What are the differences between retinal and choroidal blood supply?

Differences Between Retinal and Choroidal Blood Supply

The retinal and choroidal circulations are fundamentally distinct vascular systems: the retinal circulation originates from the central retinal artery with autoregulation and low flow, while the choroidal circulation arises from the posterior ciliary arteries with high flow, fenestrated capillaries, and sympathetic control rather than autoregulation. 1

Anatomical Origin and Distribution

Retinal Circulation

• Originates from the central retinal artery, which branches from the ophthalmic artery (the first branch of the internal carotid artery in most individuals) 1
• Supplies the inner retina, including the retinal nerve fiber layer, ganglion cell layer, and inner plexiform layer 1
• Functions as an end-artery system with limited collateral circulation 2
• In approximately 15-25% of eyes, a cilioretinal artery is present that originates from the posterior ciliary circulation rather than the central retinal artery 1

Choroidal Circulation

• Originates from the short posterior ciliary arteries (branches of the ophthalmic artery) 1
• Supplies the outer retina and photoreceptors through diffusion across the retinal pigment epithelium (RPE) 1
• Exhibits a segmental distribution with posterior ciliary arteries functioning as end-arteries 2
• Also provides the main blood supply to the anterior optic nerve 2

Blood Flow Characteristics

Retinal Circulation

• Low blood flow with efficient oxygen extraction 3
• Non-fenestrated capillaries with tight junctions forming the inner blood-retinal barrier (BRBi) 1
• Prevents permeation of molecules with diameter ≥2 nm through paracellular routes 1

Choroidal Circulation

• Extremely high blood flow at 696±110 mg/min throughout the whole choroid, comparable to or exceeding renal blood flow 1, 4
• Fenestrated capillaries that allow free passage of neutral molecules up to 500 kDa into the choroidal parenchyma 1
• The outer blood-retinal barrier (BRBo), formed by the RPE, limits entry to the neural retina based on physicochemical properties 1

Regulatory Mechanisms

Retinal Circulation

• Possesses robust autoregulation that compensates for changes in intravascular pressure 1
• Lacks autonomic innervation 3
• Regulated primarily by local factors released by endothelial cells and surrounding retinal tissue 3
• Maintains stable perfusion despite systemic blood pressure fluctuations 5

Choroidal Circulation

• Lacks autoregulatory mechanisms 1
• Regulated predominantly by sympathetic innervation 1
• Has lower mean arterial pressure compared to retinal circulation due to multiple collateral flow channels 1
• More susceptible to flow impedance when outflow resistance increases 1

Clinical Implications

Perfusion Pressure Differences

• The cilioretinal artery has lower intravascular perfusion pressure than the central retinal artery due to extensive choroidal collateralization 1
• When retinal venous pressure increases (as in central retinal vein occlusion), cilioretinal artery flow is more easily compromised because it lacks the autoregulatory compensation of the retinal circulation 1

Occlusion Patterns

• In ophthalmic artery occlusion, both retinal and choroidal circulations are affected since the obstruction is proximal to both the central retinal artery and posterior ciliary arteries 1
• The classic "cherry red spot" in central retinal artery occlusion occurs because the fovea transmits the normal red coloration of the underlying perfused choroid against the ischemic whitened inner retina 1
• In ophthalmic artery occlusion, no cherry red spot appears because both circulations are ischemic 1

Drug Delivery Considerations

• The fenestrated choroidal capillaries allow systemic drugs to freely enter the choroidal parenchyma, but the RPE (BRBo) significantly restricts entry to the retina 1
• The choroid functions as a "sink" for drug clearance due to its extremely high blood flow, rapidly removing drugs from the posterior segment 1
• Lipophilic drugs demonstrate an order of magnitude higher trans-BRB permeability compared to similarly sized hydrophilic compounds 1

Functional Distinctions

Metabolic Support

• Retinal circulation provides precise, regulated nutrient delivery to metabolically active inner retinal neurons 3
• Choroidal circulation provides high-volume oxygen and nutrient supply to photoreceptors through passive diffusion across the RPE 5, 4

Additional Choroidal Functions

• The choroid serves thermoregulatory functions, cooling and warming the retina through its high blood flow 4
• Contains secretory cells involved in modulating vascularization and scleral growth 4
• Dramatic thickness changes move the retina forward and backward for focusing, independent of accommodation 4