SafekipediaTears
Adapted from Wikipedia · Discoverer experience
Tears are a clear fluid made mostly by special glands called lacrimal glands in the eyes of all land mammals. They come in four main types: basal, closed eye, emotional, and reflex. Normally, the eyes make about 0.5 to 2.2 microliters of tears each minute, but when something irritates the eyes, this can increase a lot — up to around 300 microliters each minute.
Tears are made of water, salts, proteins, fats, and other substances that spread in layers over the eye’s surface. Each type of tear has a different makeup and purpose. For example, basal tears keep the eyes wet and comfortable, while reflex tears help wash away things that shouldn’t be there.
Tears also help protect the eyes and support the immune system. When we feel strong emotions, our bodies may produce tears as a natural way to deal with stress. Across cultures, tears carry deep symbolic significance among humans, showing feelings and connecting people in important ways.
Physiology
Tears are a clear fluid made by glands in our eyes. They help keep our eyes wet and comfortable. In 1946, a scientist named Wolff used a special tool called a slit lamp to show that tears have three layers: a thin oily layer, a watery layer, and a slimy layer. These layers work together to protect our eyes. The watery part of tears contains water, salts, antibodies, and special enzymes that can fight germs.
Tears can be made for different reasons. Sometimes, our eyes make tears to protect us from things like dust or smoke. Other times, tears are made when we feel strong emotions like sadness or happiness. Emotional tears may contain special chemicals that help balance our body’s stress levels.
Tears flow from the lacrimal glands through small tubes and collect in a special area near the inner corner of the eye. When we blink, tears spread across the eye’s surface. Extra tears, like when we cry, can travel through more tubes and end up in our nose, which is why our nose may run when we cry.
Some animals, like cats and camels, have a special third eyelid that helps keep their eyes moist and protected. Humans have a much smaller version of this eyelid.
The nerves in our face help control tear production. One nerve helps us make tears when something irritates our eyes, while another helps make the watery part of tears.
| Name | Composition | Origin | Functions |
|---|---|---|---|
| Lipid layer (TFLL) (with ~0.015–0.160 μm thickness) | The main lipid classes in whole human meibum: 1) Nonpolar lipids including wax esters (WE, 41%), cholesteryl esters (Chl-E, 31%), cholesteryl esters of (O-acyl)-ω-hydroxy fatty acids (Ch-OAHFA, 3%) and triacylglycerols (TAG, 1%); 2) Amphiphilic lipids consisting of (O-acyl)-ω-hydroxy fatty acids (OAHFA, 4%), cholesterol (Chl, 0.5%), free fatty acids (FFA, 0.1%), phospholipids (PL, 0.1%) and ceramides in various proportions (CER, 0.1%). An unknown fraction (19.2%) with nonpolar (e.g., diacylated α,ω-diols, diacylated a,b-diols and other more complex lipids), amphiphilic and non-lipid properties (e.g., denatured proteins, salts, etc.) has also been suggested. | The main source is Meibomian glands (or tarsal glands). Other proposed sources with minor roles are the Harderian (especially in rabbits), Moll, and Zeiss glands. | Diverse functions such as producing a smooth optical surface to improve the refraction of light, providing lubrication for blinks and eye movements, thickening the aqueous sub-phase due to Marangoni effect, sealing the lid margins during prolonged eye closure, providing resistance to evaporation, defending against the external environment (e.g., foreign particles and microbes), enabling the formation of a thin film, preventing its collapse, and involving in the oxygenation of the cornea. |
| Aqueous layer (with ~4 μm thickness) | Electrolytes, 60 metabolites—amino acids (1-Methylhistidine/3-Methylhistidine, arginine, asymmetric dimethylarginine/symmetric dimethylarginine, citrulline, creatine, glutamine, homoarginine, hydroxyproline, phenylalanine, proline, pyroglutamic acid, serine, taurine, theonine, tryptophan, tyrosine, urocanic acid, Valme), amino alcohols (panthenol), amino ketones (allantoin, creatine), aromatic acids (cinnamic acid, o-Coumaric acid/m-Coumaric acid/p-Coumaric acid), carbohydrates (N-Acetylneuraminic acid), carnitines (acetylcarnitine, carnitine, hexanoylcarnitine, palmitoylcarnitine), cyclic amines (Niacinamide), dicarboxylic acids (fumaric acid/Maleic acid), Nucleosides (1-Methyladenosine, adenosine, cytidine, guanosine, inosine, S-Adenosyl-homocysteine, S-Adenosylmethionine, uridine, and xanthosine), nucleotides (ADP, AMO, CMP, Cytidine diphosphate choline, GMP, IMP, UDP, UMP, UDP-N-acetylgalactosamine/UDP-N-acetylglucosamine), peptides (Oxidized glutathione), phospholipids (1-Palmitoyl-lysophosphatidylcholine), purines and derivatives (Hypoxanthine, Theobromine, uric acid, xanthine), purines and derivatives (4-Pyridoxic acid), Quaternary Amines (Acetylcholine, Glycerophosphocholine, phosphocholine), and Tricarboxylic Acids (citric acid), and other substances such as proteins (e.g., antibodies, lipocalin, lactoferrin, lysozyme, and lacritin) | Lacrimal gland | Promotes spreading of the tear film, the control of infectious agents, and osmotic regulation. |
