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Composition of Saliva


There are four major components of saliva:
  1. mucus that serves as a lubricant,
  2. -amylase, an enzyme that initiates the digestion of starch,
  3. lingual lipase, an enzyme that begins the digestion of fat, and
  4. a slightly alkaline electrolyte solution that moistens food.
Human saliva is always hypotonic to plasma. Na+ and Cl- concentrations are less than that of plasma; but K+ and HCO3- concentrations are higher in saliva than in plasma. Salivary osmolality increases as the secretion rate increases, reaching about 70% of plasma osmolality at maximal secretion rates. Also, the pH changes from being slightly acidic (at rest) to basic (pH 8) at ultimate stimulation. This increase in alkalinity is due to the increase of HCO3- in the saliva (figure). At low flow rates of saliva secretion, Na+ is actively absorbed, Cl- passively absorbed, but K+ and HCO3- are secreted as the saliva moves out of the ducts into the mouth. However, at higher flow rates, salivary ducts are not as efficient in reabsorbing Na+ and Cl- because their levels now more closely approach those of the plasma. Therefore, at high flow rates, besides being rich in HCO3- and K+, saliva also has high concentrations of Na+ and Cl- and is more basic. Amylase and mucus also increase in concentration after stimulation.
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