Page 249 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Saliva nels, resulting in a lumen-negative trans-
epithelial potential (LNTP) that drives Na pa-
+
The functions of saliva are reflected by its con- racellularly into the lumen. Water also follows
stituents. Mucins serve to lubricate the food, passively (osmotic effect). Primary saliva is
making it easier to swallow, and to keep the modified in excretory ducts, yielding second-
mouth moist to facilitate masticatory and ary saliva. As the saliva passes through the ex-
–
+
speech-related movement. Saliva dissolves cretory ducts, Na and Cl are reabsorbed and
compounds in food, which is a prerequisite for K and (carbonic anhydrase-dependent) HCO 3 –
+
taste buds stimulation (! p. 338) and for den- is secreted into the lumen. The saliva becomes
tal and oral hygiene. Saliva has a low NaCl con- hypotonic (far below 100 mOsm/kg H 2O; ! B)
centration and is hypotonic, making it suitable because Na and Cl reabsorption is greater
+
-
–
+
for rinsing of the taste receptors (NaCl) while than K and HCO 3 secretion and the ducts are
Nutrition and Digestion mouth, while immunoglobulin A and lysozyme 100µL/(min · g), these processes lag behind
relatively impermeable to water (! B). If the
eating. Infants need saliva to seal the lips when
suckling. Saliva also contains α-amylase,
secretion rate rises to values much higher than
which starts the digestion of starches in the
and the composition of secondary saliva be-
comes similar to that of primary saliva (! B).
are part of the immune defense system
(! p. 94ff.). The high HCO 3 concentration in
Salivant stimuli. Reflex stimulation of saliva
–
saliva results in a pH of around 7, which is opti-
secretion occurs in the larger salivary glands
taste of food, tactile stimulation of the buccal
lowed saliva is also important for buffering the
acidic
refluxed
into
the
mucosa, mastication and nausea. Conditioned
gastric
juices
10 mal for α-amylase-catalyzed digestion. Swal- (! D). Salivant stimuli include the smell and
reflexes also play a role. For instance, the
esophagus (! p. 238). The secretion of profuse
amounts of saliva before vomiting also pre- routine clattering of dishes when preparing a
vents gastric acid from damaging the enamel meal can later elicit a salivant response. Sleep
on the teeth. Saliva secretion is very dependent and dehydration inhibit saliva secretion. Saliva
on the body water content. A low content re- secretion is stimulated via the sympathetic
sults in decreased saliva secretion—the mouth and parasympathetic nervous systems (! C2):
and throat become dry, thereby evoking the ! Norepinephrine triggers the secretion of highly
sensation of thirst. This is an important mecha- viscous saliva with a high concentration of mucin via
nism for maintaining the fluid balance " 2 adrenoreceptors and cAMP. VIP also increases the
(! pp. 168 and 184). cAMP concentration of acinar cells.
Secretion rate. The rate of saliva secretion ! Acetylcholine: (a) With the aid of M 1 cholinocep-
tors and IP 3 (! pp. 82 and 274), acetylcholine medi-
varies from 0.1 to 4 mL/min (10–250µL/min ates an increase in the cytosolic Ca 2+ concentration
per gram gland tissue), depending on the of acinar cells. This, in turn, increases the conductiv-
degree of stimulation (! B). This adds up to ity of luminal anion channels, resulting in the produc-
about 0.5 to 1.5 L per day. At 0.5 mL/min, 95% of tion of watery saliva and increased exocytosis of
this rate is secreted by the parotid gland (se- salivary enzymes. (b) With the aid of M 3 cholinergic
rous saliva) and submandibular gland (mucin- receptors, ACh mediates the contraction of myo-
rich saliva). The rest comes from the sublingual epithelial cells around the acini, leading to emptying
glands and glands in the buccal mucosa. of the acini. (c) ACh enhances the production of kal-
likreins, which cleave bradykinin from plasma kinino-
Saliva secretion occurs in two steps: The gen. Bradykinin and VIP (! p. 234) dilate the vessels
acini (end pieces) produce primary saliva (! A, of the salivary glands. This is necessary because max-
C) which has an electrolyte composition simi- imum saliva secretion far exceeds resting blood flow.
lar to that of plasma (! B). Primary saliva
secretion in the acinar cells is the result of
transcellular Cl transport: Cl is actively taken
–
–
up into the cells (secondary active transport)
+
from the blood by means of a Na+-K -2Cl –
cotransport carrier and is released into the
236 lumen (together with HCO 3 ) via anion chan-
–
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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