Pulmonary Circulation — Starling Forces

Introduction

• Capillaries contain semipermeable membranes to allow the movement of fluid and solutes.
  • it is normally impermeable to large protein
  • Plasma ultrafiltrate is filtered by bulk flow through the capillary wall by the action of opposing hydrostatic and oncotic pressures
• The key features of the pulmonary microcirculation are:
  • The pulmonary capillaries (and the alveoli) have very thin walls which minimises the barrier to diffusion.
  • In the alveolar walls, the capillaries form a dense network which has been considered to be almost a continuous thin film of blood. This provides a large capillary surface area.
  • The pressures in the pulmonary circuit are much lower than in the systemic circulation and the pulmonary vascular resistance is very low. The pressure is just sufficient to perfuse the apical areas of the lungs in the erect healthy adult.

Starling Forces

• The NET flux across the membrane is the balance of hydrostatic pressure and oncotic pressure, as defined by the Classic Starling Equation:

where
J_v is the trans endothelial solvent filtration volume per second
( [ P_c – P_i ] – σ [ π_p – π_i ] ) is the net driving force
P = hydrostatic pressure
π = oncotic pressure
σ = Staverman’s reflection coefficient ie. Permeability of membrane to protein (0.5 for lung)
κ = filtration constant = L_pS = Hydraulic conductivity x Surface Area

Oncotic pressure gradient

• The interstitial oncotic pressure is high indicating significant leak of protein (mostly albumin) across the thin capillary walls under normal circumstances. The reflection coefficient has been estimated at about 0.5
• Considering the typical values and allowing for the reflection coefficient, it can be estimated that the net oncotic gradient is small but favours reabsorption.

Hydrostatic pressure gradient

• The capillaries are called intra-alveolar vessels and the presssure they are exposed to is close to alveolar pressure (zero to slightly negative, due to surfactant) and become more negative closer to the hilum
• This favours flow of fluid from the alveolar intersitium into the pulmonary lymphatics.
• The capillary hydrostatic pressure is variable because of the effects of gravity.
• The pulmonary circuit has a low resistance and about half of this resistance is due to the pulmonary capillaries which have no muscle in their walls. The capillary hydrostatic pressure is quickly affected by changes in pulmonary artery pressure and left atrial pressure without much protective buffering.

Overall Effect

The balance of Starling forces in the lung is generally stated as favouring reabsorption because of the clinical fact that the lungs are generally dry and clearly need to be to facilitate gas exchange

Safety Factors Preventing Pulmonary Oedema

• Increased lymph flow
• Decrease in interstitial oncotic pressure (oncotic buffering mechanism)
• High interstitial compliance

JC 2019

Answer by JC