A tear , poetically and regionally also called tenacious , is a salty body fluid that the tear glands of humans and mammals constantly secrete. It is used to clean the conjunctival sac and to moisten and nourish the cornea . It also improves the optical properties of the corneal surface by compensating for the physiological irregularities through differences in level. The tear secretion consists of three layers.
The daily amount of tear fluid produced is difficult to estimate, as some of the tears evaporate and some flow away through the tear ducts . The figures vary between one gram and half a liter of tear fluid daily; However, there is agreement that the production of stimuli such as foreign bodies in the eye, cold as well as crying and hearty laughter increases many times over. The reflexive process of yawning is also often associated with increased production of tear fluid and reflects an increased need for fluid in the eyes when staying and waking up. Virtually no tear fluid is produced during sleep. Tear production begins in the first few days of life, peaks in children and young adults and then declines with age. There is a regulatory mechanism between tear production and removal.
The Schirmer test enables an assessment of tear production in medical practice. A 5 mm wide and 35 mm long strip of a certain type of paper with a length of 5 mm is hung in the conjunctival sac. Normally a 15 mm long humidification zone can be detected after 5 minutes. A value smaller than 5 mm is considered to be pathological.
In humans, tears are produced by the lacrimal glands and accessory lacrimal glands (e.g. Kraus and Wolfring's glands). The tear fluid contains germicidal substances such as lysozyme and protects the surface of the eye from small foreign bodies by wiping them away with a blink of an eye .
Precorneal tear film
The precorneal tear film covering the corneal and conjunctival epithelium is composed of 3 layers:
- the superficial lipid layer, a monomolecular layer that comes from the secretion of the meibomian glands and serves to slow down the evaporation of the aqueous layer.
- the middle aqueous layer, which originates from the lacrimal glands and accessory lacrimal glands. It contains water-soluble substances (salts and proteins) and
- the deep mucous layer, which is composed of glycoprotein mucin, which covers the corneal and conjunctival epithelium in a reticulate manner and serves to wet the epithelial cell membranes by reducing the surface tension of the tear fluid.
An overproduction may occur due to irritation in various diseases of the eye. It can also be nervous or psychological (emotional) or related to thyroid diseases . The overflow of tears when the drainage via the draining tear ducts is disturbed is to be distinguished from real hypersecretion .
There is underproduction in many older people, which is symptom-free, but can also lead to a feeling of dryness. Furthermore, diseases such as Sjogren's syndrome and familial dysautonomy lead to underproduction. It is also found in various neurological lesions, in trachoma and in the removal of part or all of the lacrimal gland. Disturbances in the composition lead to dry eyes , which in extreme cases can lead to damage to the cornea with loss of transparency and even blindness.
In addition to water, the tear fluid consists of various proteins, including the immunoglobulins albumin and globulin . The antimicrobial activity of the tear fluid is achieved through the latter two. The y-globulin in normal tear fluid is IgA , IgG and IgE . It also consists of enzymes, including lysozymes , inorganic and nitrogenous substances, as well as carbohydrates and their metabolites, and 9 grams per liter (0.9%) of common salt corresponding to an isotonic saline solution . The chemical composition changes when we cry . The composition also depends on the cause of the tears; Emotional tears contain up to a quarter more protein than reflex tears.
The tear fluid has a relative density of 1.004–1.005. The refractive index is 1.336-1.337. The viscosity is between 1.26 and 1.32. The mean value of the osmolality is around 320 mmol / kg. The pH is 7.4.
Tears have inspired numerous areas related to crying, from literature, music, and the visual arts and painting to tattoos with the motif of one or more tears.
- D. Vaughan, T. Asbury: Ophthalmology: Diagnosis and Therapy in Practice. A textbook for students, assistants and doctors . Springer-Verlag, 2013, ISBN 978-3-642-69329-8 , pp. 56- ( google.com ).
- Gerhard K. Lang, Gabriele E. Lang: Ophthalmology essentials . Thieme, 2015, ISBN 978-3-13-171371-1 , pp. 44 ( google.com ).
- Thomas de Padova: Why do tears taste salty? In: Der Tagesspiegel . September 20, 2006, accessed February 18, 2011 .
- Duden .
- Tear production begins in the third week of life and amounts to approx. 1 gram daily.
- E.-J. Speckmann, W. Wittkowski: Manual anatomy. Tandem Verlag, 2009, ISBN 978-3-8331-8011-8 , p. 109. An average of half a liter of tear fluid is formed per day
- Friedrich Paulsen: Why do tears often come after a long yawn? . In: Science in Dialog.
- D. Vaughan, T. Asbury: Ophthalmology: Diagnosis and Therapy in Practice. A textbook for students, assistants and doctors . Springer-Verlag, 2013, ISBN 978-3-642-69329-8 , pp. 56 ( google.com ).
- Entry on tear fluid. In: Römpp Online . Georg Thieme Verlag, accessed on June 7, 2014.
- Why do tears taste salty? In: Der Tagesspiegel. September 20, 2006. (tagesspiegel.de)
- A. Skorucak: The Science of Tears.
- Chip Walter: Why do we Cry. In: Scientific American. Mind & Brain. Vol. 17, Issue 6, Dec 2006, p. 44.
- Julika Meinert: What close-ups of tears reveal. welt.de, October 5, 2014, accessed October 8, 2014 .