Gas Exchange Notes

Lecture 6

Gas exchange

• List the factor’s (Fick’s law) that determine diffusion rate of gas across alveolar membrane

  • 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

  • Diffusing capacity= measurement of the lung's ability to transfer gases

  • Diffusing capacity for oxygen increases progressively with increasing exercise

  • Reduced in disorders that thicken/damage alveolar walls such as pulmonary fibrosis, oedema, interstitial lung diseases, dust inhalation diseases and anaemia

  • 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/PA_CO

    • Typical value 25 ml/min/mmHg

• Describe the use of Haldane tube for sampling end-tidal (alveolar) air

  • 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

  • Originally inhaled= O₂ (21%, 160mmHg), N₂ (79%, 600mmHg), CO₂, H₂O

  • 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

  • Most ventilation in lower zone of lung, and least in the upperzone

  • This is due to gravity

  • 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

  • As you go further up the lung, blood flow decreases

  • Perfusion in the bases is greater than the apex

  • Bath theory=water container at the base, so bottom floor shower is most powerful

  • 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

  • Ratio of the amount of air reaching alveoli to amount of blood reaching the alveoli

  • measurement used to assess efficiency & adequacy of the matching of two variables

  • 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

  • In the whole lung, 4 litres of ventilation to 5 litres of blood flow= 0.8

  • Zone 1= 1 and Zone 3= 0.6

• State the effect upon V/Q ratio & the composition alveolar air of…

  • Total airway obstruction

    • “Shunt”

    • Alveolar composition= O₂ (40mmHg), CO₂ (45mmHg)

    • V/Q=0

  • Total obstruction to perfusion

    • “Deadspace ” E.g. pulmonary embolism

    • Alveolar composition= O₂ (150mmHg), CO₂ (0mmHg)

    • V/Q= α

• Define and explain venous admixture (physiological shunt)

  • PAO₂ is normally greater than Pa_O2 due to physiological shunts.

  • PAO₂ is calculated by PI_O2 – PACO₂/R

  • PaO₂ is measured using a sample of arterial blood

  • Admixture of mixed...

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