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

Abstract

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 Q ̇ s ) and plasma colloid osmotic pressure π p was investigated in rabbit knees perfused at constant blood pressure. Intraarticular pressure P j was independently controlled. Linear relations between transsynovial flow and plasma colloid osmotic pressure established that transsynovial flow obeys the Starling hypothesis. However osmotic conductance, d Q ̇ s dπ p , increased 3.9 times when P j was raised from 6 cm H 2O or subatmospheric pressure to 18 cm H 2O—the “yield phenomenon.” Comparison of the effects of π p and capillary pressure revealed no major change in the osmotic reflection coefficient of the blood—joint barrier to albumin upon raising P j. The large increase in osmotic conductance was predicted quantitatively by a previous model (prediction 3.8 ×) 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.”