Tear fluid
introduction
Tear fluid is a body fluid that is continuously formed and secreted in humans by the lacrimal glands that are located laterally above the two outer corners of the eye. By blinking regularly, the tear fluid is distributed and thus protects the eye from drying out.
Components of the tear fluid
The tear film that is deposited on the cornea consists of three layers. The mucous layer is located directly on the cornea and contains, among other things, the mucin substance mucin and glycoproteins. This layer ensures that the tear secretion can be distributed evenly over the entire eye.This is followed by an aqueous layer, which is the actual tear fluid. A layer of fat adjoins the air; it has an oily consistency and helps protect the cornea.
The tear fluid consists mainly of water. Other important components are table salt (which is why tears taste salty), various proteins (for example the antibacterial lysozyme and antibodies), sugar (glucose), some inorganic substances and nitrogenous substances. The pH value of tears is around 7.35.
Production and outflow
Most of the tear fluid is produced in the lacrimal gland (Glandula lacrimalis) located above the eye. From there it is passed through 6 to 12 ducts into the eye, where it can spread over the entire cornea by blinking.
How much tear fluid is produced per day is not that easy to say. The values in the literature vary between 1 and 500 milliliters per day. The difficulty arises from the fact that the amount of tears produced depends on a large number of different factors. On the one hand, it varies with age: children and adolescents produce more tear fluid than adults.
There are also some external stimuli that stimulate the production of tears, including foreign bodies, cold and extreme emotions such as laughing and crying. In addition, tear production drops sharply during the night, much more tear fluid is required to be awake during the day, which is why many people also have an increased secretion of tears when yawning.
The tear fluid that is formed flows through two small dots in the inner corner of the eye (one above, one below) through two thin tubes into the tear sac, which is located to the side of the bridge of the nose. From there, the fluid reaches the nasal passage, which ultimately flows into the nasal cavity, where the secretion can then flow away.
With the help of the so-called Schirmer test, a doctor can assess whether the production of tear fluid is occurring to an appropriate extent. For this purpose, a special strip of paper is hung in the patient's lower eyelid. After 5 minutes this is removed again and measured to what extent it has moistened. Values around 15 mm are normal, everything below 5 mm is considered to be pathological and should be further clarified.
How can you stimulate the production of tear fluid?
A lack of tear fluid can cause dry, irritated eyes, which are often uncomfortable. Sufficient tear fluid production is important to prevent this. In order for enough tear fluid to be produced, there must be enough fluid in the body. It is therefore recommended to drink 1.5-2 l daily. This does not apply to people with certain diseases such as a weakness of the kidneys (kidney failure) or the heart (heart failure). If you are unsure about the optimal amount to drink, speak to a doctor about it.
Smoke, drafts or dry air can also reduce the production of tear fluid. Avoid drafts and air conditioning and make sure there is sufficient humidity in the rooms in which you are staying. In addition, prolonged work on the computer can lead to reduced production, so it is best to take a break of about 10 minutes every hour.
Light massaging of the eye can also stimulate production. To do this, you can stroke the upper and lower eyelids one after the other from the nose outwards. It should not be rubbed hard, as this also heals the eyes. There are a number of drugs (e.g. anticholinergics) that have a side effect of decreased tear production. This is where stopping these drugs helps. However, such a measure should only be taken in consultation with a doctor.
Functions of the tear fluid
The tear fluid fulfills several important functions. The most important is that it serves to protect the cornea. It cleans the conjunctival sac: by moistening and blinking, smaller foreign bodies can be removed from the eye, substances such as lysozyme or lipocalin prevent pathogens from entering the eyes and flaked, dead epithelial cells can be removed. In addition, the tear film ensures that the eyelids can slide along the cornea without much friction and thus do not trigger any irritation. Since the cornea itself is not supplied by blood vessels and therefore also not supplied with nutrients, the cornea is nourished with the help of the tear fluid that flows around it. Last but not least, we can of course also use the tear fluid to express emotional states through crying.
