F6iii: Explain V/Q matching & mismatching
- V/Q is so important because it’s affecting how much O2 gets in & CO2 gets out
- Ventilation & perfusion performs as ratio
- If CO ∆, it will ∆ V/Q of all lung units
- If MV ∆, it will ∆ V/Q of all lung units
(Given the other is constant)
- A lung with V/Q inequality cannot maintain high PO2 & low PCO2 as well as a homogenous lung
- Ventilation is greater @ base → greater compliance
- Perfusion is greater @ base → ↑hydrostatic P
- ↑perfusion > ventilation from apex → base
- Normal V/Q scatter is responsible for the natural A – a gradient
→ Even though PO2 is 40mmHg higher at apex cf. base, most perfusion is coming from base where PO2 is low
→ The sigmoid ODC also means that even units with high V/Q the O2 content of blood doesn’t increase much (plateau)
- V/Q (16mL O2/100mL blood) = 1/10
- V/Q = 10/10 = 19.5mL O2/100mL blood
- V/Q = 10/1 = 20mL O2/100mL blood
→ So when blood is mixed from all of these different V/Q ratio alveoli, it brings the PO2 down
NOTE
- Supine → V/Q scatter is less because the vertical gradient is so much less
- ∴ ↓dead space
- Lateral → V/Q scatter is same, just instead of apex → base it occurs from sup → inf lung