Your Cornea Has No Blood Vessels

The crystal-clear window at the front of your eye, the cornea, achieves its transparency through a unique biological trade-off. In order to provide an unobstructed path for light to enter the eye, it completely forgoes a direct blood supply. Blood vessels, even microscopic capillaries, would scatter light and cloud our vision. Instead of relying on blood for sustenance, the cornea has evolved a clever two-part system. It absorbs vital oxygen directly from the atmosphere whenever your eyes are open, and it draws all of its other nutrients from the tear film on its surface and the aqueous humor, a clear fluid circulating within the eye behind it.

This avascular (bloodless) nature has a remarkable consequence for medicine. Because the cornea is isolated from the bloodstream, it is also largely hidden from the body's immune system, a state known as "immune privilege." This makes corneal transplants one of the most successful transplant procedures in history, as the risk of the recipient's body rejecting the donor tissue is significantly lower than with other organs. On a more common level, this is also why modern contact lenses are engineered to be highly gas-permeable. Any lens placed on the eye must allow a steady flow of oxygen to pass through, ensuring the cornea can continue to "breathe" from the air around it.