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Gas Exchange Notes

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Lecture 6 Gas exchange

List the factor's (Fick's law) that determine diffusion rate of gas across alveolar membrane o Net flux = - D.A.ΔP x
 Down gradient
 D Diffusion coefficient
 A Surface area of membrane
 ΔP Partial pressure gradient
 X Thickness of membrane Define diffusing capacity and state how it's affected by exercise fibrosis, pulmonary oedema and anaemia o Diffusing capacity= measurement of the lung's ability to transfer gases o Diffusing capacity for oxygen increases progressively with increasing exercise o Reduced in disorders that thicken/damage alveolar walls such as pulmonary fibrosis, oedema, interstitial lung diseases, dust inhalation diseases and anaemia o Clinical Test: Diffusing Capacity
 The diffusing capacity for carbon monoxide is the volume of CO transferred in milliliters per minute per mmHg of alveolar partial pressure
 = Vcarbon monoxide/PACO
 Typical value 25 ml/min/mmHg Describe the use of Haldane tube for sampling end-tidal (alveolar) air o Narrow hosepipe with a mouthpiece from which a tube is attached for the withdrawal of expired air at the end of a sudden, maximal expiration Describe the composition of alveolar air o Originally inhaled= O2 (21%, 160mmHg), N2 (79%, 600mmHg), CO2, H2O o Alveolar= O2 (13%, 100mmHg), CO2 (5%, 40mmHg), N2 (76%, 573mmHg), H2O (6%) Describe and explain the distribution (regional variation) in air flow (ventilation) in the lung o Most ventilation in lower zone of lung, and least in the upperzone o This is due to gravity o Bottom of the lungs have more room to expand and therefore receive more of the inspired air than the top of the lungs Describe and explain the distribution (regional variation) in blood flow (perfusion) in the lung o As you go further up the lung, blood flow decreases o Perfusion in the bases is greater than the apex o Bath theory=water container at the base, so bottom floor shower is most powerful o During exercise, blood flow increases &, in particular, increases in the apexes Define ventilation/perfusion (V/Q) ratio and describe the variation in different zones of lung o Ratio of the amount of air reaching alveoli to amount of blood reaching the alveoli o measurement used to assess efficiency & adequacy of the matching of two variables o At base, V/Q ratio is low and at top, V/Q ratio is low
 Zone 1 has the lowest blood flow, the lowest ventilation, and highest V/Q

• Variations in blood flow are more extreme than in ventilation, Q is decreased much more than V is decreased, so V/Q ratio is higher
 Zone 3 has the highest blood flow, the highest ventilation, and highest V/Q

• Q increased more than V increased, so V/Q ratio is higher Calculate the V/Q ratio for the whole lung and for each zone o In the whole lung, 4 litres of ventilation to 5 litres of blood flow= 0.8 o Zone 1= 1 and Zone 3= 0.6 State the effect upon V/Q ratio & the composition alveolar air of…
o Total airway obstruction

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