5.2 Die Moldau

Now, we'll go into the flow of water in the eye. From our point of view, it flows like the Vltava (the fans of Bedřich Smetana of course may disagree).

The traditional concept of water flow in the eye actually refers to the flow of the aqueous humor. Aqueous humor is produced by the ciliary body. It is a plasma filtrate that does not contain any blood cells (or we'd see them all day long). It enters the posterior aqueous chamber first, then passes through the pupil, i.e., between the anterior surface of the crystalline lens and the iris border, into the anterior (aqueous) chamber where it provides nutrients for the lens and the corneal endothelium. Within the aqueous chamber, there is also a temperature-mediated convection with warmer current rising to the top. The aqueous humor then exits through the angle into the trabecular meshwork and Schlemm's canal, then drains into the aqueous vein, and eventually re-enters the circulatory system. As we have explained in previous posts that any impedance along the pathway creates an increase in the intraocular pressure that may lead to damages to the retina.

To measure the flow rate of the aqueous humor, a suitable marker must be used. Since fluorescein can be delivered through the cornea into the anterior chamber, by using fluorophotometry, it is possible to determine the flow rate of this molecule (MW=332.32) and it is 2 microliter/min. This is, however, not a true measurement of the free water (MW=18.00) itself but only that of water in a confined space. It is similar to determining how fast the tap water is going through a garden hose. Free water, on the other hand, does not behave that way.

An imaging study has been done and its summary quoted below:

"Movement of water in the anterior chamber of the rabbit eye in response to topical agents for glaucoma therapy was investigated using high-resolution T2-weighted surface-coil magnetic resonance imaging (MRI) aided by a T2 contrast agent, oxygen-17 water. This water-tracer was applied topically which entered the anterior chamber within 10 min. The kinetics of its dissipation from the anterior chamber was recorded with serial T2-weighted MRI. The data represented the total water flow rate in the anterior chamber.

"We tested the effects on water flow of an alpha-adrenergic (phenylephrine), a miotic (pilocarpine), and a beta-blocker (timolol). The results showed that phenylephrine reduced more than 2/3 the normal water flow rate, pilocarpine accelerated the initial flow rate ca. 2-fold although oxygen-17 water did not fully dissipate, while timolol had little or no effect. These results suggest that water flow in the anterior chamber is in large a function of the status of the iris surface area and/or iris circulation. "

It should be noted, the flow rate using the water tracer, oxygen-17 water (MW=17.00) is 10 times that with fluorescein. In the eye, water is expected to interact with the transport systems of all tissues, not just the flow indexed by fluorescein. Indeed, we need to have a more comprehensive view of water flow in the eye.

Think "Die Moldau".