Aqueous humor formation

The various processes and forces underlying aqueous humor formation have been examined both theoretically, as a problem in terms of membrane behavior, and experimentally, using in vitro and in vivo techniques. Mathematical relationships have been developed for the interrelationship of facility, pseudofacility, capillary pressure, active secretion and xc. The accurate measurement of total facility (Ctot), pseudofacility (Cps), intraocular pressure (Pi) and episcleral venous pressure (Pe), allows the calculation, from a graphical analysis, of both mean capillary pressure and the active secretion component of aqueous formation. In vitro experiments on isolated ciliary body-iris preparations indicate that ultrafiltration contributes about 60% to the total volume flow of aqueous formation. The reflection coefficient of the ciliary epithelium to NaCl, sucrose and urea is 0·02, and to dextran is unity. Plasma or blood, separated from aqueous or Ringer's solution by the ciliary epithelium, exerts an osmotic pressure of 21 mmHg. In vivo determinations of the reflection coefficient show values of 0·02 for NaCl and sucrose and unity for dextran. The appropriate measurements of fluid permeability (LpA), Pe, Pi, Ctot, Ctrue were made and a plot made of CtotPi-CtrPe against LpA; the slope (26 mmHg) is capillary pressure minus colloid osmotic pressure and the intercept is secretion minus uveoscleral bulk flow. The capillary pressure is 47–50 mmHg and the secretory component to aqueous formation is 35% of the total fluid flow. These results suggest that ultrafiltration is the most important contributing factor to aqueous formation.