The osmotic relationship between blood and aqueous humor is a factor of wellrecognized importance in governing intraocular pressure.1 The aqueous is slightly hypertonic to blood.2-4 Friedenwald5 and Kinsey6 postulate that the hypertonicity of the aqueous derives from the formation of an excess of hydroxyl ions, which, in reacting first with carbon dioxide and then with sodium and other cations, maintains the ionic concentrations of the aqueous above plasma levels. The rate of aqueous flow is considered to be a function of this osmotic relationship.
Osmotic changes in the blood may alter aqueous flow.7 Many osmotic agents, especially sugars, have been used for the purpose of influencing intraocular pressure. Hertel, in 1914, demonstrated that urea reduced intraocular pressure when injected intravenously in the rabbit. In an experimental study, Fremont-Smith and Forbes9 have shown that intracranial and intraocular pressures are closely related and that they react
BUNGE RP, DANFORTH RC, SETTLAGE PH. Effects of Intravenous Urea on Intraocular Pressure in the Monkey. AMA Arch Ophthalmol. 1957;57(5):659-667. doi:10.1001/archopht.1957.00930050671004