HAP Chapter 23(2) copy1 PDF

Marieb Human Anatomy & Physiology Twelfth Edition Chapter 23 The Digestive System PowerPoint® Lecture Slides prepared by Justin A. Moore,...

Marieb Human Anatomy & Physiology Twelfth Edition Chapter 23 The Digestive System PowerPoint® Lecture Slides prepared by Justin A. Moore, American River College Copyright © 2025 Pearson Education, Inc. All Rights Reserved Function of Digestive System A healthy digestive system is essential to life – Converts foods into raw materials that build and fuel our body’s cells – Ingests (takes in) food and digests it (breaks it down) into nutrient molecules that can be absorbed into bloodstream ▪ Indigestible residues excreted Copyright © 2025 Pearson Education, Inc. All Rights Reserved Part 1: Overview of the Digestive System Two main groups of organs: – Organs forming alimentary canal, also called gastrointestinal (GI) tract or gut ▪ Continuous muscular tube running from mouth to anus ( 30 ft in cadaver, shorter in living body) ▪ Digests food into small fragments and absorbs them through mucosal lining into blood ▪ Organs: mouth, pharynx, esophagus, stomach, small and large intestine, anus – Accessory digestive organs ▪ Teeth, tongue, gallbladder ▪ Salivary glands, liver, pancreas – Use ducts to secrete digestive fluids into G I tract Copyright © 2025 Pearson Education, Inc. All Rights Reserved 23.1 What Major Processes Occur During Digestive System Activity? Processing of food involves six essential activities: – Ingestion: eating – Propulsion: movement of food through the alimentary canal, includes: ▪ Swallowing: initiated voluntarily ▪ Peristalsis: primary means of propulsion, involves alternating (involuntary) waves of contraction and relaxation – Mechanical breakdown (formerly mechanical digestion): increases surface area of nutrients, prepares food for chemical digestion by enzymes; includes: ▪ Chewing, mixing food with saliva (via tongue), churning food in stomach, and segmentation (local contractions of S I that mix food with digestive fluids) – Digestion (formerly chemical digestion): catabolism via enzymes, breaking down large food molecules into absorbable chemical building blocks – Absorption: passage of nutrients from lumen of GI tract through mucosal epithelium into blood or lymph – Defecation: elimination of indigestible substances in the form of feces Copyright © 2025 Pearson Education, Inc. All Rights Reserved 23.2 The GI Tract Has Four Layers and Is Usually Surrounded by Peritoneum Copyright © 2025 Pearson Education, Inc. All Rights Reserved Relationship of the Digestive Organs to the Peritoneum Peritoneum: serous membranes of abdominopelvic cavity that consist of: – Visceral peritoneum: covers external surface of most digestive organs – Parietal peritoneum: lines body wall; continuous with visceral layer Peritoneal cavity: (slit-like) space between two membranes containing serous fluid, which lubricates mobile organs as they change shape and move against each other Mesentery: double layer of peritoneum (layers fused back-to-back), extends from body wall to digestive organ (holding it in place) – Stores fat; provides route for blood vessels, lymphatics, nerves to reach organ – Most are dorsal (attach to posterior body wall), some have names (e.g., omenta) Intraperitoneal (peritoneal) organs: surrounded by peritoneum; anchored to body wall by mesentery Retroperitoneal organs: lie posterior to peritoneum; include: – Most of pancreas and duodenum (of small intestine), and parts of large intestine Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Peritoneum and the Peritoneal Cavity Figure 23.4 The peritoneum and mesenteries. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 23.1 Peritonitis (inflammation of peritoneum) usually caused by ruptured appendix – Also be caused by: ▪ Piercing abdominal wound ▪ Perforating ulcer ▪ Poor sterile technique during surgery – Peritoneal coverings tend to stick together, helps localize infection – Dangerous and lethal if it becomes widespread – Treatment: infectious debris removal and megadoses of antibiotics Copyright © 2025 Pearson Education, Inc. All Rights Reserved Histology of the Alimentary Canal Walls of alimentary canal have the same four basic layers, or tunics: – Mucosa, submucosa, muscularis externa, and serosa Mucosa (mucous membrane) – Innermost layer, in contact with luminal contents – Depending on organ, major functions include: ▪ Secrete mucus, digestive enzymes, and hormones ▪ Absorb end products of digestion ▪ Protect against infectious disease – Consists of three sublayers: ▪ Epithelium ▪ Lamina propria ▪ Muscularis mucosae Copyright © 2025 Pearson Education, Inc. All Rights Reserved Histology of the Alimentary Canal – Epithelium ▪ Simple columnar epithelium with mucus-secreting cells in most of tract – Stratified squamous in mouth, part of pharynx, esophagus, and anus ▪ Secretes mucus that protects digestive organs from enzymes and lubricates canal to ease passage of food ▪ May secrete enzymes and hormones (e.g., in stomach and small intestine) – Lamina propria consists of areolar connective tissue ▪ Capillaries nourish epithelium and absorb digested nutrients ▪ Scattered lymphoid follicles, part of MALT (mucosa-associated lymphoid tissue), defend against pathogens – Muscularis mucosae ▪ Thin layer of smooth muscle that produces local movements of mucosa Copyright © 2025 Pearson Education, Inc. All Rights Reserved Histology of the Alimentary Canal Submucosa also consists of areolar connective tissue – Rich supply of blood and lymphatic vessels, lymphoid follicles, and nerve fibers (submucosal nerve plexus) serve surrounding tissues – Abundant elastic fibers allow organs to stretch as a large volume of foodstuff enters