The organs associated with the digestive tract include the major salivary glands, the pancreas, the liver, and the gallbladder. Products of these organs facilitate transport and digestion of food within the gastrointestinal tract. The main functions of the salivary glands are to moisten and lubricate ingested food and the oral mucosa, to initiate the digestion of carbohydrates and lipids with amylase and lipase, and to secrete 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, is made in the liver but stored and concentrated in the gallbladder. 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 mouth produce saliva, which has digestive, lubricating, and protective functions. With a normal pH of 6.5-6.9, saliva also has an important buffering function and in some species is also important for evaporative cooling. There are three pairs of large salivary glands: the parotid, submandibular, and sublingual glands (Figure 16–1), in addition to the numerous minor or intrinsic salivary glands located throughout most of the oral mucosa which secrete about 10% of the total saliva volume.
There are three bilateral pairs of major salivary glands, the parotid, submandibular, and sublingual glands, which together produce about 90% of saliva. Their locations, relative sizes, and excretory ducts are shown here. These glands plus microscopic minor salivary glands located throughout the oral mucosa produce 0.75-1.50 L of saliva daily.
Inadequate saliva production, leading to dry mouth or xerostomia, can be caused by 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. The secretion of each gland is either serous, seromucous, or mucous, depending on its content of the glycoprotein mucin. Saliva from the parotids is serous and watery. The submandibular and sublingual glands produce a seromucous secretion, while that of the minor glands is mostly mucous. Saliva is modified by the cells of the duct system draining the secretory units, with much Na+ and Cl− reabsorbed while certain growth factors and digestive enzymes are added.
Three epithelial cell types comprise the salivary secretory units: