The veins in your eyes have different names depending on where they sit. The ones you can see on the white of your eye are called episcleral veins and conjunctival veins. Deeper inside, where light is processed into vision, the central retinal vein handles the main drainage work. A third set, called vortex veins, drains the middle layer of the eye wall. Together, these veins carry deoxygenated blood out of the eye and back toward the heart.
Veins on the Surface of the Eye
The tiny red lines you sometimes notice on the white of your eye are episcleral veins and conjunctival veins. The conjunctival veins sit in the thin, transparent membrane that covers the white part (the sclera) and lines your eyelids. Just beneath them, the episcleral veins run along the surface of the sclera itself. Both sets eventually feed into the superior ophthalmic vein, a larger vessel that carries blood out of the eye socket and toward the brain’s venous drainage system.
These surface veins are normally thin and barely visible. When they dilate, your eyes look “bloodshot.” That can happen from dry air, allergies, lack of sleep, or irritation from contact lenses. In most cases it’s harmless and temporary. Persistent dilation, though, sometimes signals higher pressure inside the eye. Episcleral venous pressure directly influences intraocular pressure (the fluid pressure inside the eyeball), so chronically swollen episcleral veins can be a sign of conditions like thyroid-related eye disease, inflammation of the sclera, or abnormal connections between arteries and veins in the orbit.
The Central Retinal Vein
The central retinal vein is the single most important vein inside the eye. It drains blood from the retina, the light-sensitive tissue at the back of your eye that makes vision possible. Smaller branch veins spread across the retina like a tree’s roots, collecting blood from the tissue, then merge into one central vessel. That vessel exits the eyeball through the optic nerve, about 10 to 12 millimeters behind where the nerve leaves the eye, running alongside the central retinal artery.
Once outside the eye, the central retinal vein either joins the superior ophthalmic vein or drains directly into the cavernous sinus, a large venous channel at the base of the skull. The pressure inside this vein roughly matches the pressure inside the eyeball, which keeps the vessel stable as blood flows through it.
Eye doctors can actually see the retinal veins during a routine eye exam using an ophthalmoscope. Retinal veins look darker and redder than their companion arteries and are slightly thicker. Arteries tend to have a brighter central light reflex (a thin bright stripe running down the middle), while veins appear more uniformly dark. Near the optic disc, where the optic nerve meets the retina, arteries and veins alternate in a spoke-like pattern.
Vortex Veins
The vortex veins drain the uvea, the eye’s middle layer, which includes the iris, the ciliary body (the muscle that focuses your lens), and the choroid (a blood-rich layer that nourishes the outer retina). Most eyes have four or five vortex veins, though the number ranges from four to eight. There is at least one vortex vein in each quadrant of the sclera. They pierce through the white of the eye toward the back and connect to the ophthalmic veins in the orbit.
You can’t see vortex veins from the outside. They sit deep within the eye wall and are only visible during surgery or specialized imaging. Their main job is handling the high volume of blood that flows through the choroid, which has one of the richest blood supplies of any tissue in the body.
How Blood Flows Out of the Eye
All of these veins funnel into a common exit path. The episcleral veins, central retinal vein, and vortex veins all drain into the superior ophthalmic vein (and to a lesser extent the inferior ophthalmic vein), which carry blood out of the eye socket. From there, blood empties into the cavernous sinus and eventually returns to the heart through the jugular veins. The eye has no valves in most of its venous system, so blood flow depends on pressure differences rather than one-way gates. This makes the eye’s veins somewhat vulnerable to pressure changes elsewhere in the body.
What Happens When Eye Veins Get Blocked
The most common serious problem involving eye veins is retinal vein occlusion, a blockage in either the central retinal vein or one of its branches. A blockage in the main vessel is called central retinal vein occlusion (CRVO), while a blockage in a smaller branch is called branch retinal vein occlusion (BRVO). Both types typically affect only one eye.
Symptoms include blurry vision or sudden vision loss (which can develop over hours or days), floaters (dark spots or lines drifting across your field of vision), and in more severe cases, pain or pressure in the eye. High blood pressure, diabetes, and glaucoma are the most common risk factors. BRVO is generally less severe than CRVO because only part of the retina loses its drainage, but both require prompt evaluation to preserve vision.
On the surface of the eye, chronically dilated episcleral veins can raise intraocular pressure high enough to damage the optic nerve over time. Known causes include blood clots in the brain’s venous sinuses, abnormal arteriovenous connections in the orbit, and Sturge-Weber syndrome. In some people, episcleral veins dilate without any identifiable cause, a condition called idiopathic dilated episcleral veins, where the venous pressure in the affected vessels measures roughly twice the normal value.