Past Papers · SAQ
Apex vs Base Physiology
2016A Q03
Exam questionCompare the physiology of the apex of the lung with the base of the lung in the upright position.
CICMWrecks answer
Master answer
Vertical gradient of pleural pressure and alveoli
Inspiration
- Inspiratory muscle contraction → Overcomes resistance to insp flow → negative alveolae pressure -1cm H2O → VT 500ml
- Weight of lung on alveoli and pleura above:
- causes gradient of pleural pressure: Apex (-10cmH2O) Base (-2.5cm H2O)
- Apical alveoli larger
- But compliance lesser (due to baseline distension)
- Lesser ventilation for same pressure
- Apex and base on different part of compliance curve
Expiration:
- Gradient of pleural pressure smaller
- Apex: -4cm H2O → on steeper part of compliance curve → ventilation better
- Base +3.5cm H2O →airway closure → gas trapping & shunt
Ventilation, perfusion, V/Q matching
Vertical segments based on V and Q
- Both V and Q ↓ as you move from base to apex.
- ↓ total number of alveoli → ↓ diffusive area → ↓ V
- ↑ compression of intra-alveolar vessels → ↑ west zone 1 → ↓ Q
- V ↓’s slower than Q
| Level | V/Q | Ventilation | Perfusion | PO2 | PCO2 |
|---|---|---|---|---|---|
| Apex | ~3 High | Lower Alveoli largest (no wt of lung) | Very Low Blood hydrostatic pressure low (high gravity) | High 132 | Low 28 |
| At level of heart | ~1 | Slightly more ventilated (Optimized) | Optimized (less effect of gravity) | 108 | 39 |
| Base | ~0.67 towards mixed venous point | Well ventilated (in normal lung): small alveoli, more compliance (effect of wt of lung) | Highly perfused Maximal Hydrostatic pressure | relativly hypoxic 89 | relatively hypercapnoeic 42 |
West Zones:
- Divided into zones based on relationship between Alveolar pressure (PA) Arterial pressure (Pa) Venous pressure (Pv) Interstitial pressure (Pi)
- West Zones 1,2,3: Due to changes in hydrostatic pressure when pumping to top of lung vs. bottom of lung
| West Zone | Pressure Relationships | Physiology | Location |
|---|---|---|---|
| Zone 1 | PA > Pa > Pv | No flow of blood, as arterial pressure completely opposed by alveolar pressure | not seen in normal lung |
| Zone 2 | Pa > PA > Pv | Resistance to flow is determined by alveolar pressure (Starling resistor effect) | about 3cm above the heart |
| Zone 3 | Pa > Pv > PA | Resistance to flow is determined by venous pressure Venous pooling causes increased distension of pulmonary capillaries | majority of healthy lung |
| Zone 4 | Pa > Pi > Pv > PA | Low lung volume causes narrowing of extra-alveolar vessels | Lung bases at low lung volume or in pulmonary edema |
JC 2019
Past papers
Exam appearances
| Exam | Exact exam wording | Candidate success |
|---|---|---|
| 2016A Q03 | Compare the physiology of the apex of the lung with the base of the lung in the upright position. | 33% |