What makes up our tears?
Put simply, there are three main layers of tears, all equally important to ensure a long lasting and effective tear film. It is also important to have the correct balance and composition of these layers to ensure the eye surface is adequately protected and nourished.
- Mucin Layer
This is the first layer at the bottom of our tears. The mucin layer is made up of mucous like structures that act like glue to stick the tears onto the eye. Without this layer, the tears lack resilience, preventing them from coating the eye completely.
- Aqueous Layer
This middle layer makes up the bulk of the tear film. The main watery layer is full of nutrients for hydrating and keeping the surface of the eye healthy, as well as antibacterial components to help build a healthy barrier of protection.
- Lipid Layer
At the surface, the top layer is composed of oil. This oil layer ensures that the watery layer does not evaporate or dry up in the wind. It also allows the tears to coat and spread across the eye evenly.
Why are tears important?
To understand Dry Eye and how we treat it, it is important to understand first why tears are important and what makes them special.
Human tears have many important roles in keeping the eyes healthy, and these roles are useful in understanding why Dry Eye can be so bothersome.
The Cornea is the front window of the eye, like a windscreen. It is important for creating crisp, clear vision. This surface needs to be smooth and even with no bumps or cracks; as well as completely transparent and regular. In the same way windows are hard to see through if cracked, scratched or frosted, a cornea must be clear to function well.
Human corneas are loaded full of nerves. This nerve sensation makes our corneas incredibly sensitive, and these nerves keep our cornea protected. The high density of nerve endings ensures any damaging insults are detected very quickly. Hence getting an eyelash or a grain of sand in your eye is so painful!
The function of our tears is to:
- Provide oxygen and nutrients to the cornea. Cells in the body require oxygen and nutrients to survive, though as the cornea is transparent, it cannot have blood flowing through it. Therefore, lots of nutrients and oxygen come from the tears. Tears help hydrate and provide the cornea with oxygen from the atmosphere. Without tears, the cells in the cornea will struggle to survive.
- Provide an even refractive surface for clear vision. An even distribution of tears helps to level out bumps or irregularities in the cornea (because it’s not perfect.) Like putting grout over cracks in the wall, it smooths the surface so when light enters the eye, it doesn’t scatter when passing through.
- Wash away debris. Dead cells, makeup, bacteria, dust and other particles from the atmosphere accumulate in and around the eye. So we do not need to wash our eyes with soap and water, tears are vital to wash away rubbish.
- Protect ocular surface from blink friction. Eyelids are important to spread tears across the ocular surface evenly, as well as assist in clear debris. A good layer of tears is required to allow the eyelids to blink over the eye smoothly without causing any damage or irritation. Think of it like wiping your windshield with dry or cracked wiper blades, or when there is no rain and the wipers grate across the car windscreen.
- Fight bacteria and bugs wanting to attack the cornea. Tears contain several mechanisms to fight bacteria and pathogens that come into contact with the ocular surface. Tears have strong antimicrobial properties which are very important to prevent sight threatening infection.
How are tears made?
Eyelids are vital for protecting and nourishing the surface of the eye. Eyelids have several types of glands, which secrete different important components of the tears, including the mucin layer and the lipid layer. They also provide a strong physical barrier to entering particles, as well as a rapid reflex to knock any nasties out of the way. Like windscreen wipers, our eyelids smooth out the tears, ensuring a nice even distribution of tears across the entire eye. Without healthy eyelids, tears cannot be produced or function properly.
The lacrimal gland, tucked away inside the eye socket, with the help of other small eyelid glands, produces the watery component of the tears known as the aqueous layer. The majority of the tear film is made up of this watery component and it keeps our eyes moist.
The mucus, or glue, layer in our tears is produced by the goblet calls on the conjunctiva. The conjunctiva is the stickly glad-wrap layer that sits on top of the whites of our eyes. The lipid, or oil, layer of our tear film is produced by little vertical glands that run along the top and bottom eyelids called the meibomian glands.
The lacrimal puncta is the tear drain, which allows tears to exit the eye. There are a few small drainage holes, allowing tears and all the other debris to wash away. Without effective drainage, tears are likely to overflow and become stagnant.