Biomedical Engineering Reference
In-Depth Information
END OF CHAPTER SUMMARY
10.1
The eye is a specialized sensing organ that is sensitive to both the intensity and the wave-
length of light. The wall of the eye is composed of three layers, which provides mechanical
support for the eye (the fibrous tunic); provides blood to the eye (the vascular tunic); or is
composed of sensing cells (the neural tunic). The eye can also be divided into two chambers,
each filled with fluids. The vitreous cavity maintains the shape of the eye and is filled with a
gel that is composed of many proteins and proteoglycans. The anterior cavity is filled with
aqueous humor that wets the cornea and lens, providing these structures with the nutrients
it needs to function. The lens is made up of the protein crystallin which helps to focus light
on the retina. The retina is composed of two sensing cell types: the rods and the cones, which
can be stimulated by changes in the incident light and transmit those changes to the brain.
10.2
Aqueous humor is formed at a rate of approximately 2
L/min within the anterior chamber.
It is secreted by the ciliary bodies, which are located behind the iris and are composed of
secretory epithelial cells. Sodium ions are actively transported out of the ciliary bodies,
which causes chloride and bicarbonate ions to diffuse out of the ciliary bodies. The efflux of
these ions causes an increase in the osmotic pressure of the fluid, and therefore, water
moves out of the ciliary bodies to balance this increase in osmotic pressure.
μ
10.3
Aquaporins are trans-membrane proteins that facilitate the movement of water across the
cell membrane wall. Under normal conditions, water ions can diffuse across the cell mem-
brane freely, however, this is a relatively slow process. Through the addition of aquaporin
channels to the cell membrane, water movement occurs significantly faster. Interestingly,
aquaporins contain structures that can regulate the movement of water through the chan-
nels. A structure termed the NPA motif forms a bottleneck within the aquaporin channel.
This constriction causes water ions to move through the channel in a single file. This is
likely caused by an electric field that is generated within the channel. A second filter,
termed the aromatic/arginine selectivity filter, interacts with water molecules to facilitate
their transport through the channel. Other compounds cannot interact with this filter and
cannot pass through the channel.
10.4
Aqueous humor flows from the ciliary processes, between the ciliary ligaments into the
anterior chamber. Within the anterior chamber, aqueous humor can either pass through the
pupil to wet the lens or remain in the anterior chamber to wet the cornea. After passing
through the anterior chamber, aqueous humor passes through the trabeculae that act as a
filter for the aqueous humor and then enters the canals of Schlemm. The canals of Schlemm
connect back to the vascular tunic so that aqueous humor is reabsorbed into the cardiovas-
cular system. A small portion of aqueous humor diffuses into the posterior chamber. In gen-
eral, the flow of aqueous humor can be quantified through a mass balance equation:
ρ
vA
in
5
ρ
vA
out
10.5
Intraocular pressure maintains the proper shape of the eye helps aqueous humor flow and
is a measure of the hydrostatic pressure within the eye. Intraocular pressure can be calcu-
lated from
F
C
1
p
EV
p
IO
5
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