In 1933 one of us1 reported on the urea content of aqueous humor and of blood withdrawn simultaneously from the same animal. The concentration of urea in the aqueous humor was found to be about 18 per cent lower than that in the blood stream when equal volumes of fluid were compared. Calculations of the amount of urea per hundred cubic centimeters of water in aqueous humor and in blood showed that the aqueous contains about 25 per cent less urea than the blood. Since the urea in the blood is not bound by the proteins but is free and diffusible, it seemed probable that the membrane separating the two fluids is not inert but shows selective permeability and holds back the urea in the blood stream. The other alternatives are that the urea is destroyed in the anterior chamber after the aqueous has been formed, that the urea
MOORE E, SCHEIE HG, ADLER FH. CHEMICAL EQUILIBRIUM BETWEEN BLOOD AND AQUEOUS HUMORFURTHER STUDIES. Arch Ophthalmol. 1942;27(2):317–329. doi:10.1001/archopht.1942.00880020101008
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