Oxygen is a vital life-giving molecule that sustains life on Earth. All mammals, including humans, aerobically respire to produce energy for living. Within the human body, oxygen availability helps organisms reproduce, grow, and break down food into energy we need to survive. This essay delves into the intriguing working mechanism of cornea and how they exchange oxygen from the atmosphere to maintain ocular health.
Physiology and the function of the cornea
The cornea is the clear, dome-shaped tissue covering the outermost layer of our eyes, functioning as a window of our ocular system. Its primary function is to control and focus the entry of light, accounting for between 65 and 75% of the eye’s total refractive power (Eye Associates of New Mexico, 2023). Focusing the light rays on the retina with minimal scatter and optical degradation enables us to see clearly. Moreover, the cornea is responsible for the protection of the eye’s inner structures. The cornea’s three layers, the epithelium, stroma, and endothelium, shield the interior of the eye from dust, pathogens, and debris (Cornea Research Foundation of America, 2022).
Image 1: Structure of the Eye
Why does the cornea breathe?
Unlike other parts of the body’s tissue, the cornea does not possess lungs or a respiratory system, nor does it have blood vessels that supply blood via oxyhemoglobin. The cornea is indeed non-vascular, but it still needs oxygen. Therefore, its oxygen source must come from diffusion. The epithelium, the cornea’s outermost layer, is in direct contact with the tear film, which contains dissolved oxygen from the atmosphere. Every time one blinks, a thin transparent layer called a ‘tear film’ spreads across the cornea. This film consists of three distinct layers: the oily layer, the watery middle layer, and the inner mucus layer. Passive Diffusion occurs when the oxygen molecules in the tear film diffuse across the tear film-cornea interface down the concentration from a region of higher partial pressure to that of lower partial pressure within the corneal tissue (NIH, 2019). Not just that, the endothelium, the cornea’s innermost layer, optimises the oxygen supply by actively regulating the moisture level of the ocular surface. It acts as a pump to maintain corneal deturgescence, efficiently evacuating excess fluid from the stroma. Since oxygen is more soluble in a slightly dehydrated corneal matrix, the pump action of endothelial cells keeps the stromal environment favourable for oxygen diffusion.
Implication of corneal oxygenation
Recent studies have provided light on the significance of corneal oxygenation in our bodies. The oxygen tension in the cornea, or the amount of oxygen present, is crucial in maintaining ocular health since low oxygen levels in the cornea can lead to hypoxia. This disease produces symptoms such as graying, tunnel vision, and blurred sight.
Significantly, wearing a contact lens can interfere with the oxygen supply within the cornea. Research has shown that contact lenses covering the entire cornea surface are essentially cutting off the oxygen supply to the cornea, thus reducing the tissue oxygen tension, often enough to the level below the crucial hypoxia threshold (Mertz, 1991). This interference can lead to complications such as corneal neovascularisation, where blood vessels grow through the cornea to supply oxygen.
To enhance corneal oxygenation, researchers continuously explore new technology, such as breathable contact lenses or surgical techniques, to improve long-term ocular health.
In summary, the cornea does not technically ‘breathe’ like humans. Yet, its intricate and complex system allows oxygen exchange to maintain transparency and functionality. Thus, understanding the complexities of how the cornea retains its oxygen supply is critical for progressing eye care and protecting our vision.
Eye Associates of New Mexico. (n.d.). The function of The Cornea | What is the Cornea? | Eye NM. [online] Available at: https://www.eyenm.com/cornea-center-albuquerque/what-is-the-function-of-the-cornea/#:~:text=The%20cornea%20acts%20as%20the.
What is the cornea? (2022) Cornea Research Foundation of America – What is the Cornea? Available at: https://www.cornea.org/Learning-Center/How-the-Eye-Works.aspx#:~:text=The%20cornea%20is%20the%20clear,you%20to%20have%20good%20vision. (Accessed: 13 October 2023)
Sridhar, M.S. (2018) Anatomy of the cornea and ocular surface, Indian Journal of Ophthalmology. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5819093/ (Accessed: 13 October 2023).
How tears work (no date) National Eye Institute. Available at: https://www.nei.nih.gov/learn-about-eye-health/healthy-vision/how-eyes-work/how-tears-work (Accessed: 13 October 2023). Hypoxia – considerations in contact lens use under adverse conditions … Available at: https://www.ncbi.nlm.nih.gov/books/NBK234101/ (Accessed: 13 October 2023).
Chair of Biology Society