The organs associated with the digestive tract include the major salivary glands, pancreas, liver, and gallbladder. Products of these organs facilitate movement and digestion of food within the gastrointestinal tract. The main functions of the salivary glands include moistening and lubricating ingested food and the oral mucosa, beginning digestion of carbohydrates and lipids with amylase and lipase, and secreting innate immune components such as lysozyme and lactoferrin.
The pancreas secretes digestive enzymes that act in the small intestine and hormones important for the metabolism of the absorbed nutrients. Bile, whose components are necessary for digestion and absorption of fats, makes up the exocrine secretion of the liver but is stored and concentrated in the gallbladder for later delivery to the duodenum. The liver also plays a major role in carbohydrate and protein metabolism, inactivates many toxic substances and drugs, and synthesizes most plasma proteins and factors necessary for blood coagulation.
Exocrine glands in the oral mucosa produce saliva, which has digestive, lubricating, and protective functions. With a normal pH near neutrality saliva has an important buffering function in addition to its varied hydrolytic, mucous, and immune components. The large salivary glands occur as three pairs: the parotid, submandibular, and sublingual glands (Figure 16–1). Numerous minor or intrinsic salivary glands located throughout most of the oral mucosa secrete about 10% of the total saliva volume.
Major salivary glands.
Three large bilateral pairs of major salivary glands develop in the oral cavity, the parotid, submandibular, and sublingual glands, which together produce about 90% of saliva. The figure indicates their locations, relative sizes, and excretory ducts. These glands plus microscopic minor salivary glands scattered throughout the oral mucosa produce 0.75–1.50 L of saliva daily.
Inadequate saliva production, leading to dry mouth or xerostomia, can occur with various factors affecting the major salivary glands, such as mumps viral infection, radiation of the glands, or the normal side effect of drugs such as antihistamines.
A connective tissue capsule surrounds each major salivary gland. The parenchyma of each consists of secretory units on a branching duct system arranged in lobules, separated by septa of connective tissue. Each gland produces a largely serous, seromucous, or mucous secretion, depending on its content of the glycoprotein mucin. The parotid glands release a serous, watery saliva. The submandibular and sublingual glands produce a seromucous secretion, while that of the minor glands is mostly mucous. Cells of the duct system draining the glands’ secretory units modify saliva, reabsorbing and adding various electrolytes.
Three epithelial cell types comprise the salivary secretory units:
Serous cells are polarized protein-secreting cells, usually pyramidal in shape, with round nuclei, well-stained RER, and apical secretory granules ...