Tears are a fluid produced by the body that serves many purposes for eye health and comfort. These secretions are more than just water; they are a complex mixture that continuously bathes the eye’s surface. This constant flow helps maintain clear vision and protects the delicate structures of the eye. Understanding how tears are produced and their various functions reveals their importance in daily life.
How the Body Produces Tears
The primary structures responsible for tear production are the lacrimal glands, located above and to the outer side of each eyeball within the eye socket. The main lacrimal gland, shaped like an almond, is divided into two parts: a larger orbital part and a smaller palpebral part.
The lacrimal glands continuously secrete lacrimal fluid. This fluid is released through several small ducts into the superior conjunctival fornix, an area where the conjunctiva lining the upper eyelid meets the conjunctiva covering the eyeball. Blinking then spreads this fluid evenly across the entire surface of the eye, from the outer corner towards the nose.
After lubricating the eye, tears collect at the inner corner of the eye. From there, they enter the tear drainage system through two tiny openings, called puncta, located on the inner edges of both the upper and lower eyelids. These puncta lead into small channels called lacrimal canaliculi, which direct the tears into the lacrimal sac. Finally, tears flow from the lacrimal sac down through the nasolacrimal duct, a tube that empties into the nasal cavity, which is why crying often causes a runny nose.
The Many Functions of Tears
Tears serve multiple functions, categorized into three distinct types: basal, reflex, and emotional tears. The tear film, which covers the eye, consists of three layers: an inner mucous layer, a middle aqueous layer, and an outer lipid layer that reduces evaporation.
Basal tears are produced continuously to keep the eyes moist and healthy. They provide constant lubrication, preventing the delicate surfaces of the conjunctiva and cornea from drying out. These tears also deliver nutrients and oxygen to the cornea, which lacks blood vessels, and contain antibacterial properties, such as lysozyme, to protect against infections.
Reflex tears are produced in response to irritants like dust, smoke, or strong odors, such as those from onions. Their purpose is to wash away foreign bodies and harmful substances that may enter the eye. This rapid and increased production of tears helps to flush out anything that could cause damage or discomfort.
Emotional tears, often associated with strong feelings like sadness or joy, have a different chemical composition than basal or reflex tears. While their exact biological purpose is still being researched, they are believed to play a role in human emotional expression and potentially in physiological regulation, such as releasing stress hormones.
When Tear Production Goes Wrong
Disruptions in tear production or drainage can lead to various uncomfortable eye conditions. These issues often arise from underlying factors that affect the lacrimal glands or the drainage pathways.
Dry eye syndrome occurs when the eyes do not produce enough tears or when the tears produced are of poor quality and evaporate too quickly. Common causes include aging, certain medications, environmental factors like wind or dry air, and medical conditions such as autoimmune diseases. Symptoms often include a gritty sensation, burning, redness, and blurred vision, as the eye surface lacks adequate lubrication and protection. Diseases that damage the lacrimal gland can directly lead to chronic dry eye.
Excessive tearing, also known as epiphora, is another condition where tear production is not optimal. This can result from overproduction of tears, often in response to irritation or inflammation, or more commonly, a blockage in the tear drainage system. If the puncta, canaliculi, lacrimal sac, or nasolacrimal duct become obstructed, tears cannot drain properly into the nasal cavity and instead overflow onto the face.