Pulmonary Circulation — Gravity & Pressure

Gravity Dependent Processes

• Low pressure circuit:
  • pressure drop of ~10mmHg from RV (15mmHg) to LA (5mmHg)
  • PAP is just sufficient to raise blood to top of lung
• Blood flow in upright lung varies due to gravity effect (hydrostatic Pressure)
• Lung divided into vertically into 4 zones (West Zones)
  • Zone 1 not present in normal lungs
    • manifests in Positive pressure ventilation, Hypovolaemia, eg. haemorrhage
  • Zone 4 appears in the presence of interstitial pressures or at very low lung volumes
• Normal balance between the zones varies based on:
  • changes in posture
  • changes in airway pressures due to anatomical changes / disease, ventilation changes
  • changes in lung volume:
    • At extremes pulmonary capillaries are linearly stretched and collapse as may occur with hyperinflation.
    • At very low lung volumes, extra-alveolar blood vessels become compressed and flow reduces (Zone 4).
  • changes in pulmonary vascular pressures related to cardiac factors or pulmonary vascular disease
• There are alternate theories explaining the heterogeneity in pulmonary perfusion and ventilation in both health and disease, which suggest that gravity has a less important role in the variation of regional distribution
  • for e.g: The underlying structure of the bronchial and pulmonary vascular anatomy with non-symmetrical branching (Structural theory)

West Zone 1

• Not present in the normal lung
• Nil flow of blood to apical zones
• ↑alveolar dead space → High V/Q ratio
• Increased in:
  • Hypotension, PE, Hyperinflation of lung with PPV
    • Minimal change in P_aO₂ with increased West Zone 1 as Hb fully loaded 
    • Slight increase in P_aCO₂ as there is decreased apical perfusion, however this is compensated for by the increased RR chemoreceptor change: Minimal change in P_aCO₂

West Zone 2

• Starling Resistor Mechanism
  • ↑ hydrostatic P or ↓ Alveolar pressure → Pa > PA → capillary opens → flow → ↓ hydrostatic P ° to flow → Pa < PA → capillary closes
• Blood flow tends to ‘flutter’ (pulsatile flow)
  • decreased in diastole, increased in systole.
  • Increased in inspiration, decreased in expiration
• Present from the apex of the upright lung → 10cm above the level of the heart

West Zone 3

• Blood flow is constant due to the maintenance of arterio-venous pressure gradient
• Alveolar pressure is not a factor determining blood flow in this zone
• 10cm above the heart → bases of upright lung 
• Effect of weight of lung ↓PA
• Hydrostatic pressure is maximal (~25mmHg)
• Low V/Q ratio
• Contributes to venous admixture

West Zone 4

• Interstitial pressure acts as a Starling resistor for pulmonary blood flow.
  • It is seen when interstitial pressure is high (e.g due to pulmonary oedema).
• Also seen at very low lung volumes, where extra-alveolar blood vessels become compressed and flow reduces.

Effect of ↑P in pulmonary vessels

• Pulmonary circulation is low pressure circuit – ↑pressure 2° ↑PAP / PVP (LA) pressure
• Response in pulmonary vasculature is:
  • Recruitment of closed capillaries
  • Distension (↑caliber) of vessels
    • Increased pressure delivered to the arterioles causes dilation and decreases resistance to flow, increasing flow by as much as twice what would be expected due to pressure alone
    • Pulmonary arteries are approximately ½ as distensible as veins; this buffers pressure changes transmitted to the alveolar capillaries and also permits the arteries to adopt a reservoir function

• Net effect of recruitment and distension is bufering of pressure rise (↓pressure of pulmonary circulation) until these processess are exhausted or in the presence of pulmonary capillary disease

Gladwin 2016

Answer by Gladwin