Compare the physiology of the apex of the lung with the base of the lung in the upright position.
The majority of candidates gave extensive detail on West’s zones of the lungs and did not
describe other parameters that vary from base to apex. Ventilation, resistance, compliance,
alveolar and lung size all vary. Some candidates mixed up the changes at the apex versus the
base.
In answer to the college's complaint that these answers were strongly West-dominated, the table here is offered with all possible differences in characteristics one could think of. The expectation is that no individual candidate will ever be able to reproduce the entire content of this table. Nor could that possibly be the requirement for a passing grade.
| Domain | Apex | Base |
| Shape | Conical | Irregular cylinder |
| Size | Relatively small fraction of the total lung volume | The bases represent the majority of the lung volume |
| Pleural pressure | Low (~ -7cm H2O) | High (~ -3cm H2O) |
| Anatomical boundaries | Ribcage, mediastinum, pleura superiorly, midzones of lung inferiorly | Ribcage, mediastinum, midzones of lung superiorly, diaphragm inferiorly |
| Changes with respiratory cycle | Minimal expansion | Significant expansion due to increase in ribcage diameter and diaphragmatic contraction |
| Alveolar size | Large, well-distended | Small, mid-collapsed |
| Alveolar compliance | Poor compliance (alveoli are almost maximally inflated) |
Good compliance (alveoli are mid-inflated) |
| Airway resistance | Low (traction pulls small airways open) |
High at expiration Low with inspiration |
| Ventilation | Comparatively low | 50% higher than at the apex |
| Pulmonary blood flow | Low (due to gravity and increased vascular resistance) | High (due to gravity and low vascular resistance) |
| Pulmonary vascular resistance | High | Low (due to increased flow) |
| Main source of resistance to blood flow | Alveolar pressure (i.e. Zone 1) |
Pulmonary venous pressure (i.e. Zone 3) |
| V/Q ratio | High (~3 in healthy lung) | Low (~0.6 in healthy lung) |
The entries in the table above come from literature references so numerous and so scattered that, rather than reproduce them all here, it would be easier to link to chapters of Deranged Physiology which may act as bibliographies: