Question 3

Compare the physiology of the apex of the lung with the base of the lung in the upright position. 

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College Answer

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 


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. 

Differences between the Apex and Base

of an Upright Lung

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
(combination of alveolar deflation and pressure from weight of overlying lung)

Low with inspiration
(alveolar traction pulls small airways open)

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: