Digestive Tract

Digestive tract, including gross anatomy and histology of the mouth, stomach, small intestine and large intestine.

1. Digestive System Objectives: Discuss the general functions and anatomy of the digestive tract Describe the individual organs of the system, including a discussion of the gross and microscopic anatomy.

2. Digestive System Muscular, hollow tube (= “digestive tract”) + Various accessory organs consists of:

3. Function ingestion mechanical digestion chemical and enzymatic digestion secretion absorption compaction excretion and elimination The function of the system as a whole is processing food in such a way that high energy molecules can be absorbed and residues eliminated. Individual parts function in:

4. Muscularis externa Histological Organization Tube made up of four layers. Modifications along its length as needed. 1 2 3 4

5. The 4 Layers of the Gut 1) Mucosa Epithelial cells – protection, absorption 2 ) Submucosa – made up of loose connective tissue contains submucosal plexus and blood vessels 3) Muscularis externa – smooth muscle, usually two layers - outer layer: longitudinal inner layer: circular 4) Serosa outer layer, functions in protection

6. Ingestion Ingestion is the first step in the process of digestion. Ingestion means that food is taken into the mouth, chewed, and swallowed.

7. Oral Cavity Also called the mouth, the oral cavity is the place where ingestion happens. Hard and soft palates - form roof of mouth Tongue - skeletal muscle Salivary glands - three pairs Teeth

8. Oral Cavity Digestion starts here by two means: Mechanical – teeth tear, shred, and grind food. Chemical – enzymes in saliva break down food molecules such as proteins and complex sugars.

9. Structure of Teeth Crown - exposed surface of tooth Neck - boundary between root and crown Enamel - outer surface Dentin – bone-like, but noncellular Pulp cavity - hollow with blood vessels and nerves Root canal - canal length of root

10. Types and Numbers of Teeth Dental succession: Deciduous (baby, primary) teeth - 20, replaced by Permanent teeth - 32 teeth

11. Types and Numbers of Teeth All teeth are formed before milk teeth or primary teeth are lost. It is the action of adult teeth moving into place that causes primary teeth to fall out.

12. Three pairs of Salivary Glands Salivary glands help with: digestion lubrication ( swallowing) moistening (tasting ) They secrete enzyme-filled saliva to help break down food

13. Ingestion The tongue is a muscle that pushes your food toward teeth to be mechanically processed and shapes your food into small rounded pieces to be swallowed.

14. Swallowing As you swallow, your tongue pushes the food to the back of the mouth. Since both air and food go through the pharynx, a flap of tissue called the epiglottis covers the larynx to prevent food from travelling into the lungs

15. Lesser curvature Greater curvature Cardia - end under the heart Fundus - bulge above the esophageal opening Body - largest region Pylorus - J curve, inferior end, terminates in Cardiac and Pyloric sphincters (importance?) Rugae – highly extendable interior folds Figs 25-10/11 Gross Anatomy of the Stomach

16. Histology of Stomach Type of epithelium lining stomach? Gastric pits – shallow pits, external half rapidly reproduces for replacement Gastric glands – deep in lamina propria, 3 types of cells Parietal cells (produce HCl and intrinsic factor) Chief cells (produce pepsinogen ) Enteroendocrine cells – G cells ( several hormones including gastrin which stimulates both parietal and chief cells) Fig 25.13

17. Regions of Small Intestine SI is longest part of dig. tube Duodenum (short, 12 inches) fixed shape & position Jejunum (2.5 m long) Most of digestion Ileum (longest at 3.5 m) Most of absorption, ends in Ileocecal valve – slit valve into large intestine (colon)

18. Plicae circulares – circular pleats around the interior of the small intestine. Slows movement of food, increases surface area of intestine. Villi – minute finger-like projections, contain capillaries & lacteals. Further increase surface area. Microvilli – sub-microscopic size, projections on single cells. Aid in absorption of nutrients. Intestinal glands (crypts) - intestinal juice production - Cell regeneration Structure of Small Intestinal Wall Fig 25.15

19. Histology of Small Intestinal Wall

20. Histology of S.I. Wall ( cont )

21. Cecum – pocket at proximal end with Appendix Colon Ascending colon - on right, between cecum and right colic flexure Transverse colon - horizontal portion Descending colon - left side, between left colic flexure and Sigmoid colon - S bend near terminal end Regions of Large Intestine Fig 25-17 Rectum – terminal end is anal canal - ending at the anus - which has internal involuntary sphincter and external voluntary sphincter

22. Structure of the Large Intestine

23. 1. Mucosa - abundant goblet cells (produce mucous), stratified squamous epithelium near anal canal 2. No villi 3. Longitudinal muscle layer incomplete, forms three bands or taenia coli 4. Circular muscle - forms pockets or haustra between bands Histology of Large Intestine