Editing Tears (section)

== Physiology ==

=== Chemical composition ===
Tears are made up of three layers: lipid, aqueous, and mucous.<ref name="Moshirfar2014">{{cite journal |vauthors=Moshirfar M, Pierson K, Hanamaikai K, Santiago-Caban L, Muthappan V, Passi SF |title=Artificial tears potpourri: a literature review |journal=Clin Ophthalmol |date=July 2014 |volume=8 |pages=1419–33 |pmid=25114502 |pmc=4124072 |doi=10.2147/OPTH.S65263 |doi-access=free }}</ref> Tears are composed of [[water]], [[salt (chemistry)|salt]]s, [[antibody|antibodies]], and [[lysozyme]]s (antibacterial enzymes); though composition varies among different tear types. The composition of tears caused by an emotional reaction differs from that of tears as a reaction to irritants, such as onion fumes, dust, or allergens. Emotional tears contain higher concentrations of stress hormones such as [[adrenocorticotropic|adrenocorticotropic hormone]] and [[enkephalin|leucine enkephalin]] (a natural pain killer), which suggests that emotional tears play a biological role in balancing stress hormone levels.<ref>{{cite journal|last1=Frey|first1=William H.|last2=Desota-Johnson|first2=Denise|last3=Hoffman|first3=Carrie|last4=McCall|first4=John T.|date=October 1981|title=Effect of Stimulus on the Chemical Composition of Human Tears|journal=American Journal of Ophthalmology|volume=92|issue=4|pages=559–567|doi=10.1016/0002-9394(81)90651-6|pmid=7294117}}</ref> 
{| class="wikitable"
|-
! style="width: 10%;"|Name
! style="width: 40%;"|Contents
! style="width: 10%;"|Secretors
! style="width: 40%;"|Functions
|- style="vertical-align: top;"
|Lipid layer
|[[Lipid|Oils]]
|[[Meibomian gland]]s (or tarsal glands)
|Coats the aqueous layer, provides a [[hydrophobe|hydrophobic]] barrier that envelops tears and prevents their spilling onto the cheek. These glands are located among the [[Tarsus (eyelids)|tarsal plates]], and thus deposit the tear fluid between the eye proper and the oil barriers of the lids.<ref name="ReferenceA">"eye, human."Encyclopædia Britannica from [[Encyclopædia Britannica 2006 Ultimate Reference Suite DVD]] 2009</ref>
|- style="vertical-align: top;"
|Aqueous layer
|[[Electrolyte]]s, 60 metabolites—amino acids (1-Methylhistidine/3-Methylhistidine, arginine, Asymmetric, 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., [[antibody|antibodies]],<ref name="Moshirfar2014" /> [[lipocalin]], [[lactoferrin]], [[lysozyme]],<ref>{{cite web|url=http://www.medrounds.org/ocular-pathology-study-guide/2005/10/tear-proteins.html|title=Ocular Pathology Study Guide: Tear Proteins<!-- Bot generated title -->|website=medrounds.org|url-status=dead|archive-url=https://web.archive.org/web/20060627200147/http://www.medrounds.org/ocular-pathology-study-guide/2005/10/tear-proteins.html|archive-date=2006-06-27}}</ref> and [[lacritin]])
|[[Lacrimal gland]]
|Promotes spreading of the tear film, the control of infectious agents, and osmotic regulation.
|- style="vertical-align: top;"
|Mucous layer
|[[Mucin]]s
|Conjunctival [[goblet cell]]s
|Coats the [[cornea]], provides a hydrophilic layer and allows for even distribution of the tear film.
|}

