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The Digestive System Digestive System • Also known as the gastrointestinal (GI) tract or alimentary canal • Long tube open at both ends – Also has accessory organs • Nutrients absorbed, waste eliminated – Converts food into useable form Mouth Salivary Glands Esophagus Liver Stomach Gallbladder Pancreas Small Intestine Large Intestine Appendix Anus 4 Steps to Nutrient Intake 1. Ingestion 2. Digestion - Prepares food for absorption - Mechanical and chemical breakdown of food 3. Absorption - Small intestine 4. Elimination Two Types of Digestion 1. Mechanical 2. Chemical Mechanical Digestion • Physical breakdown of food – Mouth (chewing) and stomach (churning) • Movement of food – Peristalsis - wavelike involuntary muscle contractions Chemical Digestion • In mouth, stomach, small intestine • Chemicals (e.g., HCl) and enzymes break down complex molecules – e.g., Amylase in saliva - enzyme that breaks down starch (a carbohydrate) Enzymes • Break down substances • Protein catalysts • Specific to substrates Oral Cavity (Mouth) • Mechanical digestion (teeth) and chemical digestion (saliva with enzymes) Bolus • Bolus of food forms and passes through pharynx into esophagus • Epiglottis prevents food from entering trachea Esophagus • Muscular tube that connects pharynx to stomach • Peristaltic contractions move food to stomach • Cardiac sphincter at junction prevents food in stomach from going back into esophagus Stomach • • • • J-shaped muscular organ to churn and squeeze Acts as a reservoir for food (1.5 L) Rugae (folds) allow for expansion Movement of food controlled by two sphincters: – Cardiac sphincter: Between the stomach & the esophagus Cardiac sphincter – Pyloric sphincter: Between the stomach & the small intestine Stomach • Digestion turns bolus into chyme – paste-like partially digested food • Secretes HCl (pH of 2) – Destroys microbes and breaks down food • Primarily digests protein – Pepsin, an enzyme only active at low pH Small Intestine • Major site of digestion and absorption • 80% of nutrient absorption occurs here – diffusion • About 6 m long, SMALLER diameter than large intestine, 3 sections • Bile produced by liver breaks down fats • Enzymes secreted by pancreas break down carbs, fats and proteins Structure of Small Intestine Microvilli Villus Villi and microvilli - increase surface area, maximize absorption Duodenum • First section of small intestine • Receives pancreatic secretions for further chemical digestion • Basic pH – Bile from gall bladder and bicarbonate ions from pancreas neutralize acidic chyme from stomach Jejunum and Ileum • Make up the other two sections of the small intestine. • Where most nutrient absorption takes place. • Peristaltic contractions continue to propel chyme through tract. Large Intestine • • About 1.5 m long and has a LARGER diameter than the small intestine. Consists of four sections: 1. 2. 3. 4. Caecum Colon Rectum Anus. Large Intestine • Main function is to reabsorb water • Bacteria produce vitamins which are absorbed • Compacts unabsorbed material into feces • Defecation: - Rectal sphincter passes feces into rectum for temporary storage - Anal sphincter passes feces from rectum out anus Animated Digestion Video! Graphic Digestion Video!