them, and then regain their normal shape after it passes through Muscularis externa (muscularis) – Responsible for segmentation and peristalsis – Contains an inner circular layer and an outer longitudinal layer of smooth muscle ▪ Circular layer thickens to form sphincters that act as valves to regulate movement of foodstuff from organ to organ Copyright © 2025 Pearson Education, Inc. All Rights Reserved Histology of the Alimentary Canal Serosa is the visceral peritoneum – Outermost layer of intraperitoneal organs – Areolar connective tissue covered with mesothelium (simple squamous epithelium) Serosa replaced by adventitia (dense connective tissue) in organs outside of the abdominopelvic cavity (e.g., esophagus) – Binds organ wall to surrounding structures Retroperitoneal organs have both an adventitia (side facing body wall) and a serosa (side facing peritoneal cavity) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Basic Structure of the Alimentary Canal Figure 23.5 Basic structure of the alimentary canal. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Blood Supply: The Splanchnic Circulation Splanchnic circulation includes: – Arteries that branch off aorta to serve digestive organs ▪ Normally receive 25% of cardiac output, more after a meal ▪ Include: – Celiac trunk (branches), which serves spleen, liver, and stomach – Superior and inferior mesenteric arteries, which serve intestines – Hepatic portal circulation ▪ Delivers nutrient-rich venous blood from digestive organs to liver Copyright © 2025 Pearson Education, Inc. All Rights Reserved Enteric Nervous System Consists of more neurons than the spinal cord Consists of enteric neurons – Communicate with each other to regulate digestive activities – Form two major intrinsic nerve plexuses that control GI tract motility ▪ Submucosal nerve plexus ▪ Myenteric nerve plexus (lies between layers of muscularis) Enteric nervous system participates in both short and long reflex arcs – Short reflexes: mediated by intrinsic nerve plexuses in response to stimuli within GI tract – Long reflexes: mediated by CNS and extrinsic autonomic nerves in response to stimuli arising within or outside of tract ▪ Parasympathetic inputs stimulate digestive processes ▪ Sympathetic inhibits them Copyright © 2025 Pearson Education, Inc. All Rights Reserved Neural Reflex Pathways Initiated by Stimuli Inside or Outside the Gastrointestinal Tract Figure 23.7 Neural reflex pathways initiated by stimuli inside or outside the gastrointestinal tract. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Basic Concepts of Regulating Digestive Activity Three key concepts regulate digestive activity – Neurons (intrinsic and extrinsic) and hormones control digestive activity ▪ Nervous system controls – Intrinsic controls: involve short reflexes (enteric nervous system only) – Extrinsic controls: involve long reflexes ▪ Hormonal controls – Hormones from stomach and small intestine stimulate targets (muscles, glands) in same or different organs to affect secretion or contraction Copyright © 2025 Pearson Education, Inc. All Rights Reserved Part 2: Functional Anatomy of the Digestive System Copyright © 2025 Pearson Education, Inc. All Rights Reserved 23.4 Ingestion Occurs Only at the Mouth Mouth (oral cavity or buccal cavity) is where food is chewed and mixed with enzyme-containing saliva that begins process of digestion; also initiates propulsive process of swallowing Associated organs include: – Tongue – Salivary glands – Teeth Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Tongue The tongue occupies the mouth floor; composed of interlacing bundles of skeletal muscle Functions: – Grip, reposition, mix food (with saliva) during chewing to form bolus (compact mass) – Initiate swallowing (push bolus posteriorly into pharynx) – Speech and taste Copyright © 2025 Pearson Education, Inc. All Rights Reserved Dorsal Surface of the Tongue, and the Tonsils Figure 23.9 Dorsal surface of the tongue, and the tonsils. Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Salivary Glands Salivary glands secrete saliva, a fluid that: – Cleanses mouth; dissolves food chemicals for taste and moistens food to compact it into a bolus; begins breakdown of starch via amylase Most saliva secreted (when we eat or anticipate it) by major or extrinsic salivary glands – Located outside of oral cavity (develop from oral mucosa, remain connected by ducts); these paired glands include: ▪ Parotid gland: anterior to ear. Duct opens into oral vestibule next to second upper molar ▪ Submandibular gland: medial to body of mandible – Duct opens at base of lingual frenulum ▪ Sublingual gland: anterior to submandibular gland under tongue – 10–20 ducts open into floor of mouth Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Salivary Glands Figure 23.10a The salivary glands. Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Salivary Glands Composed of two types of secretory cells – Serous cells produce watery secretion with enzymes, ions, tiny bit of mucin – Mucous cells produce mucus Parotid and submandibular glands contain mostly serous cells, sublingual gland contains mostly mucous cells Composition of saliva – Mostly water (97–99.5%), therefore hypo-osmotic; slightly acidic (pH 6.75 to 7) – Electrolytes: Na , K  , Cl , PO 43 , HCO3  – Salivary amylase and lingual lipase – Proteins: mucin, lysozyme, and IgA – Metabolic wastes: urea and uric acid Saliva protects against microorganisms because it contains: IgA, lysozyme, defensins Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Salivary Glands Control of salivation – Average output 1500 ml/day (much higher when major glands stimulated) – Salivation controlled primarily by parasympathetic division of A NS ▪ Ingested