Thick, colourless fluid constantly present in the mouth, composed of water, mucus, proteins, mineral salts, and amylase, an enzyme that breaks down starches. One to two litres are produced daily by the salivary glands. Small amounts are continually discharged into the mouth, but the presence, smell, or even thought of food increases flow. Saliva's main function is to keep the inside of the mouth moist, making speech more fluid, dissolving food molecules for taste, and easing swallowing. It also helps control the body's water balance, since lack of it stimulates thirst when water intake has been low. Saliva reduces dental caries and infection by removing food debris, dead cells, bacteria, and white blood cells.
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Saliva is the watery and usually frothy substance produced in the mouths of humans and most other animals. Saliva is produced in and secreted from the salivary glands. Human saliva is composed mostly of water, but also includes electrolytes, mucus, antibacterial compounds, and various enzymes. As part of the initial process of food digestion, the enzymes in the saliva break down some of the starch and fat in the food at the molecular level. Saliva also breaks down food caught in the teeth, protecting them from bacteria that cause decay. Furthermore, saliva lubricates and protects the teeth, the tongue, and the tender tissues inside the mouth.
Various species have evolved special uses for saliva that go beyond predigestion. Some swifts use their gummy saliva to build their nests. Some Aerodramus swiftlet nests are made only from saliva and used to make bird's nest soup. Cobras, vipers, and certain other members of the venom clade hunt with venomous saliva injected by fangs. Some arthropods, such as spiders and caterpillars, create thread from salivary glands.
The mouth of animals is the habitat of many bacteria, some pathogenic. Some diseases, such as herpes, can be transmitted through the mouth. Animal (including human) bites are routinely treated with systemic antibiotics because of the risk of septicemia.
The saliva stimulated by sympathetic innervation is thicker, and saliva stimulated parasympathetically is more watery.
Parasympathetic stimulation leads to acetylcholine (ACh) release onto the salivary acinar cells. ACh binds to muscarinic receptors and causes an increased intracellular calcium ion concentration (through the IP3/DAG second messenger system). Increased calcium causes vesicles within the cells to fuse with the apical cell membrane leading to secretion formation. ACh also causes the salivary gland to release kallikrein, an enzyme that converts kininogen to lysyl-bradykinin. Lysyl-bradykinin acts upons blood vessels and capillaries of the salivary gland to generate vasodilation and increased capillary permeability respectively. The resulting increased blood flow to the acinar allows production of more saliva. Lastly, both parasympathetic and sympathetic nervous stimulation can lead to myoepitheilium contraction which causes the expulsion of secretions from the secretory acinus into the ducts and eventually to the oral cavity.
It is produced at a rate of 1-1.5 L/day. 20mL/hr at rest, 250 mL/hr under stimulated conditions. While sleeping, salivary flow drops to almost zero.
It is a fluid containing:
Different reagents used to determine the content of saliva 1. Molisch test gives a positive result of purple color that is costituent to the presence of carbohydrates