Intraocular Fluid Flow - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 284

Ciliary bodies

Filtration and diffusion
into retinal capillaries

Lens

Vitreous
humor

Aqueous humor

Canals of Schlemm

FIGURE 10.2

Anatomical structures salient for the formation of aqueous humor. Aqueous humor is formed at
a rate of approximately 2 L per minute within the ciliary bodies. Aqueous humor then flows between the lens

and the iris to fill the anterior chamber. Aqueous humor can leave the anterior chamber via the canals of
Schlemm or diffuse into the posterior chamber. There is a much slower flow through the posterior chamber, and
fluid within this chamber can leave by diffusion into the retinal capillaries. Arrows depict the movement of aque-
ous humor through the eye. Adapted from Guyton and Hall (2000).

μ

trabeculae act as a filter for the aqueous humor, removing any debris that may enter
the aqueous humor. The debris may come from a bacterial infection or hemorrhaging of
the vascular system within the ciliary processes. After passing through the trabeculae, the
aqueous humor passes into the canals of Schlemm. Each canal of Schlemm is a thin-walled
vein that connects to the extraocular veins with the vascular tunic. The canals of Schlemm
extend around the entire eye, collecting aqueous humor from all locations. Each canal of
Schlemm is lined by highly permeable endothelial cells which allow the passage of red
blood cells (if any enter the aqueous humor) and proteins into the vascular network of the
eye so they do not accumulate within the eye. Although the canal of Schlemm is a blood
vessel, it is typically never filled with blood because the flow rate of aqueous humor is
very high. Under normal conditions, the rate of removal of the aqueous humor is equal to
the rate of formation (i.e., approximately 2

μ L/min). This can be quantified by a mass bal-

ance of the aqueous humor:

5 ρ vA out

ð : Þ

ρ vA in 10 1

The flow of aqueous humor through many of the portions of the eye can be represented
by a simple pressure-flow relationship. In particular, an accurate representation of flow
through the canals of Schlemm can be calculated from

p IO 2 p ð x Þ
R

dQ
dx 5

ð 10 : 2 Þ

where Q is the flow rate along the canals of Schlemm, p IO is the intraocular pressure, p(x)
is the local pressure within the canal, and R is the resistance to flow primarily generated
from the trabecular mesh. In this formulation, x is the distance along the canal's length.
The flow through the canal is a low Reynolds number flow, and the pressure gradient can
then be approximated from the flow between two parallel infinite plates, assuming a two-
dimensional uniform problem. The pressure gradient can therefore be calculated from

dp
dx 5

12 μ Q ð x Þ

zh 3

ð 10 : 3 Þ

ð x Þ