food stimulates chemoreceptors and mechanoreceptors in mouth, which signal the salivatory nuclei in brain stem ▪ Causes increased parasympathetic impulses via facial (VII) and glossopharyngeal (IX) nerves to glands ▪ Results in secretion of watery (serous), enzyme-rich saliva – Stimuli: smell, sight, thought of food ▪ Also irritation of lower GI tract by bacterial toxins, spicy foods, or acidity – Sympathetic division causes release of thick, mucin-rich saliva ▪ Strong sympathetic stimulation can inhibit salivation (as can dehydration via low blood volume/pressure) Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Teeth Teeth Masticate (chew): process that tears and grinds food into smaller fragments Dentition and the dental formula – Deciduous dentition: 20 deciduous teeth erupt over months 6–24; also called primary, milk or baby teeth – Permanent dentition: 32 deep-lying permanent teeth enlarge and develop while milk teeth roots resorbed, causing them to fall out (at 6–12 yr) ▪ Usually, all but third molars (wisdom teeth) erupt by end of adolescence; third molars may or may not emerge (at 17  25 yr) Teeth classified according to shape and function: – Incisors: chisel shaped for cutting or nipping off pieces of food – Canines (cuspids or eyeteeth): fanglike teeth that tear or pierce – Premolars (bicuspids) and molars: broad crowns with rounded cusps to grind or crush; molars best grinders (upper/lower lock together, creating huge forces) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Human Dentition Figure 23.11 Human dentition. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 23.4 Decaying deciduous teeth can be painful and may lead to serious infection Deciduous teeth serve as important “place holders” for developing permanent teeth – Can be kept healthy by brushing and limiting exposure to sugary liquids, especially from prolonged bottle feeding Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Teeth Tooth structure – Two major regions: ▪ Crown: exposed part above gingiva (gum) – Covered by enamel, the hardest substance in body Heavily mineralized with calcium salts and hydroxyapatite crystals – Enamel-producing cells degenerate when tooth erupts, so no healing if areas decay or crack; needs artificial filling ▪ Root: portion embedded in jaw-bone Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Teeth Root connected to crown at the neck (constricted region) Cementum: calcified connective tissue; covers and attaches root to periodontal ligament – Periodontal ligament anchors tooth in bony socket (alveolus). – Gingival sulcus: groove where gingiva borders tooth Dentin: bonelike material under enamel; maintained by odontoblasts of pulp cavity – Bulk of tooth, acts as shock-absorber – Surrounds pulp cavity ▪ Contains pulp (connective tissue with blood vessels and nerve fibers) – Nourishes tooth, provides sensation Root canal: narrow extension of pulp cavity through root Copyright © 2025 Pearson Education, Inc. All Rights Reserved Longitudinal Section of a Canine Tooth Within Its Bony Socket (Alveolus) Figure 23.12 Longitudinal section of a canine tooth within its bony socket (alveolus). Copyright © 2025 Pearson Education, Inc. All Rights Reserved Tooth and Gum Disease Dental caries (cavities): demineralization of enamel and dentin from bacterial action – Dental plaque (film of sugar, bacteria, and debris) adheres to teeth – Bacteria break down sugars, producing acids that dissolve calcium salts, exposing organic tooth matrix ▪ Tooth proteins digested by proteolytic enzymes released from bacteria – Prevention: daily brushing and flossing Gingivitis: red, sore, swollen gums (may bleed) resulting from infection – Plaque calcifies to form calculus (tartar) – Calculus disrupts seal between gingivae and teeth, so anaerobic bacteria infect gums – Infection is reversible if calculus removed Copyright © 2025 Pearson Education, Inc. All Rights Reserved Digestive Processes of the Mouth Mouth and its accessory organs are involved in (four of six) digestive processes 1. Ingests 2. Begins mechanical breakdown (via mastication) 3. Initiates propulsion (swallowing) 4. Starts digestion of polysaccharides (via salivary amylase) Copyright © 2025 Pearson Education, Inc. All Rights Reserved 23.5 The Pharynx and Esophagus Move Food from the Mouth to the Stomach Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Pharynx Food passes from mouth into oropharynx and then into laryngopharynx – Allows passage of food, fluids, and air – Lined with stratified squamous epithelium and mucus-producing glands – External muscle layers consists of two skeletal muscle layers ▪ Inner layer cells run longitudinally ▪ Outer layer cells encircle the wall, forming the pharyngeal constrictor muscles Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Esophagus Muscular tube ( 10 in) that runs from laryngopharynx to stomach – Collapsed when not propelling food – Pierces diaphragm at esophageal hiatus to join stomach at cardial orifice, which is surrounded by lower esophageal sphincter (also called gastroesophageal, or cardiac, sphincter) ▪ Keep orifice closed (assisted by diaphragm) when not swallowing ▪ Mucus cells on both sides of sphincter protect esophagus from acid reflux Esophagus has all four basic GI tract tunics; features of interest include: – Mucosa: nonkeratinized stratified squamous epithelium; changes to simple columnar at stomach – Submucosa: esophageal glands secrete mucus (lubricant) to aid passage of bolus – Muscularis externa: skeletal muscle superiorly, smooth muscle inferiorly, mixed in the middle – Adventitia (instead of serosa) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Structure of the Esophagus Figure 23.13 Microscopic structure of the esophagus. