It has been established that the aqueous humor contains less urea than the circulating blood. Urea should be present in equal amounts in aqueous and blood if the aqueous is formed by dialysis alone, since urea is a crystalloid and a nonelectrolyte. It is, however, possible that after the aqueous is formed other processes alter its chemical composition so that it can no longer be recognized as a dialysate. In an attempt to explain this discrepancy a number of factors were explored and reported in a previous paper.1
As a result of those studies the low urea content of aqueous was attributed to a lack of permeability of the blood-aqueous barrier to urea. Under normal conditions the membrane separating aqueous from blood does not allow urea to pass through it readily, so that the concentration is always about 20 per cent less in aqueous than in blood. This is
SCHEIE HG, MOORE E, ADLER FH. PHYSIOLOGY OF AQUEOUS IN COMPLETELY IRIDECTOMIZED EYES. Arch Ophthalmol. 1943;30(1):70–74. doi:10.1001/archopht.1943.00880190088008
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