.5 down to 14.5 mL O2 per 100 mL blood). i. To extract this amount of O2, PO2 must be reduced to 40 mm Hg ii. Therefore, tissue O2
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