Digestive System, Chapter 23 PDF

The Digestive System Chapter 23 23.1 – Overview of the Digestive System Break down food to release and absorb nutrients Alimentary canal or gastrointestinal (GI) tract is about 25 ft. long from mouth to anus, has 4 layers Pharynx, esophagus, stomach, small and large intestines Accessory digestive or...

The Digestive System Chapter 23 23.1 – Overview of the Digestive System Break down food to release and absorb nutrients Alimentary canal or gastrointestinal (GI) tract is about 25 ft. long from mouth to anus, has 4 layers Pharynx, esophagus, stomach, small and large intestines Accessory digestive organs – teeth, tongue, salivary glands, liver, gallbladder, pancreas 23.1 – GI Tract Layers Mucosa – mucous membrane epithelium in contact with food Epithelium – non-keratinized, stratified squamous in mouth, esophagus and anal canal. Columnar in stomach and intestines. Interspersed with goblet (secret mucous) and enteroendocrine cells. Lamina propria – loose connective tissue with blood and lymphatic vessels to transport absorbed nutrients, contain MALT Muscularis mucosa – smooth muscle always in a state of tension 23.1 - Layers Submucosa – dense connective tissue between mucosa and muscularis, has blood and lymphatic vessels, and has a collection of nerves called the submucosal plexus Muscularis – inner circular layer and outer longitudinal layer in the small intestine, mechanical digestion, exposes food to more chemicals, and peristalsis (moving food along the canal). Skeletal muscle at both ends (swallowing and defecation). Stomach adds a third layer of oblique muscle Serosa – most superficial, only in the region inside the abdominal cavity, loose connective tissue holding the canal in place 23.1 – Nerve Supply Gives food taste, lets you feel food, avoid biting yourself as you chew Enteric nervous system runs from esophagus to anus and is separated into 2 plexuses. Myenteric for motility and submucosal for regulating digestive secretions Autonomic nervous system extrinsically innervates the alimentary canal. Sympathetic nerves restrict enteric neurons. Parasympathetic nerves increase GI secretion and motility. 23.1 – Blood Supply Transport protein and carbohydrate to the liver, hepatic portal system Deliver nutrients and oxygen to organs of alimentary canal While resting and digesting 25% of blood pumped enters arteries serving the intestines 23.1 – The Peritoneum Holds digestive organs in place, 2 different regions Parietal peritoneum – lines the abdominal wall Visceral peritoneum – envelopes the abdominal organs Peritoneal cavity – space between parietal and visceral 23.2 – Digestive System Processes and Regulation Mouth – ingest, chew, and mix food. Begin chemical breakdown of carbs and lipids (activates in stomach). Move food to pharynx. Moistens and tastes food, cleans and lubricates teeth, and antimicrobial activity. Pharynx – Propel food to the esophagus. Lubricates food and passage Esophagus – Propel food to stomach. Lubricates food and passage Stomach – mix food with gastric juices to form chyme. Begin chemical breakdown of proteins. Release food to duodenum. Absorb some fat soluble substances (alcohol, aspirin). Antimicrobial functions. Stimulates protein digesting enzymes and secretes intrinsic factor for vitamin B12 absorption in small intestine. 23.2 Small Intestine – Mix chyme and digestive juices. Propels food slowly. Absorb macro and micro nutrients. Segmentation. Optimal medium for enzymatic activity. Large Intestines – further breakdown of food residue. Absorb residual water, electrolyte, and vitamins produced by enteric bacteria. Propel and eliminate feces. Food residue is concentrated and temporarily stored. Mucus eases passage of feces through colon. 