Digestive system – Alimentary canal Chapter 14

4 Functions Ingestion – taking in food Digestion – mechanical and chemical breakdown Absorption – movement of nutrients into blood Defecation - removal of indigestible residue

Alimentary canal Aka gastrointestinal (GI) tract Continuous tube open at both ends About 9m long in a cadaver; when living shorter due to muscle tone Food is technically outside the body

Mouth (oral cavity) Mastication (chewing) occurs as saliva mixes w/ food Deglutition – swallowing No absorption – exceptions: some heart medications Parts Lips Cheeks –walls Hard palate – anterior roof Soft palate – posterior roof Uvula – projection of soft palate Vestibule – btw lips/cheeks and teeth/gums Oral cavity proper – area w/in teeth Tongue Attaches to 2 bones – hyoid, styloid process Lingual frenulum – secures to floor Palatine tonsils – posterior – sides Lingual tonsils – on base of tongue

Pharynx Undergoes peristalsis Oropharynx – posterior to oral cavity Laryngopharynx – joins esophogus

Esophagus Joins pharynx and stomach About 25cm long 4 tissue layers (tunics) – esophagus to large intestine Mucosa – inner Lines lumen Submucosa – contains blood and lymphatic vessels, nerve endings Muscularis externa – inner circular layer & outer longitudinal layer of smooth muscle Serosa – outermost – visceral peritoneum Connected to parietal peritoneum by mesentery

Stomach Digestion No absorption – exception aspirin and alcohol 4-6 hours On left side of cavity Regions Cardiac region – around cardioesophogeal sphincter Where food enters Fundus – lateral to cardiac region Body Pyloric antrum – most activity here Pylorus – terminal – around pyloric sphincter valve

Stomach cont. Length about 25cm Diameter varies May hold about 4L of food Forms rugae (folds) when empty Greater curvature – convex lateral surface Lesser curvature – concave medial surface Joined to liver by lesser omentum (peritoneum) Greater omentum – hangs down to insulate, cushion, and protect organs Contains an extra muscle layer – an oblique layer Allows churning and mixing

Stomach cont. Lining has gastric pits w/ gastric glands that secrete gastric juices Chief cells – protein enzymes – pepsinogens Parietal cells – hydrochloric acid Mucous neck cells – produce protective mucous Enteroendocrine cells – gastrin Chyme – processed food that enters intestine Irritation in stomach may trigger emesis (vomiting)

Stomach cont. Heartburn – cardioesophogeal sphincter doesn’t close tightly and stomach acid enters esophagus Ulcers – insufficient mucus – hydrochloric acid and pepsin eat away mucosa

Small intestine Digestion and Absorption; 3-6 hrs Average length 2.5-7m Extends from the pyloric sphincter to the ileocecal valve 3 divisions Duodenum – “12 finger lengths long” Bile duct and pancreatic duct join to form the hepatopancreatic ampulla and enter duodenum via the duodenal papilla Allows pancreatic enzymes and bile to enter and complete digestion Jejunum – “empty” Ileum – “twisted intestine”

Small intestine cont. Pyloric sphincter – controls movement of food into intestine Wall modifications Microvilli – aka brush border Tiny projections to increase surface area Villi Fingerlike projections Contain capillaries and a lacteal (modified lymphatic capillary) – absorbed nutrients move into capillaries

Small intestine cont. Circular folds – aka plica circulares Deep folds in mucosa and submucosa layers – increase surface area Peyer’s patches – collections of lymphatic tissue – increase toward end of small intestine to keep bacteria out of blood

Large intestine 12-24 hours About 1.5m long Extends from ileocecal valve to the anus Bacteria breakdown few remaining nutrients Produce vitamin K and some vitamin B with a byproduct of methane and hydrogen sulfide Absorbs water from indigestible food Gets rid of wastes (feces) = defecation

Large intestine cont. Subdivisions Cecum – sac-like Appendix – hangs from cecum Appendicitis – build up of bacteria Colon Ascending colon – up right side of body – turns at right colic (hepatic) flexure Transverse colon – moves across body – turns at left colic (splenic) flexure Descending colon – moves down left side

Large intestine cont. Sigmoid colon – enters pelvis Rectum Anal canal – opens to exterior at anus Voluntary sphincter (external anal sphincter) – skeletal muscle Involuntary sphincter – smooth muscle Diarrhea – food is rushed through large intestine and there isn’t time for water to be absorbed – may result in dehydration Constipation – food moves too slowly and too much water is absorbed

Accessory organs – salivary glands Empty into mouth Parotid glands – anterior to ears – mumps Submandibular glands – floor of mouth Sublingual glands – floor of mouth All produce saliva – mixture Mucus – moistens and binds food into a bolus Serous fluid – contains salivary amylase – begins starch digestion

Accessory organs - teeth 1st set – deciduous teeth – baby teeth; milk teeth – 20 Should have full set by 2 years Begin to lose btw 6-12 years old 2nd set – permanent teeth – 32 Should have all but third molars (wisdom teeth) by end of adolescence Wisdom teeth emerge btw 17-25 Types – pairs listed for top ½ of mouth Incisors (2 pr) – cutting Canines (1pr)– tearing/piercing Premolars (2 pr) and molars (3 pr) – grinding

Teeth cont. Parts Crown – exposed portion Above gingiva (gum) Covered by enamel Dentin – bulk of tooth – under enamel Pulp cavity – contains blood vessels and nerves Neck – connects crown to root Root – embedded in jawbone Cementum – covers outer surface and attaches to periodontal membrane (holds tooth in place) Root canal – where pulp cavity enters root

Accessory organs Pancreas – produces digestion enzymes Also produces insulin and glucagen Liver – largest gland in body – 4 lobes Lies over stomach Hangs from diaphragm and abdominal wall by falciform ligament Produces bile – leaves via the common hepatic duct – breaks down fat Composed of bile salts and phospholipids which aid digestion Salts emulsify fats by breaking large globules into smaller ones

Accessory organ - gallbladder Small, green sac on the inferior surface of the liver Connected to liver via the cystic duct Stores bile until needed Gallstones – form when bile is stored too long or too much water is removed cholesterol crystallizes