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BISC 110 (14)
Lecture 2

Week 2

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Biological Sciences
BISC 110
Derek Bingham

Week 2 Digestion 1 Digestion is the process of breaking down food by mechanical and chemical action into substances for absorption and use by body cells. Mechanical digestion:  Reduces particle size via physical movement o e.g., teeth grinding, intestinal segmentation  Increases surface area for digestive enzyme access Chemical digestion:  Breakdown of macromolecules into smaller molecules using enzymes o Proteases break down proteins into amino acids. o Amylases break down complex carbohydrates (e.g., starches) into simple sugars (e.g., glucose). o Lipases break down triglycerides into monoglycerides and fatty acids. Gastrointestinal tract organs: 1. Mouth 2. Pharynx 3. Esophagus 4. Stomach 5. Small intestine 6. Large intestine Accessory organs: 1. Teeth 2. Salivary glands 3. Liver 4. Gallbladder 5. Pancreas Layers of the gastrointestinal tract (deep  superficial): 1. The MUCOSA is made up of three layers: a. Epithelium b. Lamina propria – CT that contains capillaries, lacteals, glands, and lymphocytes c. Muscularis mucosae – thin muscle layer responsible for “gentle agitation” of the mucosal surface 2. The SUBMUCOSA contains blood vessels, lymphatic vessels, and the submucosal plexus. 3. The MUSCULARIS includes the myenteric nerve plexus and has up to three layers of muscle a. Oblique – only present in the stomach b. Longitudinal c. Circular 4. The SEROSA secretes a lubricating serous fluid and has two components: a. CT b. Epithelium Week 2 Digestion 2 Peritoneum:  The serous membrane lining the abdominal cavity and covering the abdominal organs The visceral layer adheres to abdominal organs (the serosa of the digestive tract). The parietal layer attaches to the wall of the abdominal cavity. Two main portions: 1. The OMENTUM is a portion of the peritoneum that folds over the small intestine and colon, contains many lymph nodes, and stores fat. 2. The MESENTERY is a portion of the peritoneum that binds the small intestines to the posterior wall of the abdominal cavity. Week 2 Digestion 3 Buccal Cavity (Mouth):  Extends from the mouth opening to the oropharynx Salivary Glands:  Exocrine glands that lie outside the mouth and secrete saliva through ducts into the oral cavity Saliva: 1000 to 1500 mL per day Regulated by the AUTONOMIC NERVOUS SYSTEM in response to touch, smell, or thought of food  Parasympathetic – stimulates saliva release for swallowing, speech  Sympathetic – inhibits saliva release during stress and sleep Lack of saliva production during sleep  less bacteria killers  bad breath Teeth:  Incisors cut & nip, canines tear & shred, and molars crush & grind. Components: 1. The CROWN projects into the oral cavity above gum margins. 2. The ENAMEL is a very hard calcium phosphate that covers the dentin of the crown. 3. DENTIN is similar to bone but has no cellular component. 4. The PERIODONTAL LIGAMENT has collagen fibres that join bone to tooth (gomphosis). Week 2 Digestion 4 Digestion in the Mouth  MECHANICAL  Food is maneuvered by the tongue, ground by teeth, mixed with saliva, and shaped into a bolus for swallowing.  CHEMICAL  Salivary amylase starts carbohydrate breakdown in the mouth STARCHES ARE CONVERTED USING AMYLASE: 1. Smaller disaccharides (maltose) 2. Trisaccharides 3. Dextrins ( >5 glucoses) Esophagus:  Tube consisting of skeletal and smooth muscle  Posterior to trachea  Pharynx  esophagus  diaphragm  stomach  Purely for movement of food ANATOMY (DEEP  SUPERFICIAL): 1. Lumen 2. Mucosa a. Epithelium b. Lamina propria c. Muscularis mucosae 3. Submucosa 4. Muscularis a. Circular layer b. Longitudinal layer 5. Adventitia Swallowing  Passage of food from mouth to tummy takes 4 to 8 seconds  Liquid takes ~1 second Week 2 Digestion 5 THREE STAGES: 1. Voluntary phase – bolus of food is moved to the back of the mouth by tongue movement 2. Pharyngeal phase – when food moves through the pharynx: a. Breathing stops b. Nasopharynx is blocked by the soft palate and uvula c. The larynx is blocked by the epiglottis 3. Esophageal phase – when food moves through the esophagus by peristalsis (alternating waves of muscular contraction and relaxation  squeezes food from one part of the system to the next). Peristalsis The circular muscle above contracts to push the bolus downwards The longitudinal muscle below contracts to shorten and widen the esophagus The wave of contraction continues as the bolus moves into the new section Stomach A J-shaped enlargement of the GI tract with three muscle layers (oblique, longitudinal, circular) Week 2 Digestion 6 Sphincters  Ring of smooth muscle  Cardiac
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