=== Drainage of tear film ===
The [[lacrimal gland]]s secrete lacrimal fluid, which flows through the main excretory ducts into the space between the eyeball and the lids.<ref name=":0">{{Cite web|url=https://www.targethealth.com/post/tears-2|title=Tears|date=July 2, 2018|website=Target Health Blog|access-date=2019-07-07}}</ref> When the eyes blink, the lacrimal fluid is spread across the surface of the eye.<ref name=":0" /> Lacrimal fluid gathers in the [[lacrimal lake]] which is found in the medial part of the eye. The lacrimal papilla is an elevation in the inner side of the eyelid, at the edge of the lacrimal lake.<ref name=":0" /> The lacrimal canaliculi open into the papilla.<ref name=":0" /> The opening of each canaliculus is the lacrimal punctum. From the punctum, tears will enter the [[lacrimal sac]],<ref name="ReferenceA" /> then on to the [[nasolacrimal duct]], and finally into the [[nasal cavity]].<ref name=":0" /> An excess of tears, as caused by strong [[emotion]], can cause the nose to run. Quality of vision is affected by the stability of the tear film.<ref>{{cite journal|vauthors=Szczesna DH, Jaroński J, Kasprzak HT, Stenevi U|year=2006|title=Interferometric measurements of dynamic changes of tear film|journal=J Biomed Opt|volume=11|issue=3|pages=34028|bibcode=2006JBO....11c4028S|doi=10.1117/1.2209881|pmid=16822077|s2cid=24410898 |doi-access=free}}</ref>

=== Types ===
There are three basic types of tears: basal, reflex and emotional.<ref name="onlinelibrary.wiley.com" />
{| class="wikitable"
|-
!Category
!Description
|-
| Basal tears
|In healthy [[mammal]]ian 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]], [[immunoglobulin]]s, [[glucose]], [[urea]], [[sodium]], and [[potassium]]. Some of the substances in lacrimal fluid (such as lysozyme) fight against [[bacteria]]l [[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&nbsp;grams (0.03–0.04-ounce avoirdupois) of tears are secreted; this rate slows with age.<ref name="ReferenceA" /> 
|-
| 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, [[perfume]]s 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]].{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}} 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.<ref name="ReferenceA" /> 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, [[anger]], [[suffering]], [[mourning]], or [[Pain|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.<ref>Skorucak A. [http://www.scienceiq.com/Facts/ScienceOfTears.cfm "The Science of Tears."] ScienceIQ.com. Accessed September 29, 2006.</ref>
|}

=== Nictitating membrane ===
Some mammals, such as [[cat]]s, [[camel]]s, [[polar bear]]s, [[Pinniped|seals]] and [[aardvark]]s, have a full translucent third eyelid called a [[nictitating membrane]], while others have a [[Vestigiality|vestigial]] nictitating membrane.<ref>{{Cite journal |last=Barasa |first=A. |date=2003-06-01 |title=Morphology and structure of the nictitating membrane cartilage in mammals |url=https://europepmc.org/article/med/14717063 |journal=Morphologie |volume=87 |issue=277 |pages=5–12 |issn=1286-0115 |pmid=14717063}}</ref> The membrane works to protect and moisten the eyelid while maintaining visibility. It also contributes to the aqueous portion of the tear film and possibly immunoglobulins.<ref>{{Cite web |title=Nictitating Membrane - an overview {{!}} ScienceDirect Topics |url=https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/nictitating-membrane#:~:text=The%20nictitating%20membrane,%20or%20third,or%20may%20not%20be%20pigmented. |access-date=2022-10-18 |website=www.sciencedirect.com}}</ref> Humans and some primates have a much smaller nictitating membrane; this may be because they do not capture prey or root vegetation with their teeth, so that there is no evolutionary advantage of the third eyelid.<ref>{{Cite web |title=Why do cats have an inner eyelid as well as outer ones? |url=https://www.scientificamerican.com/article/why-do-cats-have-an-inner/ |access-date=2022-10-18 |website=Scientific American |language=en}}</ref>[[File:Crying-girl.jpg|thumb|A toddler producing tears due to emotional stress or pain|150px]]

=== Neurology ===
The [[trigeminal]] V<sub>1</sub> (fifth cranial) nerve bears the sensory pathway of the tear reflexes. When the trigeminal nerve is cut, tears from reflexes will stop, while emotional tears will not. The great (superficial) petrosal nerve from cranial nerve VII provides autonomic innervation to the lacrimal gland.<ref>{{cite journal |title=Neural Regulation of Lacrimal Gland Secretory Processes: Relevance in Dry Eye Diseases |pmc=3652637 |year=2009 |last1=Dartt |first1=D. A. |journal=Progress in Retinal and Eye Research |volume=28 |issue=3 |pages=155–177 |pmid=19376264 |doi=10.1016/j.preteyeres.2009.04.003 }}</ref> It is responsible for the production of much of the aqueous portion of the tear film.