Re-Evaluation of Starling Forces Balance and Lymphatic Clearance in the Lung

Abstract

It is generally accepted that transvascular fluid flux (Q_F) across the pulmonary microvasculature is governed by the forces summarized in Starling’s equation [1, 2]:

$$ {Q_F} = {K_F}\left[ {\left( {{P_{{mv}}} - {P_{{pmv}}}} \right)\sigma \left( {{\pi_{{mv}}} = {\pi_{{pmv}}}} \right)} \right] $$

(1)

where, as proposed by Staub, P_mv and π_mv represent the microvascular, and P_pmv and π_pmv the perimicrovascular hydrostatic and colloid osmotic pressures, respectively [3]. The proportionality constant K_F is commonly known as the filtration coefficient and represents the microvascular conductance to transvascular fluid exchange for any imbalance of forces. Because the normal pulmonary microvascular membrane allows for some protein exchange, the transvascular osmotic gradient is corrected accordingly with the use of the reflection coefficient (a). The Starling equation is a linear relationship, and predicts that, unless there is complete balance among the forces, Q_F must occur at a rate proportional to the force imbalance. Yet, since the classic experiments of Guyton and Lindsay, it is known that P_mv can vary over a wide range without net fluid accumulation in the lung [4].