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Deglutition (Swallowing) Figure 23.14 Deglutition (swallowing). Copyright © 2025 Pearson Education, Inc. All Rights Reserved Deglutition (Swallowing) Figure 23.14 Deglutition (swallowing). Copyright © 2025 Pearson Education, Inc. All Rights Reserved Deglutition (Swallowing) Figure 23.14 Deglutition (swallowing). Copyright © 2025 Pearson Education, Inc. All Rights Reserved Deglutition (Swallowing) Figure 23.14 Deglutition (swallowing). Copyright © 2025 Pearson Education, Inc. All Rights Reserved Deglutition (Swallowing) Figure 23.14 Deglutition (swallowing). Copyright © 2025 Pearson Education, Inc. All Rights Reserved Deglutition (Swallowing) Figure 23.14 Deglutition (swallowing). Copyright © 2025 Pearson Education, Inc. All Rights Reserved 23.6 The Stomach Temporarily Stores Food and Begins Protein Digestion Stomach is temporary “storage tank” for ingested food – Continues breaking down food both physically and chemically ▪ Producing paste-like chyme, and delivering it to small intestine Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy of the Stomach Empty stomach has 50 ml volume but can expand to 4 L when really distended Major regions of stomach – Cardia (cardial part): surrounds cardial orifice – Fundus: dome-shaped region beneath diaphragm, superolateral to cardia – Body: midportion – Pyloric part: funnel-shaped region, continuation of body ▪ Pyloric antrum (wider and more superior) narrows into pyloric canal that terminates at the pylorus – Pyloric sphincter (or valve) which controls stomach emptying into duodenum Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy of the Stomach Two curvatures: – Greater curvature: convex lateral surface of stomach – Lesser curvature: concave medial surface of stomach – Lesser omentum: runs from lesser curvature to liver – Greater omentum: drapes inferiorly from greater curvature over intestine, spleen, and transverse colon ▪ Contains fat deposits and lymph nodes ANS innervation: sympathetic via celiac plexus and parasympathetic via vagus nerve Blood supply: celiac trunk (gastric and splenic branches), veins of hepatic portal system Copyright © 2025 Pearson Education, Inc. All Rights Reserved Anatomy of the Stomach (1 of 2) Figure 23.15a Anatomy of the stomach. Copyright © 2025 Pearson Education, Inc. All Rights Reserved  Microscopic Anatomy of the Stomach Stomach wall contains basic four tunics; however, muscularis and mucosa are modified Muscularis externa Allow stomach not only to churn, mix, and move chyme, but also pummel it, increasing physical breakdown and movement into small intestine Mucosa also modified – Consists of simple columnar epithelium entirely composed of mucous cells – Dotted with gastric pits, which lead into tubular gastric glands that produce gastric juice ▪ Pits formed by mucous cells (like surface); gland cells vary based on region: – E.g., mostly mucous cells in cardia and pylorus, but many hormone (e.g., gastrin) secreting cells in pyloric antrum Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of the Stomach Figure 23.16a Microscopic anatomy of the stomach. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of the Stomach Types of gland cells – Glands in fundus and body are larger and produce most gastric juice – Secretory cells include mucous neck, parietal, chief, and enteroendocrine Mucous neck cells – Secrete thin, soluble, acidic mucus (function undetermined) Parietal cells – Secrete hydrochloric acid (HCl) and the glycoprotein intrinsic factor ▪ Acidity (p H 1.5–3.5) denatures proteins, activates pepsinogen, breaks down plant cell walls, and kills many bacteria ▪ Intrinsic factor required for absorption of vitamin B12 in small intestine Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of the Stomach Chief cells – Secrete pepsinogen and gastric lipases ▪ Pepsinogen first activated by HCl, then newly formed pepsin activates other molecules of pepsinogen (positive feedback mechanism) ▪ Gastric lipases account for 15% of overall GI lipolysis Enteroendocrine cells – Secrete variety of chemical messengers into IF of lamina propria, including: ▪ Locally acting paracrines (e.g., histamine and serotonin) ▪ Messengers acting both locally and as hormones (e.g., somatostatin) ▪ The hormone gastrin Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of the Stomach Figure 23.16c Microscopic anatomy of the stomach. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of the Stomach The mucosal barrier – Mucosal barrier protects stomach from own acid and proteolytic enzymes – Three factors create barrier: ▪ A thick coating of bicarbonate-rich mucus ▪ The epithelial cells of the mucosa are joined together by tight junctions ▪ Damaged epithelial mucosal cells are shed and quickly replaced – Surface epithelium replaced every 3–6 days Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 23.7 Gastritis: inflammation caused by anything that breaches stomach’s mucosal barrier Can lead to erosion of GI tract wall (peptic ulcers), called gastric ulcers in stomach – Perforations in stomach wall can lead to peritonitis and hemorrhage Most ulcers caused by acid-resistant bacteria Helicobacter pylori (H. pylori) – Can destroy the mucosal barrier – Treated with antibiotics and acid-reducing drugs Ulcers can also be caused by long-term use of NSAIDs Copyright © 2025 Pearson Education, Inc. All Rights Reserved Photomicrograph of H. Pylori, the Bacteria That Most Commonly Cause Gastric Ulcers Figure 23.18 Photomicrograph of H. pylori, the bacteria that most commonly cause gastric ulcers. