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Lecture

Diffusion and Pulmonary Circulation

2 Pages
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Department
Biology
Course Code
BIOL 2P93
Professor
Jenny Janke

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Diffusion & Pulmonary Circulation I. Diffusion a. Definitions i. Diffusion: process by which a net transfer of molecules takes place from zone of higher partial pressure to lower ii. Partial Pressure—P l = P x (fractional concentration of gas) partia total iii. Static Diffusion—reaches equilibrium w/ no net transfer of gases b. Fick’s Law  A  Vgas=  *D gas P1− P2 = flux_of _ gas T  i. D CO2= 20D ;O2 CO= D O2 c. Diffusing capacity (conductance) = 1/resistance D (gas) = (net rate of transfer) / (partial pressure gradient alv – cap) L i. Indicator of how well the lung can carry out diffusion d. Oxygen Diffusion i. Normally, capillary blood equilibrates completely w/ alveolar air even when capillary transit time is shorted by exercise ii. Diffusion defect—seldom causes systemic hypoxia at rest but may cause hypoxemia during exercise (less time for O2 equilibration) iii. Increasing PAO2 => increased gradient & better capillary oxygenation e. Measuring Diffusion Capacity i. Often use small amount of CO and look at diffusion during 10s breath hold (b/c mean partial pressure of CO in pulmonary cap. bl. is 0) ii. CO has same diffusivity as O2 => used as surrogate iii. Useful for measuring reductions in O2 diffusing capacity iv. Sensitive but non-specific for changes in gas exchange (anemia also => ↓) f. CO2 Diffusion i. Occurs 20x faster than O2 => never have clinical hypercapnia (elevation in arterial CO2 tension) due to diffusion defect II. Pulmonary Circulation (Raymond has removed an image blood pressures in various parts of the circulatory system.) a. Cardiac output: ranges from 5-25 L/min BUT pulmonary circulation rises minimally b/c pulmonary capillaries are distended/recruited to accommodate additional flow b. Pulmonary Vascular Resistance (PVR)—slope of pressure flow relation PVR = (Ppa – Pla) / cardiac output = pressure gradient across lung / CO c. Passive control of Pulmonary Circulation i. Lung volume alters PVR ii. Alveolar vessels—become smaller w/ lung expansion & become larger w/ lung deflation; mainly capillaries & arterioles iii. Extra-alveolar vessels—pulled open w/ lung expansion & pushed closed
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