Saliva is almost all water, yet it starts digestion and carries a painkiller
Human spit is about 99.5 percent water, but the remaining fraction is busy. It begins breaking down starch before you swallow, buffers acid that would dissolve tooth enamel, protects vitamin B12 on its way through the stomach, and even contains opiorphin, a natural pain-killing molecule. Estimates put daily output at around 1,500 millilitres.
The chemistry differs from blood in telling ways. Saliva carries less sodium than plasma, more potassium and more bicarbonate, and similar calcium. Its antibacterial arsenal includes thiocyanate, hydrogen peroxide and secretory IgA, and a millilitre may hold as many as 8 million human cells and 500 million bacterial ones, whose by-products explain why it can smell unpleasant. Haptocorrin latches onto vitamin B12 to shield it from stomach acid until intrinsic factor takes over.
Digestion starts here. Amylase, also called ptyalin, splits starch into simpler sugars such as maltose, and about 30 percent of starch digestion happens in the mouth. Saliva also binds food into a slippery bolus, carries flavour molecules to taste receptors, and keeps the mouth between roughly pH 6.2 and 7.4; salivary proteins stop its supersaturated minerals from crystallising while protecting tooth enamel. People who make little saliva often find food sticking and taste distorted.
Output depends on which glands are working. At rest the submandibular glands supply some 65 to 75 percent, but once food stimulates them the parotid glands' share climbs to about half. Both branches of the autonomic nervous system drive secretion: parasympathetic signals, via acetylcholine, favour digestion, while sympathetic norepinephrine shifts the balance between watery and protein-rich fluid. Production falls sharply during sleep, and Pavlov famously trained dogs to salivate at a bell.
Is licking a wound actually useful? Researchers in Florida found nerve growth factor in mouse saliva, and wounds treated with it healed twice as fast. Human saliva contains the same factor, but licking has not been shown to disinfect wounds; it mainly wipes away dirt. Other animals repurpose saliva entirely. Some Aerodramus swifts build nests from nothing else, the basis of bird's nest soup, and certain caterpillars spin silk from modified salivary glands.
Source: Saliva
