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THE DIGESTIVE SYSTEM Evolution of digestion Amoeba – engulfs food, lysomes Hydra – digestive sack with single opening Earthworm/bird – pharynx, esophagus, crop, gizzard, intestine, anus Chordata – more complex digestive system The Four Stages of Digestion 1. ingestion – taking in nutrients (i.e. eating) 2. digestion – breaking down complex molecules into smaller ones 3. absorption – taking up of digested molecules 4. egestion – removal of wastes 1. INGESTION and DIGESTION: the mouth • - food enters mouth • - it is broken up by teeth: – incisors cut – canines and bicuspids pierce and tear – molars crush and grind • - action of chewing stimulates salivary glands which secrete saliva • saliva: a mixture of water, mucus and enzymes (ex. amylase: breaks down starches into simpler carbohydrates) • food and saliva form a bolus that is pushed to the pharynx (dual purpose: air or food) – - flap-like epiglottis directs food down esophagus – - bolus moves down esophagus through rhythmic muscle contractions (peristalsis) until it reaches the cardiac sphincter of the stomach 2. DIGESTION a) DIGESTION: the stomach • - the stomach is the site of food storage (1.5 L capacity) and initial protein digestion • - to enter and to exit the stomach the food must pass through sphincters: constrictor muscles that surround a tubelike structure • - cardiac sphincter relaxes and lets bolus fall in • - stomach is J-shaped and contains gastric juice (secreted by stomach lining): a mixture of hydrochloric acid, enzymes that work well at low pH (ex. pepsin: begins to break down proteins into polypeptides), and mucus • - HCl breaks down fibres, including bacteria, mucus protects stomach lining from HCl (if mucus layer is destroyed, ex. Heliobacter pylori we get stomach ulcers as the lining is digested) • - some absorption starts here: water, alcohol, some medications (ex. aspirin) • - mixture of partially digested food, water, and gastric juice is called chyme • - chyme passes out of stomach through pyloric sphincter and into small intestine b) DIGESTION: the small intestine and pancreas • - measures up to 7 m in length, only 2.5 cm in diameter • - pyloric sphincter empties chyme into duodenum, the first 25 cm of the small intestine • - this is where the majority of digestion takes place • - secretions from the pancreas and liver enter the duodenum through a duct liver • four functions: synthesis and breakdown, detoxification, storage 1. synthesis: produces bile (made of bile salts), which breaks down fats, bile is stored and concentrated in gallbladder until needed - bile functions as an emulsifier, to allow the fats to travel through the digestive system and be further broken-down - if too much bile /too concentrated bile is produced the bile stored in the gallbladder can start to crystallize forming gallstones which then block the bile duct 2. breakdown: removes the highly toxic nitrogen group from amino acids to form urea (component of urine) • breaks down blood cells (stores products in gallbladder for removal – feces is brown), and any obstruction will turn skin yellow (jaundice) 3. detoxification: works to remove toxins from the body • - removes alcohol and other chemicals through detoxification 4. storage: stores carbohydrates and vitamins • pancreas • - produces sodium bicarbonate, which raises the pH of chyme from 2.5 to 9.0 • - secretes digestive enzymes, ex. lipase (breaks down fat) and trypsin (breaks down proteins) • small intestine • - secretes maltase (maltose to glucose) and peptidase (complete protein breakdown) • SO: • Carbohydrates glucose (amylase, maltase) • Proteins amino acids (pepsin, peptidase) • Fats (triglycerides) fatty acids and glycerol (bile, lipase) 3. ABSORPTION • a) ABSORPTION: small intestine • - after the duodenum, the following two sections of the small intestine are the jejunum and the ileum • - fingerlike villi increase the surface area of the small intestine, and absorb the glucose, amino acids, fatty acids and glycerol • - once absorbed the nutrients enter the bloodstream and are distributed throughout the body • b) ABSORPTION: large intestine (colon) • - water is absorbed • - harmless bacteria live in large intestine and produce vit K and B which are also absorbed 4. EGESTION: rectum and anus • - cellulose helps to void bowels, removing potentially toxic wastes • - ppl who don’t eat enough cellulose are at risk of colon cancer • entire process: 24 - 36 hours Interesting…. • - the nervous and hormonal systems act on digestion before we even eat: seeing, smelling and tasting food stimulates gastric secretions • - swallowing stimulates production of gastric juices, ex. the hormone gastrin stimulate gastric juice release before food gets to stomach • - speed of digestion also varies with food type - large meal activates stronger stomach muscle contractions and faster emptying - fatty meal: digestion slows down in response to a hormone from the small intestine (enterogasterone) to allow more time for fat digestion (we feel full longer after a fatty meal)