On the Role of Omlotic Water Transport in the Secretion of the Aqueous Huniour1

Part of the water of a hypertonic secretion will enter passively along the osmotic gradient between blood and secretion. The aqueous humour is hypertonic in several species and it is often thought that all of its water enters as a consequence of the hypertonicity. To investigate this point, the osmotic pressure difference aqueous‐plasma and the mobility of water under osmotic forces across the barrier between aqueous and plasma have been measured in anaesthetized cats. The thermo‐electric vapour pressure method was used. In steady state, the aqueous of the posterior chamber in cats was about 5 mOsm/l hypertonic to plasma. With the secretion rate of 15 μlmin water mobility would have to be about 3 μl/min/mOsm/l in order to explain the rate of water secretion by osmotic attraction. In order to measure the actual water mobility, sudden changes in the osmotic pressure of the plasma were induced by anisotonic injections and the initial rate of volume change of the intraocular contents correlated with the (measured) degree of anisotonicity of the plasma. The mobility found was far below 3 μl/min/mOsni/l. Hence, water movement is not due to osmotic attraction by the bulk of the aqueous. It is stressed that osmotic pressure measurements in vitro can be quite misleading when applied to living systems. In vitro methods use perfect membranes or their equivalents, which have no certain counterpart in vivo. Since in the living organism a steady state concentration difference can be upheld even across a leaky membrane, the magnitude and even direction of the osmotic force existing in vivo can differ markedly from that measured in vitro.