F7iii / 16B22: Outline the physiological factors that affect the diffusion of O2 & CO2 in the lung

16B22: Exam Report

Outline the physiological factors that affect the diffusion of oxygen and carbon dioxide within the lung.

43% of candidates passed this question.

Good answers to this question were those that included a definition of diffusion; an outline of Fick’s Law of Diffusion and then a further description how each of the variables in the this Law affect the diffusion of oxygen and carbon dioxide in the lung; and an outline of the other factors that affect diffusion not covered by the above. Most candidates included Fick’s Law in their answers and at least briefly expanded on the associated variables.

Few candidates defined the process of diffusion. The other common omissions were the factors that affect diffusion that aren’t directly encompassed in Fick’s Law, such as cardiac output, capillary transit time, carbonic anhydrase (conversion of HCO3 to CO2) and combination of oxygen with haemoglobin.

F7iii / 16B22: Outline the physiological factors that affect the diffusion of O2 & CO2 in the lung

Gas Carriage in Blood

  • Requires:
    1. Diffusion across BGB
    2. Reaction with blood

Diffusion

  • DIFFUSION = passive movement of particles from an area of high [ ] to an area of low [  ] across a semi-permeable membrane

NOTE: gas diffusion involves movement along a partial pressure gradient

  • DIFFUSION CAPACITY = the rate of gas transfer per minute / partial pressure difference of the gas
Diffusion Capacity
  • RATE OF DIFFUSION = determined by FICK’S LAW
Rate Diffusion

Area

  • Huge SA lungs 50m2
  • Affected by:
    • Total Lung Volume: ↑lung vol = ↑alv = ↑SA
    • V/Q mismatch: only alveoli which are perfused participate in gas exchange
    • Body size: ↑height = ↑lung vol = ↑SA
    • Posture: ↑diffusion capacity supine due to improved V/Q matching
    • Pathology: any disease that ↓alveoli no. will affect diffusion capacity e. emphysema ↓SA

Thickness

  • Normally v. thin 0.3µm
  • Any ↑thickness = ↓diffusion capacity
    • Pulmonary fibrosis
    • CHF
    • APO
  • All ↑ diffusion length & ∴↓ diffusion capacity

Diffusion Constant (D)

D
  • ↑ gas solubility = ↑ rate of diffusion
  • CO2 20 x more soluble
MW
  • Similar molecular weights (O2 & CO2)
  • But CO2 is 20 x more soluble → diffuses much quicker

Pressure Gradient

  • Partial Pressure = the individual pressure exerted by each gas
  • Maintained by ventilation & perfusion
  • CO2 46 (Pmixed) – 40 (Pα) → little in atmosphere & produced by body
  • O2 100 (PA) – 90 (Pα) → drop due to venous admixture

Rate of Reaction With Blood

[Hb]

  • Limits O2 transfer → once O2 binds Hb, ∆ allosteric binding & ↑affinity occurs
  • PaO2 = PAO2 in 0.25sec at rest
  • ∴ ↓[Hb] = ↓ O2 transfer/carrying capacity of blood because only minimal amount transported dissolved

Capillary Transit Time

  • ↓capillary transit time <0.25sec = ↓time available for oxygenation of Hb/blood
  • When there is less time for oxygenation, O2 becomes DIFFUSION LIMITED rather than perfusion limited

Cardiac Output

  • ↑CO = ↓capillary transit time
  • BUT ↑diffusion capacity with exercise of O2 from 21mL/min/mmHg → 65mL/min/mmHg
  • ↑CO = ↓cap transit = ↑recruitment → ↑V/Q matching

Carbonic Anhydrase

CO2 + H2O ⮂ H2CO3 ⮂ HCO3 + H+

  • Required for this reaction à allows large carriage of CO2 in blood