Secretion of the aqueous humour

The object of the present paper is to review and discuss with respect to aqueous humour secretion the role of ciliary epithelial transport; the ultrastructural basis of the transport system and the possible role of ultrafiltration; and the mode of action of the carbonic anhydrase inhibitor, acetazolamide.

Abstract

The aqueous humour, like the cerebrospinal fluid, shares many of the properties of the extracellular fluid in other parts of the body; thus it has a similar overall electrolyte composition with high sodium and chloride and low potassium, it is low in protein and is approximately isotonic with the plasma. The detailed "composition of the aqueous humour" is, however, more complicated than would appear at first sight since there is unidirectional bulk flow between the source of the fluid in the posterior chamber and the efflux in the iridocorneal angle and the exact composition at any point is influenced by equilibration with the surrounding tissues. Although aqueous humour formation was at one time regarded as little more than a process of dialysis or ultrafiltration from the plasma, detailed analyses have shown many small but significant differences in the solute concentrations which are incompatible with this view (cf. Davson, 1956) and it is now generally accepted that active transport across the ciliary epithelium plays an essential role in maintaining the normal rate of flow of aqueous humour throughout the anterior chamber of the eye. The object of the present paper is to review and discuss with respect to aqueous humour secretion: the role of ciliary epithelial transport; the ultrastructural basis of the transport system and the possible role of ultrafiltration; the mode of action of the carbonic anhydrase inhibitor, acetazolamide.