2DIGESTION… Digestion is the process of mechanically and chemically breaking down foods andabsorbing the breakdown products.The digestive system consists of analimentary canal and several accessoryorgans.
3Alimentary canal includes: ● mouth● pharynx● esophagus● stomach● small intestine● large intestine● rectum● anus
5GENERAL CHARACTERISTICS OF THE ALIMENTARY CANAL *The alimentary canal is a muscular tube that passes through the body’s ventral cavity. Different regions carry out different functions, but certain structural characteristics are similar throughout its length.
6Structure of the alimentary wall: 4 DISTINCT LAYERS:• MUCOSA (or mucous membrane)-surface epithelium, underlying connective tissue, and a small amount of smooth muscle-in some regions, folds and tiny projections increase the surface area (for absorption)• SUBMUCOSA-loose connective tissue, glands, blood vessels, lymphatic vessels, and nerves
84 layers of alimentary wall… ● MUSCULAR LAYER-consists of two coats of smooth muscle tissue and some nerves; produces movement of the tube● SEROSA (or serous layer)-outer covering of the tube (also called the visceral peritoneum)-protects underlying tissue-cells here secrete serous fluid, which moistens and lubricates tube’s outer surface
10Movements of the tube:*motor functions include:MIXING MOVEMENTSandPROPELLING MOVMENTS(include a wavelike motioncalled PERISTALSIS)
11MOUTH (Oral Cavity) *receives food and begins digestion Tongue: -rough projections (PAPILLAE)on surface of tongue provide friction to handle food-papillae also contain TASTE BUDS-covering the root of the tongueare masses of lymphatic tissue:LINGUAL TONSILS
12MOUTH (Oral Cavity) Palate: -forms the roof of the oral cavity -hard palate (anterior) and soft palate (posterior)-soft palate extends posteriorlyand downward as a projectioncalled the UVULA
13MOUTH (Oral Cavity) Teeth: -primary teeth (“baby teeth”) are replaced by the secondary teeth (permanent teeth)-teeth are used to mechanicallybreak food into smaller pieces(this increases the surface areaof food allowing digestiveenzymes to react moreeffectively with the foodmolecules)
14SALIVARY GLANDS*salivary glands secrete saliva, which moistens food, helps bind food particles, begins chemical digestion of carbohydrates, makes taste possible, and helps cleanse the mouth.
15SALIVARY GLANDS Serous cells: secrete a watery fluid that contains the digestive enzyme AMYLASE (starts to break down starch and glycogen into smaller disaccharides) Mucous cells: secrete a thick fluid called mucus, which binds food particles and lubricates during swallowing
16SALIVARY GLANDS There are 3 pairs of salivary glands: Parotid glands (largest); contains only serous cells2) Submandibular glands; contains both serous and mucous cells3) Sublingual glands (underneath the tongue; secretions are mostly mucus…thick and stringy!)
18PHARYNX and ESOPHAGUS (important passageways!) PHARYNX: connects the nasal and oral cavities with the larynx and esophagus
19ESOPHAGUS ESOPHAGUS: a food passageway from the pharynx to the stomach;passes through the diaphragm-at distal end, circular smooth muscle fibers thicken and close the entrance to the stomach (prevents regurgitation);-if some of the acidic stomachcontents are regurgitated(reflux) into the esophagus,may cause HEARTBURN
21STOMACH*the stomach receives food, mixes it with gastric juice, initiates protein digestion, carries on a limited amount of absorption, and moves food into the small intestine.
22STOMACH Structure / Parts: -J-shaped, pouch-like organ that sits inferior to diaphragm inupper left of abdominal cavity-has a capacity of about 1 L-thick folds in the inner lining disappear when the stomach wall stretches-separated from the small intestine by a powerful circular muscle: PYLORIC SPHINCTER
24*together, these secretions form STOMACH Gastric Secretions - produced by GASTRIC GLANDS, which have 3 types of cells:● mucous cells: secrete mucus● chief cells: secrete digestive enzymes● parietal cells: secrete hydrochloric acid (HCl)*together, these secretions formGASTRIC JUICE
26STOMACH PEPSIN: most important digestive enzyme in gastric juice -secreted by chief cells in an inactive form: PEPSINOGEN-when it comes intocontact withhydrochloric acid,it becomes theactive PEPSIN
27STOMACH*a layer of thick mucus is produced by cells in the stomach’s inner lining…this protective coating prevents the stomach from digesting itself.
28STOMACH Mixing and emptying actions: CHYME = a mixture of food particles andgastric juice produced bymixing-chyme is emptied into the small intestine where chemical digestion continues-the rate of emptying depends on the fluidity of chyme and the type of food