F6ii / 16A03: Compare the physiology of the apex with base of the lung in the upright position

16A03: Exam Report

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

33% of candidates passed this question.

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.

F6ii / 16A03: Compare the physiology of the apex with base of the lung in the upright position

Perfusion

  • Pulmonary BF operates in a low P system (mPAP 15mmHg)
  • Erect posture → 30cmH2O (23mmHg difference b/w apex + base)
  • Due to hydrostatic P
  • P arterial & venous P ↑from apex → base
  • Different areas of BF KA WEST’S ZONES

Zone 1: Pa > Pa > Pv

  • Caps collapse → nil BF
  • Alveolar DS

Zone 2: Pa > Pa > Pv

  • ↑hydrostatic P ∴↑Pa & Pv
  • BF determined by pressure different PA & Pa
  • Recruitment dominates

Zone 3: Pa > Pv > Pa

  • Further ↑hydrostatic P
  • BF depends on P difference between Pa & PV
  • Distention dominates

Zone 4

  • @ low lung volumes
  • ↓radial traction
  • ↑↑PVR
  • No BF

Ventilation

  • Ventilation increases apex → base
  • Due to greater compliance
Ventilation

Alveolar Gases/PH

  • V & Q ↑as you move from Apex → Base
  • ∴high V/Q apex, low V/Q @ base
  • PO2 decreases from apex to base
  • PCO2 increases from apex to base, but much less ∆PO2
  • ∴ ↓pH with ↑CO2 from apex → base

PO2

PCO2

pH

Apex

132

28

7.51

Base

95

42

7.39

Lung Volume

  • Increases from Apex → base, due to ↑compliance

NB:

PO2 ↓ from apex → base, even though ventilation is less at apex

This is because alveoli with no perfusion (WZ1) will have PO2/PCO2 values same as inspired gas because there’s no gas exchange to alter composition of inspired gas

Alveoli with no ventilation → have PO2/PCO2 same as mixed venous blood because trapped air in unventilated alveoli will equilibrate with mixed venous blood

Raw

  • Bronchi are supported by radial traction by lung tissue
  • ∴ as lung vol ↑from apex → base → RAW ↓
Raw