Digestive System

Role of the Digestive System

•Preparation of food for absorption•Preparation of food for use in cell• Elimination of unabsorbed food

Organs of DigestionTwo basic groups of organs (See Fig. 25-1; Table 25-1)• Gastrointestinal (GI) tract

(or alimentary canal)• Accessory organs

Wall of the GI TractLayers –There are 4 layers of tissue (See Figure 25-2, page 741)1) Mucosa – inner layer• a) Mucous epithelium• b) Lamina propria• c) Muscularis mucosae

2) Submucosa• a) Connective tissue• b) Submucosal plexus

3) Muscularis• a) Inner and outer layer of

smooth muscle• b) Myenteric plexus

4) Serosa• a) Connective tissue layer• b) Peritoneum

Modifications of the Layers• Although all 4 layers are found through out, the walls vary

based on their location in the digestive tract.• See Table 25-2, page 742

Mouth – Structure of Oral Cavity

• Lips - Philtrum, Oral fissure• Cheeks – lateral boundaries of oral cavity• Hard palate consists of 4 bones: (2) maxilla and (2) palatine

bone. (See Fig. 8-5, page 219 and Fig. 25-3, 743)• Soft palate - between the mouth and nasopharynx• Fauces (opening between mouth and oropharynx)• Uvula• Tongue – remember taste buds? Also see fig. 25-4, page 744

Lingual frenulum – Fold of mucous membrane in midline of the undersurface of the tongue – anchor the tongue to the floor of mouthPlica fimbriata - In this area the tongue is highly vascular and vessels are superficial; therefore, some drugs are placed under the tongue for rapid absorption

Salivary Glands – 3 pairs

Parotid glands (Fig. 25-6)• a. Drain via the parotid duct into the

vestibule opposite upper second molars

• b. Secrete enzymes only (no mucus)Submandibular glands (Fig. 25-6)• a. Ducts opening on either side of the

lingual frenulum• b. Secrete enzymes and mucusSublingual glands (Fig. 25-6)• a. Ducts open onto the floor of the

mouth• b. Secrete mucus only

Pharynx and Esophagus

Pharynx (p. 747)• Nasopharynx• Oropharynx• Laryngopharynx

Esophagus (Fig. 25-9)• Upper third—striated muscle• Middle third—mixed striated and smooth muscle• Lower third—smooth muscle

Stomach

Size and position of the stomach•Distensible• Close to diaphragm• In the epigastrium and left

hypochondrium

Stomach – Figure 25-10, Page 749

Divisions of the stomach – Fundus, Body, PylorusCurves of the stomach - Lesser curvature, Greater curvatureSphincter muscles - Cardiac sphincter, Pyloric sphincterStomach Wall – Has the 4 layers, Note modification of Gastric Mucosa, and Gastric Muscle

Functions of the Stomach

• Serves as reservoir for food• Secretes gastric juice (enzymes and acid)• Mixes enzymes and chyme (through muscular action)

and moves chyme into small intestine• Secretes intrinsic factor (Protects vitamin B12 from acid

and needed for absorption)• Absorption (limited to certain drugs, some water,

alcohol, and some fatty acids)• Secretes hormone gastrin• Destroys pathogenic bacteria

See page 750

Small Intestines

20 feet long; fills most of abdominal cavityDivisions of the small intestine• Duodenum—10 inches long • Jejunum—8 feet long• Ileum—12 feet longWall of the small intestine (Fig. 25-13)• Plicae• Villi (Fig. 25-15)• Microvilli on cells (Fig. 25-15)• Goblet cells—produce mucus (Fig. 25-14)• Crypts (Fig. 25-15)

Large Intestine

5 to 6 FeetDivisions of the large intestine•Cecum—2 to 3 inches long, Appendix extending from it•Colon

a. Ascending colonb. Transverse colon - From hepatic flexure to splenic flexurec. Descending colond. Sigmoid colon

•Rectum —7 to 8 inches long; terminal inch called anal canal. Anal columns are vertical folds and containing hemorrhoidal arteries and veins. Hemorrhoids formed by enlarged veins.

LiverLeft lobe – 1/6th of liverRight lobe divided into three lobes• Right lobe proper• Caudate lobe• Quadrate lobe

Hepatic lobule (Figs. 25-23, page 758) – Anatomical units of the liver. Hepatic lobule function (page 759)• With the hepatic cells are many phagocytic reticuloendothelial

cells. These cells remove bacteria, worn RBCs, etc. from the blood

• Hepatic cells remove and store some vitamins• Hepatic cells detoxify poisons• Hepatic cells form bile

Liver Function

•Detoxification•Bile secretion•Metabolism of food• Storage of iron and vitamins A, B12,

and D•Production of some plasma proteins• Site of fetal hematopoiesis

Gallbladder (p 760)• 3 to 4 inches long, 1 inch wide• Capacity: 30 to 50 ml of bile• Located under the liverStructure of the gallbladder• Mucosa—(rugae)• Muscularis• Serosa

Functions of the gallbladder• Storage of bile• Concentration of bile

Pancreas – p. 761About 6 to 9 inches longLocated between the stomach and the duodenumSections of the pancreas• Head of the pancreas to the right, against the duodenum• Body in between the stomach and the duodenum• Tail extending to the leftStructure of the pancreas (Fig. 25-27)• Exocrine portion (compound acinar cells)• Produces digestive enzymes• Endocrine portion—pancreatic islets - About a million isletsFunctions of the pancreas• Acinar units secrete digestive enzymes• Beta cells of pancreatic islets secrete insulin• Alpha cells of pancreatic islets secrete glucagon

Disorders of Digestive System

• Mumps – viral disease characterized by swelling of the parotid salivary glands.

• Tooth Decay, Gingivitis, Periodontitis, Maloccusion• Gastroesophageal Reflux Disease (GERD)• Ulcers• Appendicitis• HemorrhoidsSee pages 764 to 766