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Digestive Processes in the Stomach Propulsion via peristalsis Mechanical breakdown via churning action during peristalsis Digestion of proteins – HCl denatures proteins, enhancing enzymatic digestion by pepsin – In infants, rennin breaks down milk protein (casein) Absorption of lipid-soluble alcohol and aspirin (but not nutrients) Yet, only function essential to life is secretion of intrinsic factor for vitamin B12 absorption – B12 needed for red blood cells to mature – Lack of intrinsic factor causes pernicious anemia – Treated with B12 injections Copyright © 2025 Pearson Education, Inc. All Rights Reserved  Regulation of Gastric Secretion Gastric mucosa secretes up to 3 L of gastric juice/day; regulated by: – Neural mechanisms: long reflexes (via parasympathetic fibers of vagus nerve) and short (intrinsic) reflexes both stimulate gastric secretion (via release of ACh) ▪ Sympathetic inputs inhibit secretion – Hormonal mechanisms: gastrin stimulates gastric secretion of HCl and secretion of small intestine hormones (mostly gastrin antagonists called enterogastrones) ▪ Drugs that block H2 (histamine) receptors effectively reduce HCl secretion Control of gastric secretion divided into three phases (based on location of stimuli): cephalic, gastric, and intestinal Cephalic (reflex) phase occurs before food reaches stomach (minutes long) – Stimuli associated with the head (smell, taste, sight, thought of food) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Regulation of Gastric Secretion Gastric phase begins once food enters stomach (lasts 3–4 hours), accounts for about two-thirds of gastric juice released – Stimulation ▪ Distension activates stretch receptors, initiating both long and short reflexes ▪ Chemical stimuli (partially digested proteins, amino acids) directly stimulate enteroendocrine G cells to secrete gastrin – Gastrin directly (and indirectly via triggering histamine release) stimulates parietal cells to secrete HCl – Inhibition ▪ Low pH (< 2) inhibits gastrin secretion; common between meals ▪ Inhibitory action of sympathetic division overrides vagal (parasympathetic) stimulation during times of fight-or-flight (stress, fear, anxiety) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Regulation of Gastric Secretion Intestinal phase begins with a brief stimulatory component followed by inhibition – Stimulation ▪ Partially digested food enters small intestine, causing brief release of intestinal (enteric) gastrin, encouraging continued gastric secretion – Brief, overridden by inhibitory stimuli as intestine fills – Inhibition via four main factors in duodenum ▪ Distension of duodenum from physical presence of chyme ▪ Chemical presence of acidic, fatty, or hypertonic chyme – Inhibitory effects protect intestine from excessive acidity or too much chyme – Inhibition achieved in two ways: enterogastric reflex and enterogastrones ▪ Enterogastric reflex: duodenum inhibits gastric secretion via short and long reflexes (involving sympathetic and vagus nerves) ▪ Enterogastrones: group of hormones secreted by duodenal enteroendocrine cells, most important are secretin and cholecystokinin (CCK) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Figure 23.19 Neural and Hormonal Mechanisms That Regulate Release of Gastric Juice Copyright © 2025 Pearson Education, Inc. All Rights Reserved Mechanism of HCl Secretion Parietal cells pump H (from carbonic acid) into stomach lumen via H /K  ATPases (proton pumps) – As H enters stomach, HCO3  is exported into blood (called alkaline tide) in exchange for Cl (via Cl /HCO3  antiporter) ▪ Cl diffuses into lumen to join H and form HCl Copyright © 2025 Pearson Education, Inc. All Rights Reserved Mechanism of HCl Secretion by Parietal Cells Figure 23.20 Mechanism of HCl secretion by parietal cells. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Regulation of Gastric Motility and Emptying Response of the stomach to filling – Stretches to accommodate incoming food – Two factors cause pressure to remain constant until 1.5 L of food is ingested ▪ Receptive relaxation – Reflex-mediated relaxation of smooth muscle coordinated by swallowing center of brain stem ▪ Gastric accommodation – Intrinsic stress-relaxation response of visceral smooth muscle, allows stomach to stretch without greatly increasing tension Copyright © 2025 Pearson Education, Inc. All Rights Reserved Regulation of Gastric Motility and Emptying Gastric contractile activity – Contractions most powerful in pyloric part (holds 30 ml of chyme) ▪ Each peristaltic wave reaching pylorus ( 3/min) pushes 3 ml chyme into duodenum; rest returns to stomach for more processing – Only liquids and small particles pass through pyloric valve – Enteric pacemaker cells generate subthreshold waves of depolarization ( 3/min) ▪ Called cyclic slow waves of stomach, or its basic electrical rhythm (BER) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Peristaltic Waves in the Stomach Figure 23.21 Peristaltic waves in the stomach. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Regulation of Gastric Motility and Emptying Regulation of gastric emptying – Usually empties in 4 hr, but increase in fatty chyme entering duodenum can increase time to 6 hr or more ▪ Carbohydrate-rich chyme moves quickly through duodenum – Duodenum prevents overfilling by controlling how much chyme enters Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 23.8 Vomiting (emesis) is caused by: – Extreme stretching of GI tract – Irritants such as bacterial toxins, excessive alcohol, spicy food, certain drugs Chemicals in blood and sensory impulses stimulate emetic center of medulla Excessive vomiting can lead to dehydration and electrolyte and acid-base imbalances (metabolic alkalosis) Copyright © 2025 Pearson Education, Inc. All Rights Reserved 23.7 The Liver Secretes Bile; the Pancreas Secretes Digestive