Curiosity 7 of 10
7. The cornea receives oxygen directly from the air
The cornea, the transparent part at the front of the eye, has no blood vessels. Much of the oxygen it uses comes directly from the air, dissolved in the tear film.
Transparent because it has no blood
For light to pass through without distortion, the cornea must be completely transparent. Blood vessels would interfere with vision, which is why it has none. Its fibers are arranged in a very regular way, which also helps its transparency.
With no blood to carry oxygen, the cornea depends on the air. Oxygen dissolves into the thin layer of tears covering the eye and passes into the cells. Nutrients also arrive through the fluid behind it, inside the eye.
During sleep, with the eyelids closed, oxygen comes mainly from the blood vessels on the inner side of the eyelid, and the cornea receives less than during the day. That is why sleeping in contact lenses, which already reduce oxygen flow, is usually discouraged.
The lack of blood vessels also helps in transplants. Since blood barely reaches the cornea, the body's defenses take longer to recognize the new tissue, and cornea transplants are among the most performed in the world.
The cornea also does most of the eye's focusing work, about two-thirds. The lens, located behind the iris, handles the fine adjustment for seeing up close and far away.
Did you know? The cornea is one of the body's parts with the most nerve endings per area. That is why a speck of dust in the eye is so irritating, and the blink reflex is so fast.
