DIGESTIVE SYSTEM III

VII. Digestive organs - background information. A. There are a number of organs associated with the digestive tract that assist in the digestion of food in various ways. 1. Salivary glands - Read about these glands in your text. 2. Pancreas 3. Liver 4. Gall bladder

VIII. Salivary glands - Read about these glands in your text. A. Secrete fluid and mucus to wet and lubricate ingested food B. Secrete various proteineceous enzymes (amylase, lipase, maltase) that initiate the process of digestion. 1. Labial 2. Palatine 3. Parotid 4. Buccal 5. Submaxillary 6. Sublingual 7. Anterior lingual

IX. Pancreas A. The pancreas is a mixed exocrine and endocrine organ. 1. Exocrine - zymogen secretion - precursors of enzymes like lipases and the proteolytic digestive enzymes trypsin, and chymotrypsin are synthesized and secreted by acinar cells of the pancreas 2. Endocrine - hormone secretion - insuline, glucagon and somatostatin secreted by cells of islets of Langerhans.

B. General structure 1. Surrounded by a thin connective tissue capsule. Composed of lobules containing mostly exocrine secretory glands, but also endocrine glands. 2. Lobules are separated by connective tissue septa

2. Ducts that the pancreatic acini connect to are called intercalated ducts. a. Lined by squamous to cuboidal (usually cuboidal) epithelium. b. These cells will also line the neck of the acinus, the rest of the acinus (sac) being composed of acinar and centroacinar cells. 3. The intercalated ducts connect to intralobular ducts located within the pancreatic lobes. 4. The intralobular ducts connect to larger interlobular ducts in the septa between lobes. 4. The interlobular ducts connect to the main pancreatic ducts, the major duct of Wirsung and the accessory duct of Santorini gestive/images/ff855.jpg histo_exo.html

4. All secretions eventually reach the main pancreatic ducts (duct of Wirsung and duct of Santorini)that empty into the common bile duct via the duct of Wirsung in humans. 5. Pancreatic exocrine secretion is controlled by the hormones secretin and cholecystokinin. a. These are produced by APUD (amine precursor uptake and decarboxylation) cells - enteroendocrine cells in the duodenal crypts of Lieberkuhn. duct of Wirsung duct of Santorini ventral pancreas dorsal pancreas

6. The acini are sac-like, epithelial bags that connect to the intercalated ducts. a. The cuboidal epithelium of the intercalated duct extends into the neck of the acinus. b. In deeper regions, the acinus is lined with exocrine acinar cells that synthesize and secrete zymogen - dark staining cells with granules. * Contain apical zymogen granules * These are exocytosed and the digestive enzyme precursors that they contain are carried to the common bile duct via the pancreatic ducts. c. Squamous centroacinar cells partially overlie the zymogen secreting acinar cells - light staining cells within the acinus digestive/images/ff852.jpg organology/digestive/images/ff850.jpg

6. The acinar cells are typical protein synthesis and secreting cells. a. Extensive rough ER b. Golgi apparatus that concentrates inactive enzymes into secretory granules. * Initially these are prozymogen granules. * These will mature as contents are processed further. * Mature granules contain zymogen ** zymogen granules contain precursors to a number of different enzymes (e.g. trypsin, lipase, amylase) * The granules accumulate at the apices of acinar cells where they are exocytosed into the lumen of the acinus and carried to the small intestine via the system of ducts. ** The enzyme precursors become active enzymes after they are released from the acinar cells.

C. Endocrine portion of pancreas 1. Amongst the acini of the pancreas (i.e. between them) are globlar cellular aggregations containing four different types of secretory cells and fenestrated capillaries.

a. These aggregations are the islets of Langerhans, b. They contain the cells that secrete glucagon (alpha cells, 20% 0f cells), insulin (beta cells, 70% of cells), and somatostatin (delta cells, 5% of cells). Also, C-cells (no known function, 5% of cells). Glucagon - increases blood glucose Insulin - lowers blood glucose Somatostatin - inhibits release of growth hormone c. Many fenestrated capillaries will be interspersed between these cells. d. The cytoplasm of these cells will generally stain differently than the acinar cells and will not contain the large zymogen granules that are prevalent in the acinar cells.

IX. Liver A. The liver is the second largest organ in body. It is located in the abdominal cavity just below diaphragm. B. Blood supplied from two sources 1. hepatic portal vein (75% of blood) that carries blood from small intestine to liver. a. This blood carries all digested nutrients except lipid. b. Lipid is carried to the liver via lymphatic vessels. 2. hepatic artery (25% of blood) that carries oxygenated blood to liver

C. The liver is encased in a fibroconnective tissue capsule (called Glisson’s capsule) surrounding the parenchyma that consists of the hepatocytes, associated sinusoidal capillaries, and bile ducts.

D. The liver parenchyma is composed of liver lobules. 1. These are the functional units of the liver. The structure of these lobules may be viewed in a number of different ways depending on how you define the functional unit. We'll consider the lobule in the classical sense. 2. The lobule is a four to six sided column of tissue with structures known as portal triads at a number of the corners and a central vein

The liver lobule is 3 dimensional

a. The portal triads each consist of * a branch of the hepatic artery * a branch of the hepatic portal vein * a branch of the bile duct => bile ductule * some lymphatic vessels are also present, but they are not considered part of the triad

The portal triad

3. The branch of the portal vein is the largest of these structures. a. The branches of the hepatic portal vein and hepatic artery empty into sinusoids that form an anastomosing network within the tissues of the lobule. b. These sinusoids are lined with an incomplete fenestrated endothelium with large gaps (pores) between the endothelial cells. c. The gaps (pores) in the endothelial wall allow for the passage of macromolecules and cells into and out of the sinusoids

4. Blood from the liver sinusoids passes into the central vein of the lobule that eventualy empties into the hepatic vein that carries it to the posterior vena cava.