Secretion and rate of flow of aqueous humour in the cat.

KNOWLEDGE of the means by which substances enter and leave the eye, usually referred to as " aqueous-humour dynamics ", is essential for an understanding of the factors contributing to the control of the intra-ocular pressure. In non-living systems, when a membrane separates two concentrations of a substance in a solution to which the membrane is permeable, this substance travels across the membrane until the concentrations on either side are equal; chemical equilibrium is then said to exist. In the living state this type of equilibrium is seldom found; instead, a dynamic equilibrium occurs in which there is no further change in concentration of the solute across the membrane or barrier in spite of a difference of concentrations. An equilibrium of this nature is generally referred to as a " steady-state ". The concentration of urea in the aqueous humour, for example, is less than in the blood plasma, which indicates that, so far as this substance is concerned, a chemical equilibrium does not exist. It is agreed that small quantities of aqueous humour are continually being formed and eliminated from the eye. In contrast to the complex problem of the formation of the individual components of the aqueous humour, elimination is thought to consist of a simple drainage in bulk, i.e., a unidirectional flow, involving no separation of its constituents. Experimentally, the determination of this rate of drainage has presented difficulties. The problem of maintaining the eye in a physiological condition precludes measurement of the rate of flow of the aqueous humour by the direct introduction of a cannula. The rate at which a slowly penetrating substance accumulates in the aqueous humour and the nature of the distribution ration in the steady-state may be used, with certain assumptions, to derive the approximate rate of flow of the aqueous humour, as has been shown by Biarany and Kinsey (1949). In this connection, the ratio of the concentration of a substance in the aqueous humour and in the water of the plasma when the concentration in the aqueous humour is no longer rising, is generally known as the equilibrium ratio (Req). The rates of accumulation of cysteine and ascorbic acid have been studied from this viewpoint in the present work.