Choroidal Blood Supply

The retina needs a lot of energy to function. Only the inner part of the retina has its own blood vessels. Most of the retina (especially the outer part with photoreceptors) depends on the choroid for oxygen and nutrients. (1) (2)

The choroid is one of the most highly vascular tissues in the body. It has the highest blood flow per gram of tissue. Its job is to:

• Supply large amounts of oxygen
• Remove heat produced by retinal activity
• Act as a cooling system

Unlike most tissues in the body, the choroid does not have strong local autoregulation. This is because it is separated from the retina by a barrier (RPE and Bruch’s membrane), so direct feedback control is limited. (1) (2)

There are additional challenges:

• Blood enters the eye under high pressure (about 70% of systolic blood pressure).
• The eye is a rigid, closed space — even small volume increases raise intraocular pressure significantly.
• Capillaries must maintain low pressure to prevent leakage.
• Blood flow is pulsatile (with each heartbeat), so the eye must buffer these pressure changes to protect vision.

The eye is a closed, rigid system and has two separate blood supplies (1) (2):

1. The retinal circulation is supplied by the central retinal artery, which arises from the ophthalmic artery. It enters the optic nerve, travels within it, and passes through the lamina cribrosa to reach the eye, where it supplies the inner layers of the retina. Within the retina, it divides into branches in a segmental pattern, with each branch supplying a specific retinal area.

Venous blood from the retina is collected by retinal veins, which converge at the optic disc to form the central retinal vein. The central retinal vein exits the eye through the lamina cribrosa, courses within the optic nerve, and ultimately drains into the superior ophthalmic vein, which then empties into the cavernous sinus. There are also venous connections with the facial and other orbital veins. Importantly, in the retina, arteries and veins travel together. (1)

2. The Choroidal circulation comes from the posterior ciliary arteries (PCAs)—typically 2-5 per eye, arising from the ophthalmic artery—are the primary source. Short PCAs (10-20 branches) encircle the optic disc to supply the posterior choroid and optic nerve head, while long PCAs (usually 2: medial and lateral) extend to the periphery, feeding the anterior choroid, ciliary body, and iris.

The choroid has five main layers from inner (retinal side) to outer: Bruch’s membrane, choriocapillaris, Sattler’s layer (medium and small vessels), Haller’s layer (large vessels), and the suprachoroid (loose connective tissue with melanocytes). Haller’s and Sattler’s layers form the main vascular stroma.

Terminal arterioles from Sattler’s layer enter the choriocapillaris, where they form hexagonal lobules measuring about 50–350 μm. Each lobule acts as an independent microvascular unit, supplying a specific choroidal territory with minimal interconnections between adjacent lobules. Within each lobule, blood flows centrifugally — from the central feeding arteriole toward the periphery — and then drains into collecting venules in Sattler’s layer. This segmental and radial pattern produces the characteristic lobular filling pattern seen on indocyanine green angiography (ICGA).

Choroidal arterioles from posterior ciliary arteries (PCAs) function as end-arteries despite some anatomical anastomoses, leading to patchy infarcts upon occlusion due to limited functional collaterals, particularly in elderly or diseased states. Watershed zones at PCA segment borders, converging submacularly (nasal/temporal), heighten macular ischemia risk, explaining focal lesions in APMPPE where choriocapillaris hypoperfusion causes placoid RPE/outer retinal infarcts

Deoxygenated blood from choroidal lobules collects into small veins that form 4-5 vortex veins. Vortex veins (vorticose veins) are the main drainage channels for choroidal blood, typically numbering 4–8 per eye (most commonly 4–5), with at least one in each scleral quadrant. Their exit sites lie 6 mm posterior to equator through scleral canals (ampullae, 0.5–1 mm diameter), forming quadrant watersheds (macular horizontal, vertical papillomacular). (3) They are valveless, thin-walled (no smooth muscle), they collect from equatorial Haller’s veins, course intrasclerally, and  then flow via ophthalmic veins to the cavernous sinus and internal jugular vein, with minimal anastomoses between segments.  

Thus, valveless vortex veins allow bidirectional flow, meaning blood can exit the choroid into orbital veins but also reflux back.  Normally, choroidal pressure (~20-30 mmHg) exceeds episcleral venous pressure (8-10 mmHg), driving outflow through thin-walled, collapsible vortex veins without valves to block reverse flow. The intrascleral segment acts as a Starling resistor: external scleral compression collapses the vein during high intraocular pressure (IOP), limiting inflow but permitting reflux if choroidal pressure surges (e.g., congestion). (3)

Retinal vs Choroidal Blood Flow Control

The retina has autoregulation, but no direct neural control.

This means:

• Retinal blood vessels adjust their own blood flow.
• They respond to local needs (oxygen demand, metabolic activity).
• Control happens through local chemical signals (metabolic and myogenic mechanisms).

Because retinal vessels are embedded within the retinal tissue, they can directly sense changes and adjust blood flow accordingly. On the other hand, the choroid does NOT have strong autoregulation, but it DOES have neural control. Unlike retinal vessels, the choroid is separated from the retina by:

• Retinal pigment epithelium (RPE)
• Bruch’s membrane
• Subretinal space

Because of this separation, it cannot directly sense retinal metabolic needs. Instead, choroidal blood flow is controlled mainly by the autonomic nervous system.

References: