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Magill’s Medical Guide, 9th Edition

Tears and tear ducts

by April D. Ingram

Category: Anatomy

Also known as: Lacrimal ducts, nasolacrimal ducts, tear film

Anatomy or system affected: Eyes, head, nose

Specialties and related fields: Ophthalmology, optometry

Definition: Fluid produced by the eye and the channel that carries it from the eye to the nasal cavity.

Key terms:

canaliculus: a small canal linking the upper and lower puncta to the lacrimal sac

cornea: the transparent front portion of the eye

fluorescein: a brightly colored dye used for diagnostic purposes

instill: to slowly put a liquid into the eye, drop by drop

lacrimal: pertaining to the secretion and conduction of tears

lysosyme: an enzyme found in body secretions that destroys bacteria by breaking down their walls

ophthalmologist: a medical doctor who specializes in the treatment of disorders of the eye

STRUCTURE AND FUNCTIONS

The lacrimal gland is located beneath the upper eyelid, on the outer edge of each eye, and its key function is to produce tears. Tears flow constantly across the conjunctiva, which is the front surface of the eye, in order to keep it clean and lubricated. Blinking spreads tears across the eye. This constant flushing of the eye surface also supplies oxygen and nutrients to the cornea. Tears provide an important barrier to infection as they contain the antibacterial enzyme lysosyme, which destroys microorganisms on the eye surface. As new tears are produced, old tears drain from the eye at its inner edge, called the “canthus,” by being drawn into two small holes, the upper and lower puncta, through capillary action. Tears flow through the canaliculus, entering the lacrimal sac, then to the tear duct (called the “nasolacrimal duct”), and at last into the nasal cavity. This is the reason that a surplus of tears results in a runny nose.

Tears, or tear film, are made up of three distinct layers: an outer oily layer to prevent drying, which is produced by the meibomion glands; a watery middle layer, which contains the oxygen and nutrients; and an inner mucous layer, produced by the conjunctival goblet cells, reducing the surface tension of the tears and allowing them to be spread evenly on the surface of the eye.

The body naturally produces tears in order to maintain and protect the eye, but in humans, tears are also produced in response to emotion in the form of crying. During a normal twenty-four-hour day, 0.75 to 1.1 grams of tears are secreted; however, this rate declines with age. Also, approximately 10 to 15 percent of tear secretion is lost to evaporation, dependent on the environment.

DISORDERS AND DISEASES

The most common disorder associated with tears is excessive tearing. This can occur for many reasons and may require evaluation by an ophthalmologist. An excessive amount of tears can be caused by infection, environmental irritants, glaucoma, certain medications, allergic reaction, eyestrain, dry eyes, foreign material in the eye, scratch on the surface of the eye, or other eyelid or eyelash disorders.

Dry eye, also described as “scratchy eye,” can occur when too few tears are being produced, or they are draining from the eye too rapidly or evaporating too quickly. Symptoms of dry eye may also include photosensitivity, blurred vision, redness, and an itching or burning sensation. Often the treatment is to instill drops of artificial tears or lubricating gel, to apply compresses, and to avoid potential irritants such as excessively dry or warm rooms to prevent evaporation. If the cause of dry eye is that tears are draining too quickly, then a surgeon may modify the tear duct to slow or limit tear drainage.

Another common disorder associated with tear ducts is obstruction. If there is an obstruction of the tear duct (nasolacrimal duct), then tears are not able to drain easily into the nasal cavity. Blockage of the tear duct can occur in early childhood as a congenital blockage or later in life as a result of chronic infection or irritation, injury, or tumor. In order to test the tear duct for obstruction, a drop of fluorescein dye will be instilled into the lower lid and then the patient will be observed at regular intervals to determine if the dye has traveled through to the nasal cavity. Treatment varies depending on the underlying cause.

A congenital blockage occurs in 6 to 20 percent of infants when a membrane remains between the tear duct and nasal cavity. This results in the inability for the eye to drain, and the eye appears excessively watery or filled with mucus. Often the membrane will rupture spontaneously before one year of age, thus resolving the obstruction, but sometimes a probe will need to be inserted by an ophthalmologist.

Tear duct obstruction may cause an acute infection of the lacrimal sac, located at the upper, wider end of the nasolacrimal duct. This form of infection is called “dacrocystitis,” and patients normally have an overflowing of tears (epiphora) and a swollen painful mass over the area of the lacrimal sac. Dacrocystitis may be treated with warm compresses and gentle massage but may require systemic antibiotics and drainage of the mass.

PERSPECTIVE AND PROSPECTS

Research is being conducted to determine if tears not only provide nutrients and moisture to the eye but also have diagnostic uses. Tears are created from blood that has been filtered; therefore, tears can provide medical clues as to things happening in the blood. Studies are being done to determine if biological changes in the eye and eye health can be measured through small variations in the level of inflammatory proteins found in tears. Also, in 2000, a group of contact lens researchers in Australia found that tears contain protein markers that may be used to detect certain types of cancer. Research is continuing in this area in the hope that tears may provide useful information in future diagnoses.

See also Eye infections and disorders; Eyes; Glands; Host-defense mechanisms; Ophthalmology; Sense organs; Sjögren’s syndrome; Vision; Vision disorders.

For Further Information:

1 

A.D.A.M. Medical Encyclopedia. “Dry Eye Syndrome.” MedlinePlus, November 20, 2012.

2 

“Blocked Tear Duct.” Mayo Foundation for Medical Education and Research, February 13, 2013.

3 

Cohen, Adam, Michael Mercandetti, and Brian Brazzo, eds. The Lacrimal System: Diagnosis, Management, and Surgery. New York: Springer, 2006.

4 

Howson, Alexandra, and Christopher Cheyer. “Lacrimal Duct Stenosis.” Health Library, June 4, 2012.

5 

Kohnle, Diana, and Michael Woods. “Dacryocystitis.” Health Library, November 26, 2012.

6 

Sullivan, David, Darlene Dartt, and Michele Meneray, eds. Lacrimal Gland, Tear Film, and Dry Eye Syndromes 2: Basic Science and Clinical Relevance. New York: Plenum Press, 1998.

7 

“Tear System.” American Society of Ophthalmic Plastic and Reconstructive Surgery, 2012.

8 

Van Haeringen, N. J. “Aging and the Lacrimal System.” British Journal of Ophthalmology 81 (1997): 824-26.

Citation Types

Type
Format
MLA 9th
Ingram, April D. "Tears And Tear Ducts." Magill’s Medical Guide, 9th Edition, edited by Anubhav Agarwal,, Salem Press, 2022. Salem Online, online.salempress.com/articleDetails.do?articleName=MMG2022_1329.
APA 7th
Ingram, A. D. (2022). Tears and tear ducts. In A. Agarwal, (Ed.), Magill’s Medical Guide, 9th Edition. Salem Press. online.salempress.com.
CMOS 17th
Ingram, April D. "Tears And Tear Ducts." Edited by Anubhav Agarwal,. Magill’s Medical Guide, 9th Edition. Hackensack: Salem Press, 2022. Accessed December 14, 2025. online.salempress.com.