23.2 – Accessory Organs Liver – Produce bile salts to emulsify lipids for digestion and absorption Gallbladder – stores, concentrates, and releases bile Pancreas – produce digestive enzymes and bicarbonate. Bicarbonate helps to neutralize acidic chyme and provide optimal environment for enzymatic activity 23.2 Ingestion – entry of food into alimentary canal through the mouth Swallowing – last voluntary act until defecation, propulsion Peristalsis – sequential smooth muscle contraction and relaxation to propel food 23.2 Mechanical digestion – physical process that does not change chemical nature of food (mastication, churning, segmentation). Exposes a larger surface area to digestive juices. Chemical digestion – breaking down complex food molecules into their building blocks (water, acid, enzymes, and salts) Absorption – primarily in the small intestine, taking nutrients into the bloodstream or lipids into lymphatic system Defecation – undigested materials are removed from the body 23.2 – Regulatory Mechanisms Neural control – sensors for expansion of stomach, if food particles are broken down enough, amount of liquid present, and type of nutrient. Can activate glands and muscle. 2 types of reflexes Short reflex – local stimulus and result. Expanding stomach causes increase in digestive juices. Long reflex – ANS and CNS reacting to stimuli outside the digestive tract. Smell, sight, and taste of food cause digestive juice secretion Hormonal control – gastrin to stimulate release of gastric acid. Secretin stimulates bicarbonate release from pancreas. CCK stimulates pancreatic secretion of enzymes and bile from liver and gallbladder. Gastric inhibitory peptide inhibits gastric secretion and slows gastric emptying and motility. 23.3 – Mouth, Pharynx, and Esophagus Mouth – oral cavity lined by cheeks, tongue, lips, and palate. Can handle digestion and respiration at the same time. Hard palate is anterior and soft palate is posterior on roof. Uvula drops down from the soft palate to stop food from entering the nasal cavity Tongue – ingestion, mechanical and chemical digestion (lingual lipase), swallowing, and vocalization. Attached to mandible, styloid processes, and hyoid bone. It has 2 symmetrical halves. Intrinsic and extrinsic muscles for dynamic movement. Taste buds. Lingual frenulum – connects tongue to floor of mouth. Too short is tongue tied. Severed is like Gene Simmons. 23.3 – Salivary Glands Secrete 1–1.5 L per day of saliva, even while we sleep, produce more while eating Saliva – 95.5% water, 4.5% ions, glycoproteins, enzymes, growth factors, and waste. pH of 6.35-6.85. Contains IgA. Regulated by ANS Salivary amylase – initiates the breakdown of carbohydrate, stops as it reaches the stomach Sympathetic stimulation and dehydration cause a decrease in saliva production Saliva is also produced after eating to cleanse the mouth 23.3 - Teeth 2 sets of teeth – 20 deciduous (baby) teeth and 32 permanent teeth 8 incisors – sharp front teeth 4 cuspids (canines) – pointy to tear up food, beware of vampires 8 premolars – flatter shape to mash food 12 molars – largest to crush food ready for swallowing, 3rd molars top and bottom are called wisdom teeth and they erupt toward early adulthood, they may need to be removed 23.3 – Tooth Anatomy Gingivae (gums) and the periodontal ligament hold teeth in socket Crown – part above gum line, covered by enamel over dentin Root – embedded in maxilla and mandible, covered by cementum over dentin Pulp cavity – nerves and blood vessels, surrounded by dentin Cavity – colony of bacteria feeding on sugar degrade the enamel 23.3 – The Pharynx (Throat) Handles food and air by involuntary muscle contraction Nasopharynx – most superior, only handles air Oropharynx – middle portion of pharynx Laryngopharynx – inferior, connects to esophagus and larynx Epiglottis is pulled over the larynx due to muscle contractions while swallowing Food into the trachea causes coughing to force food back up into the pharynx 23.3 – The Esophagus Muscular tube connecting the pharynx to the stomach, 10 in. long, posterior to trachea Upper esophageal sphincter – controls food into esophagus Lower esophageal sphincter – controls food entering stomach and acid leaving the stomach (GERD) Deglutition (Swallowing) – movement of food to stomach, 4-8 seconds for solid food and 1 second for very soft food and liquid, 3 phases Voluntary – skeletal muscle you control