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Anatomy 200 - Digestive System

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University of Alberta
David Begg

Digestive System Introduction - Specialized for digestion and absorption of food - Consists of the digestive tract and accessory digestive glands - Located predominantly in the abdominopelvic cavity - Walls of cavity lines by parietal peritoneum - Organs covered by layer of visceral peritoneum - Peritoneal cavity is the space between the two layers Functions - Ingestion – food and liquid taken in - Secretion – acid, buffers and enzymes - Motility – mixes and moves food along digestive tract - Digestion o Mechanical – teeth and tongue break food into smaller particles o Chemical – food broken down by acid and enzymes - Absorption – materials move across the digestive epithelium - Elimination – waste is compacted and excreted Gastrointestinal Tract - Oral Cavity – teeth, tung and salivary glands fragment and hydrate food - Pharynx – initiates swallowing - Esophagus – muscular tube that conducts food from pharynx to stomach - Stomach – muscular sac that stores food and initiate digestion - Small intestine – digestion and absorption - Large intestine – compaction and storage of feces Tongue - Composed of multiple layers of skeletal muscle o Oriented at different angles to allow movement in different directions - Positions food within oral cavity - Sensory organ (temperature and taste [sweet, salty, sour, bitter and umami]) - Organ of speech Teeth - Deciduous (temporary dentition) – 20 teeth - Permanent dentition – 32 teeth - Replacement occurs between ages 6 and 17 - Function in mechanical breakdown of food (mastication) Esophagus - Muscular tube extending from pharynx to stomach (25cm long and 2cm in diameter) - Passes along the mediastinum and through the diaphragm into the abdominal cavity - Lined by a stratified squamous (resists abrasion) non-keratinized epithelium - Heart burn – when the diaphragm in the abdominal activity is irritated and stomach contents climb up into the esophagus Muscles of esophageal wall - Upper portion – skeletal (voluntary) o Can decide to continue swallowing or bring food back up - Lower portion – smooth (involuntary) o Committed to finish swallowing Stomach - Muscular sac with 2 major functions: o Stores food o Mechanically breaks down food through contractions of muscular wall - Chemically digests food through actions of secreted acid and enzymes - Mixes food and gastric juices to form chime o Mucous, HCl and enzymes  very acidic, so it’s released in small amounts to duodenum Gastric Gland Cells - Surface mucus – secrete alkaline mucus to protect epithelium from acid - Mucus neck – even more alkaline mucus to protect glands - Parietal – secrete HCl and intrinsic factor (Vitam12 B absorption)  transported through epithelium - Chief – secrete pepsinogen (converted to pepsin by stomach acid)  pepsinogen is the non- active version of pepsin because it’s very powerful - Enteroendocrine – secrete hormones which control function of digestive tract o G cells secrete gastrin – stimulates secretion of parietal and chief cells Small Intestine - Duodenum o Short (25cm) C-shaped segment o Receives chyme from stomach o Receives secretions of pancreas and gall bladder (enzymes) - Jejunum o Second segment (2.5m) long o Region where majority of digestion and absorption take place - Ileum o Final segment (3.5m) long o Digestion and absorption completed o Empties into large intestine pH and Digestion - pH of chyme in the stomach acidic (pH 1.0-2.0) - Digestive enzymes in the small intestine have an alkaline pH optimum - How is acidic chyme neutralized in the duodenum? o Chyme is released slowly from the stomach by the pyloric sphincter and as it enters the duodenum, chyme is rapidly neutralized by alkaline secretions from the pancreas (bicarbonate ions) and alkaline mucus secreted by submucosal glands of the duodenum (Brunner’s glands) Amplification of Surface Area in the small intestine is 600 fold making the surface area ~2000 sq. feet Large Intestine - Horseshoe-shaped tube - 1.5m long and 7.5 cm in diameter - Reabsorbs water and electrolytes - Absorbs important vitamins produced by colonic bacteria (Vitamin K, B ) 12 - Compacts and stores feces Comparison of Mucosa - Stomach – secretory - Small intestine – absorptive and secretory - Large intestine – absorptive and secretory Second note package Digestive Glands - Large exocrine glands associated with the digestive tract which release their secretions into digestive tract through ducts - There are 3 major digestive glands o Salivary glands o Pancreas o Liver and Gallbladder - Salivary glands o Saliva composed of serous (watery secretions and proteins) and mucouse (much thicker than serous) secretions plus the enzyme amylase (breaks down carbohydrates) o Serous secretions moisten food
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