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Mechanisms of pleural liquid accumulation in disease

V Courtney Broaddus, MD
Section Editor
Talmadge E King, Jr, MD
Deputy Editor
Geraldine Finlay, MD


In the normal pleural space, there is a steady state in which there is a roughly equal rate of the formation (entry) and absorption (exit) of liquid (figure 1). (See "Mechanisms of pleural liquid turnover in the normal state".) This balance must be disturbed in order to produce a pleural effusion. Thus, there must be an increase in entry rate and/or a reduction in exit rate. It is likely that both mechanisms contribute to effusion formation for the following reasons:

An isolated increase in entry rate, unless large and sustained, is unlikely to cause a clinically significant effusion because the absorbing pleural lymphatics have a large reserve capacity to deal with excess pleural liquid. If, for example, artificial hydrothoraces are instilled into the pleural space of awake sheep, the exit rate can increase to 0.28 mL/kg per hour, which is 28 times the baseline rate [1].

An isolated decrease in exit rate is also unlikely to cause a large effusion because the normal entry rate is low. Even if the exit of liquid ceased entirely, accumulation of liquid would take many days to become evident. As an example, the normal entry rate of 0.01 mL/kg per hour is equivalent to a total of 12 mL/day in a 50 kg woman; at this rate of entry without any exit of liquid, it would take more than one month days for 500 mL to accumulate in the pleural space. Of note, the effusion would presumably be a transudate, since the normal liquid entering the pleural space is low in protein.


Excess liquid filters out of systemic microvessels based on a balance of hydrostatic and osmotic forces across a semipermeable membrane [2,3]. These forces are well described in the Starling equation, in which the hydrostatic forces that force water out of the vessel are balanced by osmotic forces that reabsorb water back into the vessel [4,5].

Flow  =  k  x   [(Pmv  -  Ppmv)  -  s (πmv  -  πpmv)]

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Literature review current through: Oct 2017. | This topic last updated: Apr 15, 2016.
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