Pharyngeal phase – moves food into esophagus Esophageal phase – initiation of peristalsis to push food into stomach 23.4 – The Stomach Links the esophagus to the small intestine, lots of digestion, contractions and constantly changing position and size Empty it is about half the size of your fist, can stretch to hold 4 liters Body weight does not correlate to stomach size Holds and stores food, released as chyme into small intestine Has an additional oblique muscle to churn food 23.4 - Structure Cardia – where esophagus enters Fundus – domed area, stores undigested food and gases Body – main part of stomach Pylorus – connects to duodenum Pyloric sphincter – controls emptying Without food it deflates inward and the fold is called a ruga 23.4 Gastric glands are at the bottom of gastric pits, looks like pin cushion Parietal cells – produce hydrochloric acid (activates pepsin and kills bacteria) and intrinsic factor (needed for vitamin B12 absorption) Chief cells – secrete pepsinogen (inactive form of pepsin) Mucous neck cells – secrete thin acidic mucous, role is unknown Enteroendocrine cells – secrete various hormones like gastrin 23.4 – Gastric Secretion 3 phases Cephalic phase – brief, due to sight, smell, taste, or thought of food. Conditioned response to prepare for digestion Gastric phase – lasts 3-4 hours once food enters and stretches stomach. If pH drops too low then hydrochloric acid secretion stops Intestinal phase – regulation of stomach emptying into the duodenum 23.4 – Mucosal Barrier Gastric enzymes are strong enough to digest the stomach itself, mucosal barrier protects against acidic environment Wall is covered by a thick bicarbonate rich mucus Epithelial cells meet at tight junctions to keep acid out Stem cells replace damaged cells every 3-6 days Ulcer – large erosion due to acid digesting stomach wall, 2 main reasons NSAID’s and H. pylori bacteria infection 23.4 – Digestive Function Mechanical – mixing waves to create chyme, gentle and intense waves from body toward pylorus. Forces about 3mL of chyme per wave into the duodenum, rest of chyme mixes more Chemical – acid inactivates salivary amylase and activates lingual lipase (breakdown fat). Hydrochloric acid and pepsin breakdown protein. Intrinsic factor is the one function essential to life (produce red blood cells, neurological function) Contents are emptied into duodenum 2-4 hours after eating. Carbs empty fastest followed by high protein foods. Triglycerides take the longest and fatty meals may stay in stomach for 6 hours or more 23.5 – The Small and Large Intestine Small intestine – primary digestive organ and most absorption, longest part of canal, huge surface area, 3 regions Duodenum – shortest region, contains bile duct and main pancreatic duct, has duodenal glands to secrete bicarbonate rich alkaline mucous Jejunum – middle section Ileum – longest section, thicker, more vascular, more mucosal folds, joins the large intestine at the ileocecal sphincter Contains circular folds, villi, and microvilli to enlarge surface area and enhance absorption 23.5 – Mechanical Digestion Peristalsis – moving chyme through the intestine, started by hormone motilin, takes 90-120 minutes for chyme to reach end of ileum. All food leaves small intestine in 3-5 hours Segmentation – back and forth contractions to mix chyme with digestive juices and force particles toward the mucosa to be absorbed. Most rapid in duodenum at 12 times per minute while ileum is 8 times per minute 23.5 – Chemical Digestion Finishes carb and protein digestion Most lipids are undigested and need bile and pancreatic lipase Most water is absorbed through osmosis 23.5 – Large Intestine From the appendix to the anus, finishes absorption, forms feces and eliminates it, 4 regions Cecum – first part, connected to ileum, continues absorption of water and salts, holds the appendix (lymphoid tissue, may be a reservoir for enteric bacteria) Colon – ascending, transverse, descending, and sigmoid sections. Rectum – has 3 lateral bends (rectal valves) to prevent simultaneous passage of feces and gas Anal canal – open