The major glands of the digestive system are located outside the wall of the digestive tract but are connected to its lumen via ducts. These glands include

the major salivary glands, pancreas, and liver.

MAJOR SALIVARY GLANDS

The major salivary glands are the parotid, submandibular, and sublingual glands. These produce about 1 L of saliva per day, approximately 95% of the daily salivary secretion, which they deliver into the oral cavity.

•The three pairs of salivary glands possess a secretory component that is responsible for the formation of primary saliva (isotonic saliva), which is modified by the initial portion of the duct system (striated ducts) to form the secondary saliva (hypotonic saliva).

•Saliva is a hypotonic solution whose functions include lubrication and cleansing of the oral cavity (and reducing bacterial flora by the lysozyme, lactoferrin, peroxidases, histidine-rich proteins, and immunoglobulin A [IgA] that it contains), initial digestion of carbohydrates by salivary amylase, and assisting in the process of taste (by dissolving food substances).

Saliva also acts as a buffer due to its contents of bicarbonates produced by cells of the striated duct.

•The parotid gland produces serous secretions, whereas the submandibular and sublingual glands manufacture mixed secretions (a combination of serous and mucous saliva).

PANCREAS

The pancreas is a mixed gland, in that it has exocrine and endocrine functions (see Graphic 15-1). Every day, the exocrine pancreas produces approximately 1 L of an alkaline fluid rich in digestive enzymes and proenzymes, which is delivered to the duodenum via the pancreatic duct.

•Enzymes are manufactured by the acinar cells (see Table 15-1 which lists these enzymes and their function), whereas the alkaline fluid is released by centroacinar cells and cells of the intercalated ducts.

The pancreas, unlike the salivary glands, does not possess striated ducts.

•The release of the enzymes and alkaline fluid is intermittent and is controlled by the hormones cholecystokinin and secretin, respectively, as well as acetylcholine released by nerve cells of the enteric nervous system. The two types of secretions may be delivered independent of each other.

D I G E S T I V E S Y S T E M I I I 357

These hormones are produced by the DNES cells of the epithelial lining of the alimentary tract mucosa.

The endocrine pancreas is composed of scattered spherical aggregates of richly vascularized cords of endocrine cells, known as islets of Langerhans. Five cell types are present in these structures (see Table 15-2 Hormones produced by cells of the Islets of Langerhans):

•α cells, producing glucagon,

•β cells, manufacturing insulin,

•δ cells, manufacturing somatostatin,

•PP cells, secreting pancreatic polypeptide, and

•G cells, producing gastrin.

LIVER

The liver is the largest gland of the body. It performs a myriad of functions, many of which are not glandular in nature (see Graphic 15-2). It is believed that the parenchymal cells of the liver, known as hepatocytes, have a lifespan of about 5 months and they are capable of performing each of the approximately 100 different functions of the liver.

•Since each hepatocyte is bordered by a vascular sinusoid, liver cells can absorb toxic materials and byproducts of digestion, which they detoxify and store for future use.

•Hepatic sinusoids receive oxygen-rich blood from branches of the hepatic artery and nutrient-laden blood from branches of the portal vein.

•The sinusoidal lining cells possess

large fenestrae that lack diaphragms and they display

discontinuities between adjoining cells that, although large, are too small for the passage of blood cells or platelets.

•Monocyte-derived macrophages, known as Kupffer cells, participate in the formation of the sinusoidal endothelial lining.

Kupffer cells participate in removing defunct red blood cells and other undesirable particulate matter from the bloodstream.

Fat-storing (Ito) cells are located in the space of Disse, the narrow space between the sinusoidal lining cells and the hepatocytes. Ito cells are believed to function in the accumulation and storage of vitamin A, but in the case of alcoholic cirrhosis, these cells also manufacture type I collagen, responsible for fibrosis of the liver.

Hepatocytes are arranged in radiating plates of liver cells that are arranged in such a fashion that they form hexagonal lobules (2 mm long and 0.7 mm in diameter). These structures are referred to as classical lobules (see Graphic 15-2).