Interaction of plasma colloid osmotic pressure and joint fluid pressure across the endothelium-synovium layer: significance of extravascular resistance.

Fluid exchanges between blood and a synovial joint cavity across two membranes in series--synovial capillary wall (fenestrated) and synovial intima (modified connective tissue). The relation between transsynovial absorption of intraarticular Krebs solution (flow Qs) and plasma colloid osmotic pressure pi p was investigated in rabbit knees perfused at constant blood pressure. Intraarticular pressure Pj was independently controlled. Linear relations between transsynovial flow and plasma colloid osmotic pressure established that transsynovial flow obeys the Starling hypothesis. However osmotic conductance, dQs/d pi p, increased 3.9 times when Pj was raised from 6 cm H2O or subatmospheric pressure to 18 cm H2O--the "yield phenomenon." Comparison of the effects of pi p and capillary pressure revealed no major change in the osmotic reflection coefficient of the blood-joint barrier to albumin upon raising Pj. The large increase in osmotic conductance was predicted quantitatively by a previous model (prediction 3.8 X) based on increases in extravascular (intimal) conductance as a function of extravascular pressure. It is argued that capillary endothelium is not the sole significant hydraulic resistance in this pathway. In the terminology of Intaglietta and de Plomb (1973) synovial capillaries are functionally intermediate between "tubes" and "tunnels."