Salivary Secretions Originate in the Gland Acini and Are Modified in the Collecting Ducts
The salivary gland is a typical acinar gland composed of an arborizing system of collecting ducts that end in cellular evag- inations known as acini (Figure 29-1). The cellular epithelium of the acini is functionally distinct from that of the collecting ducts.
Saliva is initially secreted into the lumen of the acini. The glandular cells lining the acini secrete water, electrolytes, enzymes, and mucus. As the newly formed saliva progresses through the collecting ducts, its composition is modified. The duct epithelium reabsorbs electrolytes, especially sodium and chloride, in a manner similar to that in the proximal tubules of the kidneys. The final product, saliva, is hypotonic and has a sodium concentration substantially less than that of extracellular fluid. The extent to which the acinar secretion is modified in the collecting ducts depends on the rate of saliva production. At high rates of salivary flow, there is little modification, which results in higher tonicity and electrolyte concentration, in comparison to low rates of flow.Most mammals have at least three pairs of salivary glands: the parotid glands, which lie just under the ear and behind the vertical ramus of the mandible; the mandibular glands, which are in the intramandibular space; and the lingual glands, which lie in the base of the tongue. Each of these glands drains into a main duct that has a single opening into the mouth. In addition to these major glands, there are minor glands in the tongue and buccal mucosa. These small, indistinct glands often have numerous secretory ducts emptying into the mouth. The concentration of mucus is different in the secretions of the various salivary glands. The parotid gland secretes watery, or serous, saliva, whereas many of the minor glands secrete highly mucous saliva. Other glands secrete a mixed type of saliva containing both mucous and serous material. Avian salivary glands secrete a copious amount of mucus to lubricate unmasticated food for swallowing.