Enzymes Liver, gallbladder, and pancreas are accessory organs associated with small intestine – Liver: produces bile (fat emulsifier) – Gallbladder: stores and concentrates bile – Pancreas: supplies most enzymes for digestion, as well as HCO3  to neutralize stomach acid Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Liver Gross anatomy of the liver – Largest gland in body; weighs 3 lbs – Consists of four primary lobes: right, left, caudate, and quadrate ▪ Gallbladder rests in recess on inferior surface of right lobe – Falciform ligament ▪ Suspends liver from diaphragm and anterior abdominal wall – Lesser omentum anchors liver to lesser curvature of stomach – Hepatic artery proper and hepatic portal vein enter liver at porta hepatis – Bile leaves liver via left and right hepatic ducts; fuse to form common hepatic duct ▪ Cystic duct fills/drains gallbladder ▪ Bile duct formed by union of common hepatic and cystic ducts Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy of the Human Liver Figure 23.23a Gross anatomy of the human liver. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy of the Human Liver Figure 23.23b Gross anatomy of the human liver. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy of the Human Liver Figure 23.23c Gross anatomy of the human liver. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of the Liver Figure 23.24a Microscopic anatomy of the liver. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of the Liver Figure 23.24b Microscopic anatomy of the liver. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of the Liver Figure 23.24c Microscopic anatomy of the liver. Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Liver Microscopic anatomy of the liver – Hepatocytes have large amounts of both rough and smooth ER, Golgi apparatus, peroxisomes, and mitochondria, allowing them to: ▪ Produce 900 ml bile per day ▪ Process bloodborne nutrients – E.g., store glucose as glycogen and use amino acids to make plasma proteins ▪ Store fat-soluble vitamins ▪ Perform detoxification – E.g., converting ammonia to urea Copyright © 2025 Pearson Education, Inc. All Rights Reserved  The Liver Bile: Composition and enterohepatic circulation – Yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, triglycerides, phospholipids, and electrolytes ▪ Only bile salts (cholesterol derivatives) and phospholipids aid digestive processes ▪ Bilirubin: main bile pigment, product of heme (hemoglobin) metabolism – Intestinal bacteria metabolize into stercobilin; gives feces brown color – Enterohepatic circulation: recycling mechanism that conserves bile salts ▪ Reabsorbed in ileum (the last part of small intestine) and returned to liver via hepatic portal vein ▪ 95% of secreted bile salts recycled (only 5% newly synthesized each time) Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Enterohepatic Circulation Figure 23.25 The enterohepatic circulation. Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Liver Homeostatic imbalances of the liver – Hepatitis: inflammation of the liver ▪ Causes: viruses; toxic effects of alcohol, drugs, and wild mushrooms ▪ Five viruses that cause hepatitis identified and named: H AV to HEV – HBV and HCV linked to chronic hepatitis, cirrhosis, and liver cancer HCV treated with 12-week combination drug therapy – Non-alcoholic fatty liver disease (NAFLD) most common liver disease in North America; caused by abnormal lipid metabolism and liver inflammation ▪ Both associated with obesity and insulin resistance; predisposes patient to develop cirrhosis or even cancer of the liver – Cirrhosis: last stage of progressive chronic inflammation of liver; usually result of alcohol use disorder, NAFLD, or viral hepatitis ▪ Fibrous (scar) tissue can obstruct blood flow and cause portal hypertension – Liver transplants only effective treatment for end-stage liver disease ▪ Liver can regenerate to its full size in 6–12 months after 80% removal Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Gallbladder Thin-walled muscular sac on inferior surface of liver (covered by visceral peritoneum) Functions: store and concentrate bile (by absorbing water and ions) Contains many honeycomb folds that allow it to expand as it fills Muscular contractions release bile via cystic duct, which leads into the bile duct Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 23.9 Gallstones: crystallized cholesterol, caused by too much cholesterol or too few bile salts – Can obstruct flow of bile from gallbladder (cholelithiasis) – Painful when gallbladder contracts against sharp crystals – Treatment: crystal-dissolving drugs, ultrasound vibrations (lithotripsy), laser vaporization, or surgery (cholecystectomy) Obstructive jaundice: bile duct blockage can cause bile salts and pigments to build up in blood, resulting in jaundiced (yellow) skin – Jaundice can also reflect liver disease Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Pancreas Location: mostly retroperitoneal, deep to greater curvature of stomach – Head is encircled by duodenum; tail next to spleen Contains endocrine and exocrine parts: – Exocrine function: produce pancreatic juice ▪ Acini: clusters of secretory acinar cells that produce zymogen granules containing inactive digestive enzymes (proenzymes) ▪ Ducts: transport secretions of acinar cells from pancreas to duodenum – Small duct cells secrete water (bulk of pancreatic juice) with HCO3  – Endocrine function: secretion of insulin and glucagon by pancreatic islets Copyright © 2025 Pearson Education, Inc. All Rights Reserved Structure of the Enzyme-Producing Tissue of the Pancreas Figure 23.26a Structure of the enzyme-producing tissue of the pancreas. