Saliva is secreted by the parotid, submandibular, sublingual and buccal salivary glands. The aqueous component is formed by primary secretion of a solution similar to extracellular fluid. This is modified as it passes along the gland ducts, with removal of Na+ and Cl-, and addition of K+ and HCO3-. Bicarbonate makes saliva alkaline and helps buffer the acid in food, protecting against dental caries.
The main organic components of saliva are:
- Mucus, for lubrication in eating and speech.
- Amylase, which digests starch at alkaline pH.
- Lysozyme, which has antibacterial actions and protects against oral infection.
Secretion of saliva is reflexly stimulated via the salivary nuclei in the medulla oblongata in response to:
- Stimulation of chemoreceptors and mechanoreceptors in the mouth.
- Activity in higher centres of the central nervous system (CNS), e.g., smelling food or thinking about food.
The efferent limb of the reflex is parasympathetic and follows cranial nerves VII and IX to reach the glands. Parasympathetic stimulation favours a rapid flow of enzyme-rich saliva. Drugs which block parasympathetic neurotransmission produce a dry mouth by inhibiting salivation.
There are 3 pairs of glands that are responsible for secreting 90% of saliva:
- Parotid glands - Serous and Amylase
- Sublingual and Submandibular glands - a mixed secretion of serous and mucous
- 10% are secreted from minor glands - mucous and mucins
In total, an average person would secrete around 1.5 litres per day.