Why are there tears in your eyes and how are they formed?

Tears come from lacrimal glands. The lacrimal gland is composed of slender tubular glands and ducts, and it is an organ that secretes tears. The lacrimal gland is located in the lacrimal gland fossa above the orbit, which is divided into two parts: the upper part is the orbit, also called the upper lacrimal gland, which is large and looks like almonds, about 12×20mm, and the lower part is the eyelid, also called the lower lacrimal gland, which is small in size. There are 10 ~ 12 drainage tubes in the lacrimal gland, and the tears are discharged from these drainage tubes after they are produced. Tears are a kind of weakly alkaline transparent liquid, of which 98.2% is water, and the rest is a small amount of inorganic salts, protein group, lysozyme, immunoglobulin A, complement system and other substances. After the tears are produced, they are discharged from the body through the lacrimal passage. The lacrimal passage consists of lacrimal punctum, lacrimal canaliculus, lacrimal sac and nasolacrimal duct. There is a tear spot on the inner side of the upper and lower eyelid margins. Tears enter the lacrimal canaliculus from the lacrimal punctum like a sewer, and enter the lacrimal sac through the canaliculus about 10mm long. Lacrimal sac is specially used to collect and store tears and prevent them from flowing out. The size of lacrimal sac is about 12×6mm. There is a tube with a length of 12 ~ 24mm and a diameter of 3 ~ 6mm under the lacrimal sac, which is the nasolacrimal duct. Tears in the lacrimal sac enter the nasal cavity through the nasolacrimal duct. Therefore, when we order eye drops, we should hold down the lacrimal sac with our fingers, just to prevent eye drops from flowing into the nasal cavity through the nasolacrimal duct, and people will shed tears when they are sad, sad and sad; People cry when they are happy. Tears seem to be a symbol of emotional changes, but in fact, tears do not completely represent emotional changes. When foreign objects such as dust fall into our eyes, a lot of tears will be produced to wash them out. Tears contain not only a lot of water, but also lysozyme, immunoglobulin, complement system, lactoferrin and β-lysin, which can inhibit the growth of bacteria. Therefore, tears also have a very important function-defense. Generally speaking, conjunctival sac is not an ideal place for bacteria to live, because the most suitable growth temperature for bacteria is 35 ~ 38℃. Because tears evaporate when you open your eyes, the surface temperature of cornea can be as low as 30℃, which is not suitable for bacterial growth. However, if eyes are closed or bandaged for a long time, the temperature of conjunctival sac will rise, which is beneficial to the reproduction of bacteria. The protective effect of conjunctival sac comes from antibacterial substances in tears. The main function of lysozyme in tears is to achieve sterilization by decomposing polysaccharides existing in bacterial cell walls. Five kinds of immunoglobulin * * * were found in tears, among which immunoglobulin A was the most. It can inhibit the adhesion of bacteria on the mucosal surface and dissolve bacteria with the participation of lysozyme, thus achieving the purpose of sterilization. Lactoferrin inhibits the metabolism of bacteria by preventing bacteria from using iron ions, thus achieving the purpose of sterilization. β -lysin is a non-lysozyme antibacterial substance, which mainly resists staphylococcus, cracks bacterial plasma membrane and assists lysozyme. In addition, tears can also wet the surface of eyeball and conjunctival sac, changing the optical properties of cornea. You see, tears have such an important role, so it is not just to express emotional changes.