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Structure of the Enzyme-Producing Tissue of the Pancreas Figure 23.26b Structure of the enzyme-producing tissue of the pancreas. Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Pancreas – Composition of pancreatic juice ( 1200  1500 ml/day) – Watery, alkaline solution (pH 8) to neutralize acidic chyme from stomach – Contains electrolytes (primarily HCO3  ) and digestive enzymes: ▪ Proteases (for proteins), made/secreted inactive to prevent self-digestion ▪ Amylase (for carbohydrates) ▪ Lipases (for lipids) ▪ Nucleases (for nucleic acids) – Proteases activated in duodenum, where they work ▪ Enteropeptidase (formerly called enterokinase): enzyme bound to apical membrane of duodenal epithelial cells, activates trypsinogen to trypsin ▪ Trypsin in turn can then activate: – More trypsinogen to trypsin – Procarboxypeptidase to carboxypeptidase – Chymotrypsinogen to chymotrypsin Copyright © 2025 Pearson Education, Inc. All Rights Reserved Activation of Pancreatic Proteases in the Small Intestine Figure 23.27 Activation of pancreatic proteases in the small intestine. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Bile and Pancreatic Secretion Into the Small Intestine Anatomy of duct systems – Bile duct and pancreatic duct (main pancreatic duct) unite in wall of duodenum at bulblike structure called hepatopancreatic ampulla ▪ Opens into duodenum via major duodenal papilla ▪ Hepatopancreatic sphincter controls entry of bile and pancreatic juice into duodenum – Accessory pancreatic duct: smaller, proximal duct that empties directly into duodenum Copyright © 2025 Pearson Education, Inc. All Rights Reserved Relationship of the Liver, Gallbladder and Pancreas to the Duodenum Figure 23.28 Relationship of the liver, gallbladder, and pancreas to the duodenum. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Mechanisms Promoting Secretion and Release of Bile and Pancreatic Juice Figure 23.29 Mechanisms promoting secretion and release of bile and pancreatic juice. Copyright © 2025 Pearson Education, Inc. All Rights Reserved 23.8 The Small Intestine Is the Major Site for Digestion and Absorption Small intestine is the body’s major digestive organ – Digestion is completed (with the help of bile and pancreatic enzymes) and virtually all absorption Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy Small intestine: 6  7 m long ( 20 ft) in cadaver, from pyloric sphincter to ileocecal valve; 2  4 m long (7  13 ft) in living person via muscle tone – Diameter about 2.5  4 cm long (1  1.6 in), about half that of large intestine Three subdivisions – Duodenum: 25 cm (10 in) long ▪ Mostly retroperitoneal, curves around head of pancreas – Jejunum: 2.5 m (8 ft) long, and ileum: 3.6 m (12 ft) long ▪ Attached to posterior abdominal wall by a mesentery (or mesentery proper) ▪ Terminal ileum joins large intestine at ileocecal valve Copyright © 2025 Pearson Education, Inc. All Rights Reserved The Small Intestine Figure 23.30 The small intestine. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy Blood supply: – Superior mesenteric artery (oxygenated arterial blood) – Veins (nutrient-rich venous blood) empty into superior mesenteric vein, which drains into hepatic portal vein (to liver) Nerve supply – Parasympathetic innervation via vagus nerve – Sympathetic innervation via thoracic splanchnic nerves Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy Modifications of small intestine for absorption – Small intestine’s length and three structural modifications provide huge surface area for nutrient absorption ▪ Modifications – circular folds, villi, and microvilli—increase surface area 600 to 200 m2 (size of a tennis court) – Circular folds: permanent folds of mucosa and submucosa ( 1 cm tall) ▪ Force chyme to slowly spiral through lumen, increasing surface area and time for absorption – Villi: fingerlike projections of mucosa ( 1 mm tall) ▪ Each villus contains a lacteal (lymphatic capillary for lipid absorption) and a dense capillary bed – Microvilli: cytoplasmic extensions of apical surface of enterocytes that create fuzzy appearance called brush border ▪ Membrane-bound brush border enzymes complete carbohydrate and protein digestion Copyright © 2025 Pearson Education, Inc. All Rights Reserved Structural Modifications of the Small Intestine That Increase Its Surface Area for Digestion and Absorption Figure 23.31a,b Structural modifications of the small intestine that increase its surface area for digestion and absorption. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy Histology of the small intestine wall – Modifications of its mucosa and submucosa reflect its function in digestion – Epithelium of villi and the tubular intestinal crypts (intestinal glands) between villi consists of five main cell types: ▪ Enterocytes: simple columnar absorptive cells with dense microvilli, bound by tight junctions; form bulk of epithelium ▪ Goblet cells: mucus-secreting cells ▪ Enteroendocrine cells: secrete enterogastrones, including CCK and secretin; mostly in crypts, some in villi ▪ Paneth cells: deep in crypts, secrete antimicrobial agents (defensins, lysozyme) that destroy certain bacteria ▪ Stem cells: deep in crypts, continuously divide and differentiate to produce other cell types; cells at tips of villi undergo apoptosis and shed – Renewing villus epithelium every 2–4 days Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 23.10 Chemotherapy targets rapidly dividing cells – Destroying cancer cells, but also rapidly dividing GI tract epithelial cells ▪ Explains resulting nausea, vomiting, and diarrhea after treatment Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy Histology of the small intestine wall (cont.)inued – MALT (mucosa-associated lymphoid tissue) protects intestine against