Biomedical Engineering Reference
In-Depth Information
which is a function of the formation/drainage rate of the aqueous humor and the cardiovas-
cular venous pressure. When the inflow flow rate of aqueous humor and the outflow flow
rate of aqueous humor are not balanced, the volume of the eye can increase and the outflow
flow rate is dependent on the resistance to outflow (this usually increases due to a block-
age). This can be quantified with
dV
eye
dt
5
Q
in
2
Q
out
which would measure the time rate of change of the eye volume. There are two typical
ways to measure the intraocular pressure, the first with a constant area and the second with
a constant force. Each of these methods assumes that the eye is a perfect spherical shell
with uniform elastic properties.
10.6
Glaucoma and cataracts are two common pathologies that affect the eye. Glaucoma is one
of the leading causes of blindness and is caused by a slow increase in intraocular pressure.
Intraocular pressure normally increases due to a blockage of the canals of Schlemm.
Cataracts are caused by an imbalance of the biochemical interactions of the lens proteins.
Due to this imbalance, the lens proteins either experience a structural change or they do not
organize properly. Either way, the lens tends to lose its transparency and light cannot be
focused on the retina properly. The retina is not normally damaged during cataracts, but it
is damaged during glaucoma.
HOMEWORK PROBLEMS
10.1
Describe the three layers of the eye, with respect to the structure/function relationship.
10.2
There is a blind spot along the retinal wall where the optic nerve attaches to the retina.
Why does this not impair vision?
10.3
Aqueous humor circulates at a constant inflow flow rate of 2
L/min and at steady state
drains from the eye at the same flow rate. This flow rate is dependent on the pressure-
volume relationship of the eye which can be described by V
5
Cp, where C is the compli-
ance of the eye and the outflow flow rate is equal to p/R, where R is the resistance to out-
flow. Calculate the compliance of the eye at steady state, knowing that the resistance to
outflow is 4.2 mmHg * min/
μ
L and the volume of the eye is 7 cm
3
. If the pressure is
increased by 10 mmHg due to a compression of the eye which reduces the overall volume
to 5.5 cm
3
, what is the outflow flow rate in these conditions?
μ
*10.4
Approximate how many aquaporin channels are required to maintain an inflow flow rate
of 2
L/min, assuming that the permeability of each aquaporin channel is 6
E
14 cm
3
/s and
that each channel allows one water molecule to pass at a time.
μ
10.5
Discuss in detail the restrictions of water movement through the aquaporin channels. Is
this an effective way to move water into/out of cells?
*10.6
Assume that there are 10,000 aquaporin channels that when combined can form aqueous
humor at a rate of 2
L/min. Assume that the diameter of each aquaporin channel is
0.25 nm. Under disease conditions, the aqueous humor drains at a rate of 1.95
μ
μ
L/min.
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