The three paired salivary glands and small buccal glands located throughout the oral cavity contribute to the approximately 1 liter of saliva produced in a day. Salivary flow can vary between 800 milliliters and 2 liters but this is regulated by a number of factors, especially nervous control by the brain stem.
Composition of Saliva
Saliva is a combination of a serous and mucoid fluid. The serous fluid is watery and contains the enzyme ptyalin which the starts the digestion of carbohydrates. It is secreted by all three paired salivary glands – parotid, submandibular and sublingual. The mucus component of saliva is secreted throughout the day to moisturize the lining of the mouth and increases during eating to lubricate the food in the mouth and assist with swallowing. It is secreted by the buccal glands of the mouth as well as the sublingual and submandibular glands. Mixed in with the mucus or ptyalin is a fluid that is high in potassium and bicarbonate ions – process explained below.
The composition of saliva changes throughout the day and depending on the situation. During most of the day, when not eating, the saliva is mainly composed of mucus to keep the lining of the mouth moist. This is secreted at a rate of approximately 0.5 milliliters per minute. During eating or any other situation that triggers maximal salivation, the salivary flow can increase up to 20-fold. At these times, the rapidly exiting saliva does not contain the same quantity of potassium and bicarbonate ions since the secondary process to facilitate this cannot occur at a sufficient rate.
Additional components of saliva include thiocyanate ions, lysozymes and other proteolytic enzymes, as well as antibodies. These substances play a role in preventing infections in the mouth by destroying microorganisms and digesting any food particles that are stuck between the teeth and which may be used by the bacteria as a food source.
Secretion of Saliva
Multiple acini of the salivary glands produce saliva and then secrete it into collecting ducts where it empties into the main duct of the gland. The acini produce a ion-rich solution containing potassium, bicarbonate, sodium and chloride. This is similar to tissue fluid. Depending on the needs at the time, ptyalin and/or mucus are included in this solution. This primary secretion is not the final saliva composition that exits the gland.
As the primary fluid travels through the ducts, it undergoes a significant change in composition. First the sodium ions in the fluid are exchanged for potassium across the wall of the ducts. The fluid in the ducts now contain less sodium and more potassium. This difference in ions creates a negative electrical potential within the duct and this causes chloride to be reabsorbed. Simultaneously bicarbonate ions are exchanged for the chloride ions and some bicarbonate is also actively pumped into the ducts.
The salivary fluid now has high concentrations of potassium and bicarbonate compared to the primary secretion that initially exited the acini.
Functions of Saliva
- Lubricate the mouth lining and food during chewing (mastication).
- Moisten the mucosa of the mouth.
- Ptyalin plays a small role in the digestion of starch.
- Antimicrobial action, especially against bacteria, thereby avoiding mouth infections or slowing decay in a tooth cavity.
Nervous Control of Salivation
Parasympathetic impulses travel from the salivatory nuclei in the brain stem (“salivation centers”) reach the salivary glands via the facial and glossopharyngeal nerves. This stimulates saliva production and salivary flow.
The salivation centers may be stimulated by :
- Impulses from the higher brain – example thinking of food that one finds delectable.
- Impulses from the mouth and throat – taste sensations (sour and umami) and tactile sensations (smooth objects in the mouth stimulate sflow while rough objects inhibit salivation).
- Impulses from the stomach and proximal part of the small intestine – irritation of the lining of these parts of the alimentary tract
Parasympathetic stimulation that increases salivation also increases blood flow to the salivary glands. Conversely, increased blood flow to the gland, for reasons other than parasympathetic stimulation of the gland, also increases salivation.