In the human body, virtually every living cell depends on a dense network of blood capillaries that supplies oxygen through red blood cells. However, there is an astonishing biological exception in the front window of your eyes: the cornea. If this natural lens had blood vessels, light would strike the erythrocytes, scattering and leaving us with completely blurred and unusable vision.
To overcome this evolutionary dilemma and guarantee a crystalline transparency close to 99 %, the cornea is a strictly avascular tissue that obtains the vast majority of its oxygen by breathing directly from the atmosphere. The process works through a refined physiological choreography:
- Tear diffusion: Oxygen from the outside air first dissolves in the tear film (a microlayer of just 3 to 4 micrometers) and then passively diffuses through the epithelial layers into the stroma.
- The nighttime shift: When closing the eyelids to sleep, the airflow is cut off and the oxygen concentration drops by about 65 %. During sleep, the cornea survives thanks to the capillaries of the palpebral conjunctiva (the inside of the eyelid), causing a slight swelling or physiological edema with which we usually wake up.
- Nutrition through aqueous humor: Glucose and other essential nutrients that do not come from the air are transported by the aqueous humor in the anterior chamber of the eye.
Understanding this direct respiration is crucial for eye health: excessive use of low-permeability contact lenses can suffocate the tissue, causing corneal hypoxia and prompting the eye to grow emergency new blood vessels that threaten to destroy vision.
Source: American Academy of Ophthalmology (AAO) / Journal of Optometry: 'Corneal oxygenation and metabolism'