Tear disorders
Diseases that affect the tear fluid can be divided into overactive and underactive. Usually, external stimuli such as emotional excitement, foreign bodies, chemical or physical stimuli lead to increased tear production, which reflexively trigger an increased production of tear fluid, which leads to the overflow of tears, also known as tears or epiphora. In the true sense of the word, this is not a disease, but a natural process that is supposed to protect the eye. In rare cases, however, tears can also be caused by a blockage of the draining tear ducts and must then be treated.
More often, however, there is insufficient tear fluid production. This can occur on the one hand through the use of certain medications or through external factors such as cold, dry air, smoke or wind. In addition, the amount is sometimes limited in non-specific conditions such as inflammation in the area of the eye. Inflammation of the lacrimal glands themselves also occurs, but is rare. There are also certain diseases associated with dry eyes, the most important of which is the so-called Sjogren's syndrome. This is an autoimmune disease in which several glands on the face are unable to produce, which is why patients complain of both dry eyes and dry mouth. In some cases, dry eyes do not cause any discomfort, but most of the time the eyes feel irritated and in the worst case the cornea can be damaged, which is why dry eyes are usually treated with eye drops that can be purchased in the pharmacy. Apart from that, the cause of the dry eye must of course always be looked for and an underlying disease treated appropriately if necessary.
What is the cause if the tear fluid does not drain out?
The tear fluid usually takes a very specific route. After its formation in the lacrimal gland (glandula lacrimalis) on the top-outside of the eye, it runs over the eye towards the nose. Then it flows through the upper and lower lacrimal point (punctum lacrimalis superior, punctum lacrimalis inferior) through the lacrimal tubules (canaliculi lacrimalis) into the lacrimal sac (saccus lacrimalis). From there the tear fluid flows through another duct (ductus nasolacrimalis) into the pharynx, where the tears are swallowed. If this drainage is disturbed at one point, the tear fluid “overflows” and runs out of the eye. This is known as tears (Epiphora).
The occlusion of a structure leading to the tear duct can be the cause. These occlusions can be congenital, in which case the tears appear from birth. But they can also only develop in the course of life, e.g. after inflammation or as a simple aging process. In very rare cases, a newly growing tumor can also cause the blockage. In addition, acute inflammation of the tear ducts (e.g. canaliculitis) can lead to an obstruction. In this case, side effects such as redness, pain and swelling usually occur.
Find out more about the following topics here:
- Lacrimal drainage disease
- Clogged tear duct
What causes the tear fluid to sting?
Usually the feeling that the tear fluid is burning in the eye does not come from the tear fluid. This is usually composed in such a way that it is well tolerated by the eye (e.g. less than approx. 1% salt). However, with dry eyes it can be that the eye becomes irritated if an "unexpected" amount of tear fluid wets it. It is typical to start burning when one is crying. The burning sensation should soften as the crying continues. In this case, the burning sensation should be treated like dry eyes. If symptoms persist, a doctor should be consulted.
Read more about the topic: Burning in the eye
What is artificial tear fluid?
Artificial tear fluid is the term used to refer to agents whose composition roughly corresponds to the body's own tear fluid and which are used to replace the body's own tear fluid. This can be necessary if the body's own tear fluid is not available in sufficient quantity to fulfill its tasks. Artificial tears must then be supplied to protect the eye from infections and to avoid irritation.
These products are also known as tear substitutes. They always contain water and fats (lipids), which form a kind of protective film and prevent the water from evaporating immediately. In addition, sugar (glucose), salts and proteins are often found. In addition, many artificial tear fluids contain hyaluronic acid. This binds water and thus ensures that the eyes are moistened. Preservatives are another additive that is often found. However, they can further irritate the eyes. Artificial tear fluid can be found in various forms, e.g. as sprays, drops or gels. They are all brought into or out of the eye from the outside. Which application form is most suitable depends on the frequency of use, but also on individual preferences.
You can also find out more about the topic: tear duct disease
