TWIL #069 - The Cornea Gets Its Oxygen Directly from the Air
The cornea is the only tissue in the human body that gets its oxygen not from blood vessels, but directly from the atmosphere - which is why wearing contact lenses for too long can suffocate it.
- #biology
- #medicine
- #science
The cornea is the transparent dome at the front of the eye that refracts light before it enters the lens. It must be transparent to function - which means it cannot contain blood vessels, because vessels would scatter light and impair vision. Like cartilage, it is avascular.
How it gets oxygen: Without blood supply, the cornea obtains oxygen by direct diffusion from the atmosphere. The tear film on the surface of the eye absorbs oxygen from the air and delivers it to the corneal cells. The inner layers of the cornea also receive some oxygen from the aqueous humour (the fluid inside the eye), but the primary source for the outer layers is atmospheric.
Contact lenses and corneal hypoxia: Extended contact lens wear reduces the amount of oxygen reaching the cornea by creating a barrier between the tear film and the air. The cornea responds to oxygen deprivation (hypoxia) by growing new blood vessels - a process called corneal neovascularisation or corneal pannus. These vessels impair transparency and can cause permanent vision damage. This is why contact lens wear time is limited, why silicone hydrogel lenses (which are far more oxygen-permeable) were a significant advancement, and why sleeping in contacts is strongly discouraged.
Transparency mechanism: The cornea's transparency depends on the highly regular arrangement of collagen fibrils in the stroma (the thick middle layer). The fibrils are uniformly spaced at intervals smaller than the wavelength of visible light, so they don't scatter light. The corneal cells actively pump out water using sodium-potassium pumps to maintain this precise hydration state - if these pumps fail (as in corneal disease), the tissue swells, the fibril spacing becomes irregular, and the cornea becomes cloudy.
Transplants: The cornea is one of the most successfully transplanted tissues, with a very low rejection rate compared to organ transplants, because the immune system has limited access to avascular tissue. Donor corneas can also be stored for weeks under proper conditions.