| Mucous layer (with ~2.5–5 μm thickness) | Mucins | Conjunctival goblet cells | Coats the cornea, provides a hydrophilic layer and allows for even distribution of the tear film. |
| Category | Description |
|---|---|
| Basal tears | In healthy mammalian eyes, the cornea is continually kept wet and nourished by basal tears. They lubricate the eye and help keep it clear of dust. Tear fluid contains water, mucin, lipids, lysozyme, lactoferrin, lipocalin, lacritin, immunoglobulins, glucose, urea, sodium, and potassium. Some of the substances in lacrimal fluid (such as lysozyme) fight against bacterial infection as a part of the immune system. Lysozyme does this by dissolving a layer in the outer coating, called peptidoglycan, of certain bacteria. It is a typical body fluid with salt content similar to blood plasma. Usually, in a 24-hour period, 0.75 to 1.1 grams (0.03 to 0.04 oz) of tears are secreted; this rate slows with age. |
| Reflex tears | The second type of tears results from irritation of the eye by foreign particles, or from the presence of irritant substances such as onion vapors, perfumes and other fragrances, tear gas, or pepper spray in the eye's environment, including the cornea, conjunctiva, or nasal mucosa, which trigger TRP channels in the ophthalmic nerve. It can also occur with bright light and hot or peppery stimuli to the tongue and mouth. It is also linked with vomiting, coughing, and yawning. These reflex tears attempt to wash out irritants that may have come into contact with the eye. |
| Emotional tears (psychic tears) | The third category, in general, referred to as crying or weeping, is increased tearing due to strong emotional stress, pleasure (sometimes called 'tears of joy'), anger, suffering, mourning, or physical pain. This practice is not restricted to negative emotions; many people cry when extremely happy, such as times of intense humor and laughter. In humans, emotional tears can be accompanied by reddening of the face and sobbing—cough-like, convulsive breathing, sometimes involving spasms of the whole upper body. Tears brought about by emotions have a different chemical makeup than those for lubrication; emotional tears contain more of the protein-based hormones prolactin, adrenocorticotropic hormone, and Leu-enkephalin (a natural painkiller) than basal or reflex tears. The limbic system is involved in the production of basic emotional drives, such as anger, fear, etc. The limbic system, or, more specifically the hypothalamus, also has a degree of control over the autonomic system. The parasympathetic branch of the autonomic nervous system controls the lacrimal glands via the neurotransmitter acetylcholine through both the nicotinic and muscarinic receptors. When these receptors are activated, the lacrimal gland is stimulated to produce tears. |
Human culture
Crying often brings tears, and it happens when people feel strong emotions like sadness, anger, happiness, or fear. These feelings can come from many things, such as listening to music or talking with friends.
Babies cry a lot, especially when they can't talk yet. Their crying helps parents and caregivers know what they need. Mothers, in particular, can sense their baby's crying and feel their hearts slow down and then speed up again.
In some places, people think crying in public is not proper, except when someone close to them has passed away. In many Western cultures, it's more common to see women and children crying than men. Crying can also be a way to ask for help and make others want to support you.
Some therapies suggest that crying can be good for your health and mind. Sometimes, people might pretend to cry, which is called "crocodile tears." There's also a condition where people cry while eating, linked to recovery from a face nerve problem.
Pathology
Bogorad's syndrome, also called "crocodile tears syndrome", is a rare condition that can happen after Bell's palsy or damage to the facial nerve. In this condition, the nerves that control salivation become mixed up with nerves that control tear production. This can cause someone to produce tears when they eat or smell food, even though their eyes are not sad or upset. This condition was first described in 1926 by a scientist named F. A. Bogorad.
Dry eye, known medically as keratoconjunctivitis sicca, is a common problem where the eyes do not make enough natural tears. Even though the eyes feel dry, people with this condition may find their eyes watering in response to irritation from the lack of tears. This happens because the tears are not properly coated, causing them to spill over the face.
Familial dysautonomia is a genetic condition that may stop people from producing extra tears when they feel strong emotions. Sometimes, blockages in the small openings near the eyes can cause normal tears to overflow, making it look like someone is crying all the time. This can affect how others see them.
Pseudobulbar affect is a condition where a person may laugh or cry suddenly and without feeling the usual emotions. It often happens to people with certain brain injuries. Scientists think this is caused by damage to parts of the brain that control emotions. This condition is different from feeling sad or depressed, as it does not come with other symptoms like trouble sleeping or changes in appetite.
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