Ocular Circulation

Question 1

The provision of nutrition and removal of waste is facilitated in the eye by which two systems of blood vessels?

Answer 1

uveal and retinal vessels

Question 2

What three sources help supply nutrients to the inner retina?

Answer 2

• diffusion from choroidal vessels
• inner retina vessels in certain species
• diffusion of nutrients from the anterior uveal vessels through the vitreous.

Question 3

What determiners perfusion pressure, and how is this evaluated in the eye?

Answer 3

perfusion pressure between arteries and veins in a circulatory system. In the eye the IOP approximates venous pressure, therefore the ocular perfusion pressure is the pressure of small arteries entering the eye minus the IOP. (clinically this is important because either lowering systemic blood pressure or increasing IOP will both decrease ocular perfusion pressure)

Question 4

In most species, where does blood flow to the ciliary body originate from?

Answer 4

combination of vessels from the iridal major arterial circle, branches of the anterior ciliary arteries, branches of the long posterior ciliary arteries.

Question 5

Which two species are found to have autoregulation of blood flow to their iris and ciliary body?

Answer 5

cat and non-human primate (limey for other speakers as well)

Question 6

carbon dioxide does what to anterior uveal vessels?

Answer 6

causes vasodilation

Question 7

stimulation of sympathetic alpha receptors in the anterior uveal vessels lead to what?

Answer 7

vasoconstriction

Question 8

stimulation of parasympathetic muscarinic receptors in the anterior uveal vessels lead to what?

Answer 8

vasodilation

Question 9

Which specific prostoglandins lead increase blood flow to the eye?. By how much do the increase blood flow, and by what mechanism?

Answer 9

• E1 and F2alpha

- vasodilation leads to a two to three fold increase in blood flow to the anterior uvea.

Question 10

What is the approximate diameter of choroidal blood vessels in most of of the species we study?

Answer 10

15-50 micrometers

Question 11

Ture of False - choroidal vessels are fenestrated and relatively highly permeable?

Answer 11

true

Question 12

Normal physiologic leakage of proteins from the choroidal vessels into the choroidal tissue leads to what?

Answer 12

a high osmotic gradient between the choroidal tissue and retina, which can help remove fluids from the retinal tissue and potential space between the retina and RPE - this facilitating retinal attachment.

Question 13

Characterize the apparent compartmentalization of choroidal blood flow, and how this might manifest itself clinically?

Answer 13

Regions of the choroid appear to be supplied by a single choroidal artery or short posterior ciliary artery. This can account for “watershed zones” seen with choroidal ischemia after marked elevation of IOP in glaucoma.

Question 14

What is the rate of uveal blood flow in the cat?

Answer 14

1.2mL/min

Question 15

What is the mean combined retinal and choroidal circulation time in the cat?

Answer 15

3-4 seconds (I take this to meant that blood pumped into the choroidal and retinal circulation takes 3-4 seconds to pass through the entire circuit before exiting)

Question 16

Due to the high flow rate of blood through the the urea, what happens to oxygen extraction from blood?

Answer 16

It decreases (only 5-10% of oxygen is extracted from blood due to the high flow rate. If flow rate drops, oxygen extraction increases)

Question 17

The high flow rate to the uvea (specifically the choroid) has two distinct advantages, which are?

Answer 17

• protected oxygen delivery to the retina in times of decreased flow rate.
• protects the eye from light generated thermal damage

Question 18

What are the possible effects of sympathetic stimulation of choroidal blood vessels?

Answer 18

Vasoconstriction with up to 60% reduction in blood flow.

Question 19

Do choroidal blood vessels possess auto regulatory mechanisms.

Answer 19

little if any

Question 20

In the cat, what % of oxygen delivered to the retina comes from the retinal vessels?

Answer 20

20% (other 80% is from choroidal diffusion)

Question 21

What is the theoretic oxygen diffusion maximum through retinal tissue?

Answer 21

143 micrometers

Question 22

How does the theoretical oxygen maximum diffusion affect those species with anangiotic fundi?

Answer 22

The retina is typically thin (less than 143 um) with short photoreceptors, no tapetum, high glycogen levels in Muller cells, and no retinal taper.

Question 23

How does the partial pressure of oxygen in retinal vessels effect retinal vascular autoregulation?

Answer 23

• increased P02 and decreased PCO2 cause vasoconstriction.

- decreased P02 and increased PC02 cause vasodilation

Question 24

in the cat, at what PC02 does vasodilation reach its maximum, and what effect does this have on blood flow?

Answer 24

maximum reached at PC02 of 75-85 mmHg, which increased blood flow from 15 to 50 mL/min

Question 25

What is retrolental fibroplasia?

Answer 25

Condition in immature eyes with developing blood vessels where high oxygen consumption leads to vasoconstriction and incomplete vascular development and vessel obliteration. After withdrawal of oxygen, vasoproliferation leads to retinal edea, hemorrhage, and tractional retinal detachment.

Question 26

Alpha receptor stimulation in retinal vessels lead to what?

Answer 26

vasoconstriction

Question 27

How does regional blood flow in the retina appear to differ from optic disc to the periphery?

Answer 27

regional blood flow decreased from optic disc to periphery

Question 28

How does regional blood flow to the retina differ in humans?

Answer 28

blood flow to the temporal portion of the retina is three times higher than to the nasal portion. (no difference between dorsal and ventral)

Question 29

What factors may be released from the retinal vascular endothelium during stress (i.e. - chemical stimuli, changes in blood pressure and flow, oxygen levels)?

Answer 29

nitric oxide, endothelins, prostoglandins, and renin-angiotensin products

Question 30

In the human, cat and rabbit, over what range of IOP does the optic nerve head possess autoregulation of blood flow?

Answer 30

~30-75mmHg (most effective in people between 6-30)

Question 31

Where does the optic nerve head typically receive it’s blood supply from?

Answer 31

short posterior ciliary arteries

Question 32

In the dog, what is the retrolaminar pressure for ganglion cell axons passing through the lamina cribrosa?

Answer 32

~7mmHg, which corresponds directly to cerebrospinal pressure