Chapter 16 The Digestive System

Functions of the Digestive System take in food break down food absorb digested molecules provide nutrients eliminate wastes

Two Divisions of the Digestive System alimentary canal (digestive tract) digests and absorbs oral cavity to anus accessory digestive organs

Organs of the Digestive System Figure 14.1

Digestive Tract muscular tube approximately 9 M long open at both ends Wall has 4 layers with some variation due to specific function.

Layers of Digestive Tract innermost: mucosa epithelial cells which line cavity attached to connective tissue surrounded by thin muscular layer protects tissues of canal carries out secretion and absorption

Layers of Digestive Tract next: submucosa thick layer of loose connective tissue housing blood vessels, lymph vessels, and nerves nourishes surrounding layers of tract

Layers of Digestive Tract next: muscular layer inner: circular smooth muscle outer: longitudinal smooth muscle propels food through digestive tract contains lots of nerves to control movement and secretion

Layers of Digestive Tract outermost: serosa visceral peritoneum protects underlying tissue secretes serous fluid to keep tract from sticking to other tissues of abdominal cavity

Layers of Alimentary Canal Organs Figure 14.3

Mouth (Oral Cavity) Anatomy teeth – 32 total in adults incisors, premolars, molars, wisdom teeth tongue – thick, muscular organ functions in digestion, taste and speech attaches to posterior part of mouth and by frenulum covered by mucous membrane houses taste buds within papillae salty, sweet, sour, bitter

Mouth (Oral Cavity) Anatomy tonsils lingual and palatine palate – roof of oral cavity hard = bone soft = muscle and connective tissue separates oral and nasal cavities uvula – fleshy projection of the soft palate

Salivary Glands produce saliva – mixture of mucous and serous fluids keeps mouth moist contains enzymes that aid in chemical digestion 3 pairs in oral cavity parotid – located anterior to ears secrete serous and mucous fluids submandibular – base of jaw ducts open below tongue secrete more serous than mucous fluids sublingual – smallest, below tongue duct opens into floor of oral cavity secretes thick, stringy mucous fluid

Saliva produce 1 liter a day serous fluid: parotid and submandibular glands contains amylase, enzyme to break down starches lysozyme: reduces bacterial growth in mouth mucous: contains mucin for lubrication regulated by parasympathetic stimulation (involuntary nervous system) increase in secretions with sight, smell, thought of food

Processes of the Mouth mechanical digestion mastication (chewing) of food mixing of food with saliva by tongue chemical digestion salivary amylase breaks down starches allow for the sense of taste NO food absorption occurs in mouth.

Swallowing tongue, soft palate, pharynx and esophagus phase 1: voluntary (mouth) Bolus (food mass) is forced into pharynx by tongue. phase 2: involuntary (pharynx) Pharynx connects mouth and esophagus. Receptors in pharynx sense food which triggers swallowing reflexes. Epiglottis covers larynx. phase 3: involuntary (esophagus) Esophagus connects pharynx to stomach. Peristalsis moves food from esophagus to stomach. Lower esophageal sphincter helps prevent regurgitation of food.

Stomach Anatomy located on the left side of the abdominal cavity Food enters at the cardiac opening. Slide 14.15a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Stomach Functions acts as a storage tank for food site of food breakdown start of protein digestion very little absorption delivers chyme (processed food) to the small intestine Slide 14.18 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Regions of the stomach Cardiac region – near the heart Fundus – most superior part Body Pylorus – funnel-shaped terminal end Food empties into the small intestine at the pyloric sphincter. Slide 14.15b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Stomach Anatomy Slide 14.17 Figure 14.4a Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Muscles of the stomach different than rest of digestive tract 3 layers outer longitudinal layer middle circular layer inner oblique layer produce churning action – mechanical digestion

Specialized Mucosa of the Stomach gastric glands – secrete gastric juice 3 types of cells mucus cells – produce a sticky alkaline mucus which protects stomach lining chief cells – secrete protein-digesting enzymes (pepsinogen which becomes pepsin) parietal cells – secrete hydrochloric acid rugae – internal folds of the mucosa allows stomach to stretch when full

Other Stomach Secretions gastrin – hormone that increases stomach secretions intrinsic factor – aids in absorption of vitamin B12 cholecystokinin – hormone that stimulates gastric secretions Production of gastric juices is regulated by hormones and the nervous system. Humans produce about 2 L per day.

How Stomach Digests Food As food enters digestive tract, gastrin is produced which causes production of gastric juices. Food mixes with gastric juices and forms chyme which passes to pyloric region. As food empties, secretions from stomach wall are inhibited by nervous system.

