Retinal Anatomy

Question 1

The retina is the most metabolically

active part of the eye.

Answer 1

•The photoreceptors are nourished by
the retinal pigmented epithelial cells that
receive their blood supply from the
underlying choriod.
•  90% of the blood that circulates to the
eye goes into the choroid, reflecting the
demands of the photoreceptors and
their supporting retinal pigmented
epithelium.

Question 2

Retinal Blood supply

Answer 2

•  The retina has a dual blood supply,
•  the outer third being supplied by the choroidal capillaries
•  the inner two thirds being supplied by branches of the
central retinal artery.
•  The outer retina is avascular deriving its nutrients from choroid.
•  Capillaries are absent from the fovea, its nutrients come from
choroid

•  The retina has the highest oxygen
consumption (by weight) of any human tissue.
•  Retinal blood flow is auto-regulated resulting
in flow varying by nutritional demands.
•  The choriodal circulation has a high flow rate,
low oxygen exchange, and a fenestrated
(leaky) capillary bed.
•  The retinal circulation has a low flow rate, a
high oxygen exchange, and continuous (not
leaky) capillary bed.

•  85% of the blood flow to the eye passes
to the choroid and only 4% to the retina.
•  The venous blood of the choroid has
almost as much oxygen as the arterial
blood.

Question 3

•  Capillaries may be

Answer 3

–fenestrated (have “holes”) as in the choroid

–  continuous (no “holes” as in the retina)

Question 4

•  Capillaries drain into post-capillary venules.

Answer 4

– Endothelial wall of capillary is one cell thick
–  The wall is encased in a basement membrane that
supports the endothelium.
– Pericytes control the diameter of the capillary and lie
outside the basement membrane and surround the
capillary.
– Pericytes are able to contract enabling the capillary to
maintain its shape in response to increased intra-luminal
pressure

Question 5

Central Retinal Artery

Answer 5

•  The central retinal artery is an end
artery with no anastomotic connections.
•  It derives from the ophthalmic artery, ~
1 cm posterior to the eye and enter the
inferio-nasal aspect of the optic nerve.
•  The central retinal vein accompanies
the central retinal artery.

•  After passing through the lamina cribrosa, the
artery divdes into four branches that supply
non-ovelapping sectors of the retina.
– Superio-temporal retinal artery
– Inferio-temporal retinal artery
– Supero-nasal retinal artery
– Inferrio-nasal retinal artery
•  These arteries are end-arteries and if they are
occluded then the retina they supply infarcts.

Question 6

Branches of the central retinal artery

Answer 6

Superio-nasal artery
Optic Disc
Fovea
Superio-temporal artery
Inferio-temporal artery
Superio-nasal artery

Question 7

Retinal Blood Supply

Answer 7

Two capillary layers supply the retina as deep as the inner
nuclear layer.
The inner plexus lies in the ganglion cell layer and the outer
plexus lies in the inner nuclear area.
Adjacent to the fovea there may only be one capillary layer
where around the optic disc there may be four.

Question 8

Choroidal Blood Supply

Answer 8

•  Choroid blood vessels are derived from the
short posterior ciliary arteries of the ophthalmic
artery and recurrent branches arising from the
anterior ciliary arteries.
•  The choriocapillaris consists of wide-bored
fenestrated capillaries that provide nutrition to
the outer retina.