Fluid dynamics of gingival tissues

Gingival hydraulic interstitial pressure was measured with glass micropipettes in 18 anesthetized rabbits at the level of the free gingiva, attached gingiva and oral mucosa facing the incisors and molar teeth. Samples of gingival interstitial tissue fluid were also collected by inserting nylon wicks in the subepithelial layer of the oral mucosa. Colloid osmotic pressure of interstitial fluid samples was measured with an osmometer whose membrane had a molecular cutoff of 30 kD. Hydraulic interstitial pressure from the free gingiva, at an average distance of 300 microns from sulcular space, was -1.3 +/- 0.9 (SD) cmH2O. Mean colloid osmotic pressure of gingival tissues interstitial fluid was 13.1 cmH2O, corresponding to a protein concentration of 2.8 g/dl. The thickness of the sulcular epithelium and of the oral gingival epithelium (data from 2 rabbits) were approximately 100 microns; the minimal distance of microvessels from the surface of the sulcular epithelium was approximately 150 microns. Based on hydraulic and colloid osmotic data, the Starling balance of pressures causes fluid filtration from gingival capillaries to gingival interstitium; however, across the sulcular epithelium, the pressure gradient sustains fluid absorption from the sulcus into the gingival interstitium. Plasma proteins may leak from microvessels into gingival interstitium, down convective bulk flow and via a concentration dependent diffusive component. At sulcular level, proteins may leak into the sulcus down a concentration gradient. Thus, at sulcular level a peculiar condition occurs in that there is an absorption gradient for water but a filtration gradient for plasma proteins.