Saliva

Saliva does several things at once. It moistens food. It starts breaking down carbohydrates thanks to salivary amylase, which is an enzyme, that is, a protein that splits molecules. It holds microbes in check. And it works as a buffer: it raises pH, which is the measure of how acidic something is, with help from bicarbonate, phosphate and other ions.

It does not come out finished all at once, but in two stages. First the acinar cells, the ones at the far end of the gland, release a primary saliva that is isotonic, meaning it has the same salt concentration as the body's fluid, and that carries amylase, mucus and extracellular fluid.

That saliva then travels down the ductal tree and is modified along the way, like water that changes as it runs through a pipe that takes things out and puts things in. Sodium is reabsorbed, potassium and bicarbonate are secreted, and chloride is absorbed passively. What reaches the mouth is hypotonic: it has less salt than the body's fluid.