Stomach Movement 2 types resulting from stomach wall muscle contractions mixing – from weak contractions mix ingested food with stomach secretions to form chyme peristalsis – from stronger contractions force chyme toward and through pyloric sphincter

Structure of the Stomach Mucosa Figure 14.4b, c Slide 14.20b Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Small Intestine absorbs products of digestion receives secretions from pancreas and liver transports remaining residue to large intestine 6 meters long ileocecal junction – joins ileum and large intestine Ileocecal sphincter and ileocecal valve keep materials moving in right direction.

Subdivisions of the Small Intestine Duodenum attached to the stomach curves around the head of the pancreas 25 cm long Common bile duct from liver and pancreatic duct join each other and empty into duodenum. Jejunum 2.5 meters; attaches anteriorly to the duodenum Ileum 3.5 meters, extends from jejunum to large intestine Slide 14.22 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Subdivisions of the Small Intestine Duodenum, jejunum and ileum have same structure except: there is a gradual decrease in diameter decrease in thickness of the walls decrease in # of circular folds decrease in # of villi Slide 14.22 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Chemical Digestion in the Small Intestine 3 modifications to increase surface area up to 600 times circular folds – run perpendicular to long axis of tract villi – tiny finger-like projections formed by mucosa microvilli – extensions of villi found in absorptive cells

Mucous Lining of Small Intestine composed of simple columnar epithelium absorptive cells have microvilli produce digestive enzymes absorb digested food goblet cells produce protective mucus granular cells protect intestinal epithelium from bacteria endocrine cells produce regulatory hormones

Secretions of the Small Intestine lubricate and protect intestinal wall from acidic chyme and digestive enzymes keep chyme in liquid form produced by mucosa, pancreas, liver peptidases – break proteins into amino acids disaccharidases – break down complex sugars into monosaccharides Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Absorption in the Small Intestine major function of small intestine most in duodenum and jejunum monosaccharides – absorbed through active transport or facilitated diffusion by villi amino acids – absorbed through active transport by villi and carried away in blood fatty acids – dissolve into cell membranes of villi and carried away by lymphatic vessels water – absorbed through osmosis into villi electrolytes – absorbed by active transport by villi

Movement in the Small Intestine mixing and propulsion – mechanical breakdown peristaltic contraction – causes chyme to move along tract; occur along entire digestive tract segmental contraction – short movements to mix contents

Large Intestine larger in diameter, but shorter than the small intestine 1.5 meters extends from ileocecal valve to anus major functions: absorption of water, electrolytes, vitamins K and B elimination of indigestible food (feces) production of mucus for lubrication No digestion of food occurs here. Bacteria produce vitamin K and B.

Divisions of the Large Intestine cecum – saclike first part of the large intestine appendix accumulation of lymphatic tissue hangs from the cecum colon ascending transverse descending S-shaped sigmoid (in pelvis) rectum anus external body opening contains 2 sphincters

Large Intestine Slide 14.28 Figure 14.8 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Structures of the Large Intestine 3 bands of longitudinal muscle arranged in teniae coli create haustra (pocket like sacs in wall) large numbers of goblet cells

Absorption in the Large Intestine no digestive enzymes absorption of water and electrolytes and vitamins K and B

Propulsion in the Large Intestine mass movements slow, powerful contractions move contents toward anus occur two to three times per day cover more distance than peristaltic contractions Presence of feces in the rectum causes a defecation reflex. Internal anal sphincter is relaxed. Defecation occurs with relaxation of the voluntary (external) anal sphincter.

Pancreas located behind the stomach produces digestive enzymes that break down all categories of food trypsin – protein digestion pancreatic amylase – starch digestion lipidase – fat digestion secreted into the duodenum Alkaline fluid introduced with enzymes neutralizes acidic chyme.

Liver located on the right side of the body under the diaphragm consists of four lobes capable of regeneration connected to the gall bladder via the common hepatic duct metabolizes carbohydrates, lipids, proteins stores glycogen, vitamins A,D and B12, iron and blood filters blood, removes toxins and damaged blood cells secretes bile

Bile yellowish, green liquid includes water, bile salts, bile pigments, cholesterol and electrolytes Bile salts emulsify fats. aid in absorption of fatty acids, cholesterol and some vitamins

Gall Bladder sac found beneath liver stores bile from the liver releases bile into the duodenum in the presence of fatty food release controlled by sphincter muscles Crystallized bile causes gallstones.

Processes of the Digestive System Figure 14.11