The choroid is a thin, blood vessel-rich layer in the back of your eye that keeps the retina nourished, oxygenated, and at the right temperature. It wraps around the rear two-thirds of the eyeball, sandwiched between the sclera (the white outer wall) and the retina (the light-sensing layer). Despite being only about a third of a millimeter thick, the choroid has the highest rate of blood flow per unit weight of any tissue in the body.
Where the Choroid Sits in the Eye
Picture the back of your eye as three nested layers. The outermost is the sclera, the tough white shell that gives the eyeball its shape. Just inside that sits the choroid, packed with blood vessels and dark pigment. The innermost layer is the retina, which converts light into electrical signals your brain reads as vision. The choroid is essentially the retina’s life-support system, positioned right where it needs to be to deliver blood and absorb excess light.
In a healthy adult eye, the choroid measures roughly 270 to 450 micrometers thick at its thickest point beneath the center of the retina, with an average around 360 micrometers. That thickness matters: it reflects how many blood vessels are layered inside.
Feeding the Retina
The retina’s light-detecting cells, especially the photoreceptors in the outer retina, are energy-hungry. They don’t have their own direct blood supply. Instead, the choroid’s dense network of tiny capillaries delivers oxygen and nutrients upward through a thin barrier called Bruch’s membrane. This capillary bed, called the choriocapillaris, sits right against Bruch’s membrane and uses specialized transport mechanisms to shuttle proteins and other molecules across to the retinal pigment cells above.
The same system works in reverse for waste. As photoreceptors burn through energy and shed cellular debris, the retinal pigment layer passes those waste products back down through Bruch’s membrane into the choroidal bloodstream, which carries them away. When this waste-removal process slows down, typically with age, deposits called drusen can accumulate between the layers, a hallmark of age-related macular degeneration.
Cooling the Retina
Light that enters your eye gets focused onto a tiny area called the macula, and that concentrated light generates heat. Your retina can’t tolerate much temperature change without its cells becoming damaged, so the choroid acts as a cooling system. Its high-volume blood flow absorbs heat from the retinal pigment layer and carries it away, much like a radiator circulating coolant. Research published in the journal Ophthalmology found that this heat-dissipating role may be a primary reason the choroid maintains such an unusually high blood flow rate in the first place, not just to deliver nutrients.
Blocking Stray Light
The choroid is noticeably darker than surrounding eye tissues because it contains large amounts of melanin, the same pigment that colors skin and hair. This pigmentation serves an optical purpose: it absorbs stray light that passes through the retina, preventing it from bouncing off the sclera and back into the eye. Without this light-absorbing layer, scattered photons would create internal glare, reducing contrast and making images look washed out. Think of it like the matte black interior of a camera body, which exists for the same reason.
Regulating Eye Pressure and Fluid Balance
The choroid plays a role in maintaining the pressure balance inside the eye. Fluid dynamics between the choroidal capillaries and the surrounding tissue follow a balance of hydrostatic and osmotic pressure. When that balance is disrupted, whether through inflammation, changes in blood pressure, or other factors, fluid can accumulate in the space above the choroid, a condition called choroidal effusion. This fluid buildup can push against the retina and distort vision.
Influencing Eye Growth and Myopia
One of the choroid’s more surprising roles involves signaling whether the eye should grow longer or shorter during development. In children and young adults, the choroid appears to help regulate the remodeling of the sclera, influencing how the eyeball elongates. In animal studies, when the eye is pushed toward nearsightedness, the choroid thins before the eye actually stretches longer, suggesting it acts as an early mediator in the process.
This relationship holds in humans too. People with normal vision tend to have a choroidal thickness around 250 micrometers at the center of the retina, while those with high myopia can see their choroid thin to less than 30 micrometers. The more severe the nearsightedness, the thinner the choroid becomes. This thinning isn’t just a consequence of the eye stretching; it likely contributes to the vision problems that come with high myopia, since a thinner choroid delivers less blood to the retina.
What Happens When the Choroid Fails
Because the choroid is the retina’s primary supply line, problems in the choroid tend to show up as vision problems quickly. One of the most common is choroidal neovascularization, where abnormal new blood vessels grow from the choroid through Bruch’s membrane and into the retina. These fragile vessels leak fluid and blood, damaging the macula. In a study of 100 people with this condition, 59% reported seeing flickering or flashing lights in the affected eye, and 12% experienced formed visual hallucinations, such as seeing patterns, faces, or objects that weren’t there.
Choroidal thinning is also increasingly recognized as an early marker for conditions like age-related macular degeneration and pathologic myopia. Clinicians now routinely measure choroidal thickness using imaging scans to track disease progression, since changes in the choroid often precede visible damage to the retina itself.