At WiseGEEK, we're committed to delivering accurate, trustworthy information. Our expert-authored content is rigorously fact-checked and sourced from credible authorities. Discover how we uphold the highest standards in providing you with reliable knowledge.
The cornea is undoubtedly one of the human body’s most incredible structures. It’s the transparent portion of the eye that covers the iris, pupil, and anterior chamber, helping to refract light and contributing most of the eye’s focusing power.
To allow you to see, the cornea must be transparent, so it is avascular, which means it doesn’t have any blood vessels. Yet it is a living tissue that requires oxygen to survive. This raises an interesting question: how does oxygen reach the cornea if it doesn’t have a direct blood supply? After all, the vast majority of tissues depend on oxygen transported around the body by blood.
Diffusion is the key to providing the cornea with the oxygen and nutrients it needs to stay healthy. On the outside of the eye, tear fluid comes into contact with the air and brings essential nutrients to the cornea. Inside the eye, the aqueous humour delivers oxygen via simple diffusion to the back of the cornea.
When you are asleep, the cornea still has access to oxygen via exposure to the palpebral conjunctiva (the lining of your eyelids). This extensive blood vessel network comes into contact with the tear film on your cornea, thus allowing oxygen and nutrients to reach the cornea even when your eyes are closed. However, when closed, your eyes only receive around one-third of the oxygen they get from the air when you are awake.
When the cornea is deprived of oxygen from outside the eye, it can develop blood vessels—a process known as corneal neovascularization—which can severely impact your vision. In extreme cases, this can happen to contact lens users who keep their lenses in while sleeping, though it is less common with silicon hydrogel lenses. When awake, contact lenses move a tiny amount (around a millimeter per blink) to allow oxygen in. Yet this doesn’t happen when you’re asleep, and neither does the cornea come into contact with the palpebral conjunctiva. Wearing contact lenses overnight is also associated with infections, corneal ulcers, and keratitis.
The incredible cornea:
- Due to the density of sensory nerve fibers, the cornea is an extremely sensitive part of the body. Studies suggest that the density of pain receptors in the cornea is several hundred times higher than in the skin and at least 20 times higher than in dental pulp.
- In addition to being one of the most sensitive body parts, the cornea is the fastest-healing tissue in the human body. Many corneal abrasions will heal in less than 36 hours.
- The cornea is not the human body's only avascular tissue. Cartilage, hair, nails, tooth enamel, and outer skin layers also lack blood vessels.