Function and Rate of Production
The aqueous provides nutrients to the avascular cornea and
lens. The primary difference between blood plasma and
aqueous is in the concentration of ascorbate and of protein.
Ascorbate concentration is approximately 20 times
higher in aqueous than in blood plasma and must be
actively transported into the aqueous. Ascorbate is thus
supplied to both cornea and lens and is important as a free
radical scavenger helping to guard these tissues against oxidative
damage. The protein content in plasma is 200 times
greater than in aqueous, a consequence of the tight junctional
barrier. The low concentration of protein causes
minimal light scatter and thus maximum light transmission
is maintained through the aqueous. The aqueous also
carries waste products from the cornea and lens and therefore
has a high concentration of lactate, a metabolic waste
product of the anaerobic glycolysis of the lens and cornea.
Approximately 2.5 μl of aqueous is produced per
minute.15 Aqueous production follows the circadian
rhythm with a higher rate during the day, that rate is
decreased by about 50% during the night.23 It is unclear
whether the fluctuation of IOP coincides with this production
cycle. During sleep circulating epinephrine
is decreased which may in part account for the reduction
in production but is unlikely the sole factor for the
circadian cycle.36
Although the ultrafiltration process can be influenced
by changes in IOP the effect on the rate of formation
is slight.15 Since active secretion is the primary mechanism
for aqueous formation moderate changes in blood
pressure have little effect on the rate of formation.31
Autonomic nerves located within the ciliary body can
influence aqueous production by acting on the blood
vessels, dilating them and increasing blood volume or
decreasing volume by constricting the vessels. Although
no anatomic evidence has been found identifying
autonomic innervation to the epithelia, animal studies
have found some alteration in production volumes
in response to manipulations of the autonomic signal.
Further information on the effect of aqueous production
on intraocular pressure and drug treatments that
reduce aqueous production will be found in Chapter 6.