All About Saliva

Saliva is approximately 99% water, though mucus, proteins, enzymes, antibacterial compounds, and electrolytes also make up the liquid. These substances are responsible for the various functions of saliva, and the composition is altered when it exits ducts in the mouth. Sodium is reabsorbed, and potassium and bicarbonate ions are secreted. There are as also many as 500 million bacterial cells in each milliliter of saliva. This bacteria is partially responsible for bad breath.

Three major salivary glands are primarily responsible for the production of saliva, though a few minor glands are involved as well. The submandibular glans produces around 65% of the saliva in the human mouth, the parotid produces about 20%, and the sublingual creates between five and seven percent. These glands are controlled by the autonomic nervous system, which is part of the nervous system and responsible for supplying smooth glands and muscles.

Saliva acts as a lubricant, decreasing friction and protecting the mucous membrane or lining of the mouth from trauma. Lubrication aids not only food processing but also contributes to oral health and taste perception. People with xerostomia -- reduced saliva production -- are much more prone to mouth soreness and often find food sticks to the inside of their mouths.

Saliva also binds food together when we chew. The mucus in saliva moistens the food and makes it into a slippery food bolus so that it can move easily down the esophagus. This is the beginning of the digestion process. Amylase, an enzyme in saliva, helps break down starch into simpler sugars so that it is ready for the next stage of digestion. Salivary lipase produced by the salivary glands helps begin the process of breaking down fats.

We taste food when it is dissolved in chemicals in saliva and carried to the taste receptor cells. During this process, some chemicals interact with the taste substances; for examples, proteins bind with bitter substances and ions buffer sour tastes. Saliva also protects the taste receptors from dryness and bacterial infection. Dysgeusia is a condition in which people who have little saliva have reduced taste or experience a constant, unpleasant metallic taste.

Saliva helps balance the pH of the mouth. In its resting state, saliva is slightly acidic. Various ions in saliva maintain this pH range. If the mouth becomes too acidic, this can cause demineralization and raise the risk of cavities, the dissolution of hard dental tissues. Saliva is also thought to play a role in the development of taste buds. Epidermal growth factor (EGF) is one substance in saliva. It helps heal ulcers, inhibits gastric acid, stimulates the synthesis of DNA, and protects against a range of potential injuries.

Experts estimate humans produce between three quarters to one and a half liters of saliva each day. Most people produce almost no saliva during the night. The flow rate of saliva is around 30 milliliters per hour. The average person produces about 20,000 liters of saliva in their lifetime -- enough to fill 53 bathtubs!

Some animals -- including humans -- use spitting saliva as a defense mechanism. In some countries, spitting is taboo, but even in Western cultures, it is viewed as unsavory due to the spread of bacteria it can cause. Many people also believe licking a wound can help it heal more quickly. There is currently no evidence to support saliva's ability to aid wounds beyond clearing the area of larger pieces of dirt and foreign bodies. Some birds, such as swifts, use their saliva as an adhesive to build their nests.

Carnivores and herbivores digest food differently than humans and thus have different saliva composition. In humans, amylase breaks down the starch in food, but this is unnecessary for animals who eat only meat -- the breakdown of their sustenance begins in the gut. Herbivores also begin the breakdown of cellulose in the digestive tract, after food has left the mouth.

Some alcoholic drinks are made using saliva. Masato is an ancient Peruvian drink composed of chewed pieces of cooked yuca, a plant with high starch content. The saliva breaks the starch down into sugar, which is then fermented. Chicha is a popular Latin America drink with both alcoholic and non-alcoholic forms. It can be made by fermenting chewed-up corn.