Viscous and osmotically mediated changes in fluid movement across synovium in response to intraarticular albumin

Fluid exchange in synovial joints involves both interstitial and transcapillary flow. Here the influence of intraarticular albumin, a normal component of intraarticular fluid, on net transsynovial flow was explored by infusing albumin solutions into the synovial cavity of rabbit knees and recording net flow across the synovial lining. The latter is a discontinuous cell layer possessing matrix-filled intercellular spaces and fenestrated capillaries. Albumin solutions (10-250 g.liter-1) of known oncotic pressure and viscosity were infused at a constant pressure between 3 and 18 cm H2O, either in vivo or post mortem. The rate of absorption (outflow) of Krebs solution from the cavity at the same pressure was measured as the control. Albumin reduced the net rate of absorption of fluid from the joint cavity (Qs). The reduction in Qs in vivo, expressed as a fraction of control, was greater than the fractional reduction in the fluidity of the albumin solution (i.e., 1/relative viscosity). At high albumin concentrations in vivo, the net flow reversed direction and became a net filtration into the joint cavity. The change in Qs per unit change in intraarticular oncotic pressure was then 29.7 x 10(-3) microliters.min-1.cm H2O-1. Even when there was continuous net outflow across the lining, however, as at lower albumin concentrations or higher joint pressures, the intraarticular albumin became diluted compared with the infusate. When synovial blood flow was stopped by killing the animal, albumin had less effect on net transsynovial outflow than that in vivo. Outflow was still reduced by albumin but the fractional reduction was now less than the fractional reduction in bulk fluidity. Also, intraarticular dilution was less marked postmortem than in vivo. The observations were compatible with the view that the net transsynovial flow in vivo comprised two opposing flows, namely, capillary (fenestral) filtration into the joint cavity and interstitial flow from cavity to subsynovium. These two flows were affected oppositely by albumin: filtration was enhanced by the oncotic pressure of albumin in the pericapillary interstitium, whereas interstitial flow was reduced by the effect of albumin on fluidity. The results also raised the issues of whether fenestral filtration reduces nearby interstitial albumin concentration and how interstitial and bulk-phase fluidities are related.