to exterior. Internal anal sphincter (smooth muscle) and external anal sphincter ( skeletal muscle), both are normally closed 23.5 – Bacterial flora Trillions of bacteria live in large intestine, nonpathogenic, they facilitate digestion, absorption, and synthesis certain vitamins. They may enhance immune response Bacteria is always being tested by dendrites to make sure it is not pathogenic 23.5 - Digestion Few nutrients left except for water which is reabsorbed, chyme stays for 12-24 hours Mechanical – haustral contractions (sluggish segmentation), 1 every 30 minutes and last for about 1 minute. Peristalsis is slower. Mass movement (start in transverse colon and force chyme toward the rectum), happen 3 or 4 times per day. Fiber softens stool and increases contraction power. Chemical – secrete mucous, do not secrete digestive enzymes, any digestion is due to bacteria, indigestible sugars broken down by bacteria cause methane gas and can be excessive with certain foods 23.5 Feces – undigested food residue, unabsorbed substances, bacteria, old cells from GI mucosa, inorganic salts, and some water Valsalva maneuver – voluntary procedure to remove feces, increases intra-abdominal pressure Feces in anal canal opens internal sphincter and gives you the choice to open the external sphincter, if you choose not to in a few seconds the reflex contractions will stop until the next mass movement Delayed defecation for too long causes more water to be absorbed making the feces firmer, can lead to constipation. If it moves to quickly it is diarrhea. Bowel movements range from 2-3 per day to 3-4 per week 23.6 – Accessory Organs 23.6 - Liver Hepatic portal vein delivers nutrients from small intestine. Nutrients, drugs, and toxins are all absorbed and processed. Hepatocytes – main cells of liver, produce bile which moves to common hepatic duct and joins with common bile duct into small intestine. Process nutrients. Bile – for emulsification (breakdown) of lipids in small intestine, just like dish soap works on fats in water. Bile salts are reabsorbed in the ileum. Extra bile is stored in gallbladder. Bilirubin – main bile pigment, gives stool its color after encountering intestinal bacteria 23.6 – The Pancreas Exocrine and endocrine functions. Pancreatic juice digests sugar, protein, and fat. It buffers acidic juice and inactivates pepsin to create and optimal environment 23.6 – The Gallbladder Stores, concentrates, and propels bile into duodenum through common bile duct 23.7 – Digestion and Absorption Digestion is accomplished by enzymes through hydrolysis ATP required for all absorption Body is able to absorb monosaccharides (glucose, galactose, and fructose), it can break down disaccharides (sucrose and maltose) and it can break down polysaccharides (glycogen and starch). Can not break down fiber. Carb digestion begins in the mouth and in the small intestine pancreatic amylase does most of the work Most carbs are absorbed in the jejunum, we absorb about 120 grams per hour (60 while exercising). 23.7 Proteins are broken down into amino acids, starts in the stomach and continued by pancreatic juice in the small intestine Most proteins are absorbed in the jejunum (95-98%) Lipids mainly breakdown in the small intestine due to pancreatic lipase. Each triglyceride is broken down into 2 free fatty acids and a monoglyceride. A micelle encloses fats for absorption. Nucleic acid is broken down by pancreatic nuclease into pentoses, phosphates, and nitrogenous bases. 23.7 Minerals – electrolytes are absorbed and balanced by the sodiumpotassium pump requiring ATP. Iron gets stored bound to ferritin in intestinal epithelial cells if not needed. Most iron is lost as cells die and slough off. Women have more iron transport proteins due to menstruation. Ca++ absorption is regulated by hormones. Vitamins – fat soluble vitamins are absorbed along with lipids in micelles. Water soluble absorb by diffusion except for B12 which is taken up by endocytosis in the terminal ileum. Water – about 9 liters each day enters the small intestines and 90% is absorbed there.