Duncan Smith asked:
Where does saliva come from? Is it filtered out of blood in a similar way that kidneys do?
Saliva is made in the same way that you make tears. You have glands which have a rich blood supply. When the blood goes through it passes through very thin walled capillaries which are a little bit leaky. Itís exactly the same as if you go to the garden centre and you see those watering systems where the pipes have small holes in. You turn the tap on and the water going through squeezes some of the water out of the pipe. Why do you make saliva, in other words watery liquid and not blood? The answer is the walls of these tiny capillaries are just big enough for water molecules but theyíre too small to let the bigger protein and cellular components of blood leach out. Some things can get out: the water and some antibodies but thatís about it.