Canonical Question
Diffusing capacity
Master answer
- Diffusion capacity:
- “The volume of gas which will diffuse through a membrane each minute for a partial pressure difference of 1mmHg (ml/min/mmHg)”
- Normal diffusion capacity for oxygen (DLO2): 20ml/min/mmHg
- Fick equation:
\[ Flow \; of \; gas \; ∝ \; {{A} \over {T}} \; . \; D \; (P_1- P_2) \]
- Thickness of membrane (normal: 0.3μm)
- Surface area of membrane (normal: 50-100m2)
- Age, posture, degree of inflation (1 mark for any)
- Partial pressure gradient across membrane
- Rate of blood flow through lungs
- (no points for mentioning diffusion constant of gas – question relates to only oxygen)
- Rate of oxygenation of reduced Hb
- Shift of oxygen dissociation curve (pH, temperature, PCO2, 2,3-DPG)
- Haematocrit
- Abnormalities of haemoglobin
In exercise
- DLO2 increases to 30-60ml/min/mmHg
- Increased VT and RR
- Increased alveolar ventilation → partial pressure gradient of O2 across capillary increases (significant as flow of erythrocytes through pulmonary vasculature likely to increase)
- Recruitment of dormant pulmonary capillaries (increased surface area for diffusion)
- due to: Increased lung inflation, Increased pulmonary arterial systolic pressure
- Perfusion increases from 21ml/min/mmHg up to 65ml/min/mmHg
- Better matching of ventilation-perfusion
Mooney 2016
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2012A Q20 | List the physiological factors affecting the diffusion of oxygen across the alveolar membrane (70% of marks). Describe how this changes with exercise (30% of marks). | historical_member | — |