The Starling equation, which describes net transvascular liquid flow, does not include the possibility that a reduction in
plasma protein concentration may have a direct effect on lung liquid conductance or microvascular
protein permeability. Nevertheless, both effects have been reported. Since these results were not predictable, we wondered whether the changes were due to the decrease in
plasma protein concentration or to the process by which
protein depletion was accomplished (batch
plasmapheresis which involves considerable handling of blood). To separate these factors, we did control (
sham) and
protein removal
plasmapheresis in awake sheep by two
plasmapheresis methods (batch and continuous-flow). We monitored pulmonary hemodynamics, measured lung lymph flow, and determined
protein concentrations in lymph and plasma. We calculated or measured the
protein osmotic pressures of plasma and lymph. After control
plasmapheresis, lymph flow increased and lymph:
plasma protein concentration decreased but had returned to baseline levels by 4 hours. After
protein removal
plasmapheresis, the changes persisted for 24 hours. However, lung microvascular conductance (filtration coefficient) was not increased, except during the first 4-hour period. The changes in lymph flow and
protein concentration ratio are explained using a simple two-pore model. We conclude that, over the range studied,
hypoproteinemia does not increase lung microvascular liquid conductance or
protein permeability.