Choroid/Retina Flashcards
What is the mean arterial pressure of the arteries entering the eye?
65 mmHg
What is the pressure in the episcleral veins leaving the eye?
15 mmHg
What is perfusion pressure in the eye?
Approximately 50 mmHg
How is ocular perfusion pressure (OPP) calculated?
OPP = diastolic blood pressure - IOP
What effect does a decrease in diastolic BP have on OPP?
OPP decreases
What is autoregulation in the context of blood vessels?
The process by which blood vessels alter their diameter to increase or decrease resistance to blood flow
Which cells are most likely responsible for autoregulation within the blood supply of the retina and optic nerve?
Pericytes
Fill in the blank: A smaller blood vessel diameter offers more _______ resulting in a decrease in blood flow.
resistance
Which arteries offer more resistance: retinal arteries or choroidal arteries?
Retinal arteries
What does transmural pressure describe?
The pressure across the blood vessel wall
How is transmural pressure calculated?
Pressure inside the vessel - Pressure outside the vessel
What is critical closing pressure?
The pressure at which the blood vessel collapses and blood flow stops
What effect does elevated IOP in acute glaucoma have on blood flow through the central retinal artery?
Reduces blood flow
What happens to the perfusion pressure of retinal tissue when blood flow is reduced due to elevated IOP?
Decreases
How do retinal vessels respond to changes in transmural pressure during elevated IOP?
Increase their vessel diameter through autoregulation
What can happen if IOP remains acutely elevated for too long?
Central retinal artery reaches its critical closing pressure, resulting in CRAO
What is the immediate threat to vision in acute glaucoma?
CRAO
Automatic Nervous System Control
Sympathetic Innervation
Sympathetic fibers are prevalent throughout the uveal tract but they DO NOT
innervate the central retinal artery past the lamina cribrosa and therefore do
not influence retinal blood flow (18).
Sympathetic innervation causes vasoconstriction of uveal blood vessels.
It is important in maintaining adequate blood flow through the uvea during
sudden increases in blood pressure.
A sudden spike in blood pressure increases the force of blood floW
through the small vessels of the uvea. The sympathetic system
responds by causing constriction of the blood vessels
resulting in a
compensatory reduction in blood flow.
Parasympathetic lnnervation
Parasympathetic fibers from the oculomotor and facial nerves are also
prevalent throughout the uveal tract. Parasympathetic innervation is most
prominent in the anterior uvea and has a minimal influence on choroidal and
retinal blood flow.
Parasympathetic innervation causes vasodilation of the uveal blood vessels
in response to sudden decreases in blood pressure.
Unlike the choroid
retinal vasculature is NOT under autonomic control.
Unique Environment in the Eye
The following unique characteristics of the eye allow for appropriate structure and
function:
IOP must be greater than the episcleral venous pressure so aqueous humor
can drain from the anterior chamber
through the corneoscleral meshwork
and into the venous system.
IOP must be greater than the intracranial pressure in order to maintain an
axoplasmic (i.e.
pressure) gradient that flows from the optic nerve towards
the brain.
Papilledema results from a reversal in the axoplasmic gradient between
the eye and the brain due to an increase in intracranial pressure. This
reversal causes cerebrospinal fluid to spill from the subarachnoid space
onto the optic disc margins and the surrounding RNFL.
• IOP must be lower than the pressure in the retinal and uveal arteries
allow-
ing nutrients to be delivered from the choriocapillaris to the RPE cells.
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