Salivary Gland Physiology
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salivary glands
Secrete saliva, which:
- Lubricates ingested food
- Protects the mouth from acidic food and pathogens
- Initiates carbohydrate digestion
Sites of Salivary Secretion
1. Parotid gland
2. Sublingual gland
3. Submandibular gland
Minor (Intrinsic) Salivary Glands in the:
1. Tongue
2. Pharynx
Saliva Composition
- Mostly water: hypotonic relative to plasma
- High bicarbonate and K+ concentrations: (relative to plasma), which neutralize acidic foods in the mouth.
- Low Na+ and Cl- concentrations: (relative to plasma).
- The digestive enzymes:
- Mucin, which forms thick mucus to moisten and lubricate food as well as aid bolus formation.
- Lysozyme and IgA antibodies, which lyse bacteria and protect the mouth against microorganisms.
Saliva Formation
Acinar region = "secretory region"
- Secretes the initial saliva
- Water, bicarbonate, K+, Na+, and Cl- are secreted into the acinar region.
Initial salivary secretion is a plasma-like solution (isotonic relative to plasma.
Ductal region = "modifying region"
- Transport saliva to the oral cavity.
- Modify the ionic composition of the initial saliva by selectively absorbing and/or secreting water and electrolytes.
- Na+ and Cl- are reabsorbed, which makes their concentration in saliva lower relative to their plasma concentrations.
- Bicarbonate and K+ are secreted into the duct, which makes their concentration in saliva higher relative to their plasma concentration.
Final saliva that enters the oral cavity is hypotonic.
Note: Salivary ducts are nearly impermeable to water. Tight junctions between ductal cells prevent any additional water leakage into or out of salivary ducts, which makes final salivary secretion more dilute than the initial secretory product.
FLOW RATE
Flow rate of saliva through salivary ducts effects it final ionic composition because it affects the time the saliva is in contact with the surface of the ductal epithelium and, thus, the degree of absorption and secretion that occurs along the length of the duct.
Note: bicarbonate secretion is not effected by changes in flow rate; saliva is almost always rich in bicarbonate.
High Flow Rate
- Higher concentration of ions relative to water.
- Final saliva is isotonic to plasma (it's most similar to the initial saliva secretion) because there is less time for reabsorption and secretion.
Final saliva has higher Na+ and Cl- concentrations and lower K+ concentrations than at average flow rates.
Low Flow Rate
- Water concentration is high and the ionic concentration is low.
- Final saliva is hypotonic to plasma (and the least like the initial saliva secretion); this allows more time for Na+ and Cl- reabsorption as well as K+ secretion into the ducts.
Final saliva is much more dilute than at a normal flow rate.
Although we only discussed water and electrolyte secretion from salivary acinar cells, bear in mind, they also secrete the digestive enzymes (salivary amylase and lingual lipase), mucin, lysozyme, and IgA antibodies, which are present in the final saliva.
Regulation of salivary secretion
Note: Unlike other GI accessory glands, salivary secretion is not under hormonal regulation and only involves neural regulation.
Parasympathetic branches of the autonomic nervous system stimulate the majority of salivary secretion but that the sympathetic nervous system also plays a minor role in secretion, as well.
- Parasympathetic nervous system stimulates secretion of watery (serous), enzyme-rich saliva via cranial nerves IX, X and VII.
- Sight, smell, and taste of food stimulate saliva secretion via parasympathetic activation. Conditioned reflexes and nausea do, as well.
Also:
- Sympathetic activation inhibits salivary secretion.
- Sympathetic nervous system also does stimulate a viscous, mucin-rich saliva