microorganisms; includes: ▪ Individual lymphoid follicles ▪ Peyer’s patches (aggregated lymphoid nodules): located in lamina propria, but can protrude into submucosa – Found in increasing numbers in distal ileum (lots of bacteria there) – Lamina propria also contains abundant plasma cells that secrete IgA (antibodies) – Submucosa of duodenum contains elaborate duodenal glands that secrete alkaline (bicarbonate-rich) mucus to neutralize acidic chyme Copyright © 2025 Pearson Education, Inc. All Rights Reserved Digestive Processes in the Small Intestine Chyme entering the small intestine contains partially digested carbohydrates and proteins, and largely undigested fats – 3–6 hours to pass through small intestine as nutrients and most water absorbed Sources of enzymes for digestion – Bile, HCO3  , enzymes are imported from liver and pancreas – Membrane-bound brush border enzymes perform final steps of digestion Regulating chyme entry – Chyme entering duodenum usually hypertonic, so its delivery must be slow to prevent osmotic loss of water from blood – Low pH of chyme must be raised, and it must be mixed with bile and pancreatic juice to continue digestion – Feedback via enterogastric reflex and enterogastrones carefully control movement of food into duodenum to avoid overwhelming it Copyright © 2025 Pearson Education, Inc. All Rights Reserved Digestive Processes in the Small Intestine Motility of the small intestine – Two motility patterns: ▪ Segmentation: principal form of motility after meal, for mixing ▪ Migrating motor complex: form of peristalsis, primary pattern between meals – After a meal ▪ Initiated by intrinsic pacemaker cells, alternately contracting/relaxing rings of smooth muscle move chyme back-and-forth (also slowly moving it toward cecum) ▪ Intensity controlled by long and short reflexes and hormones – Parasympathetic increases motility; sympathetic decreases it – Between meals ▪ True peristalsis, initiated by rise in hormone motilin, every 90–120 min ▪ Each wave starts distal to previous; called migrating motor complex (MMC) ▪ Complete trip from duodenum to ileum takes 2 hr Copyright © 2025 Pearson Education, Inc. All Rights Reserved Digestive Processes in the Small Intestine Ileocecal valve control – Ileocecal valve relaxes to admit chyme into cecum (of large intestine) via: ▪ Gastroileal reflex – Also increases force of segmentation ▪ Gastrin – Also increases motility of ileum – Ileocecal valve flaps close when chyme exerts backward pressure ▪ Prevents regurgitation into ileum Copyright © 2025 Pearson Education, Inc. All Rights Reserved 23.9 The Large Intestine Absorbs Water and Eliminates Feces Large intestine extends from ileocecal valve to anus – Much shorter than small intestine (1.5 m v s 6 m), but ersu about twice the diameter (7 cm) – Major digestive functions: ▪ Absorb most of the remaining water from indigestible food residues – Also absorb metabolites produced by resident bacteria ▪ Store residues temporarily, then eliminate them as semisolid feces (or stool) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy Subdivisions of large intestine: cecum, appendix, colon, rectum, and anal canal – Cecum: saclike, first part of large intestine – Appendix: contains masses of lymphoid tissue (part of MALT) ▪ Bacterial storehouse capable of recolonizing gut when necessary ▪ Twisted shape of appendix makes it susceptible to blockage Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 23.11 Appendicitis: acute inflammation of appendix – Usually results from a blockage (often by feces) that traps infectious bacteria – Most common in adolescence when entrance to appendix is at its widest Symptoms: pain in umbilical region, moving to lower right abdominal quadrant – loss of appetite, nausea, and vomiting are also seen Venous drainage can be impaired, leading to ischemia and necrosis (tissue death) – Ruptured appendix can cause peritonitis as bacteria leak out into peritoneal cavity Treatment: surgical removal (appendectomy), or in select cases, with antibiotics Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy Subdivisions of large intestine (cont.) inued – Colon has four main regions ▪ Ascending colon: travels up right side of abdominal cavity to liver – Ends in right-angle turn called right colic (hepatic) flexure ▪ Transverse colon: travels across abdominal cavity to spleen – Ends in another right-angle turn, left colic (splenic) flexure ▪ Descending colon: travels down left side of abdominal cavity ▪ Sigmoid colon: S-shaped portion that travels through pelvis to join rectum – Rectum: runs just in front of sacrum; three transverse folds (rectal valves) stop feces from being passed with gas – Anal canal: last segment of large intestine, external to abdominopelvic cavity ▪ Open to exterior at anus; two sphincters close anus except during defecation – Internal anal sphincter (smooth muscle) – External anal sphincter (skeletal muscle) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy of the Large Intestine Figure 23.33a Gross anatomy of the large intestine. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy of the Large Intestine Figure 23.33b Gross anatomy of the large intestine. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Gross Anatomy Relationship of the large intestine to the peritoneum – Colon is retroperitoneal, except for transverse and sigmoid colon ▪ These intraperitoneal regions (transverse and sigmoid) anchored to posterior abdominal wall by mesenteries called mesocolons – Rectum also retroperitoneal Copyright © 2025 Pearson Education, Inc. All Rights Reserved Mesenteries of the Abdominal Digestive Organs Figure 23.34a Mesenteries of the abdominal digestive organs. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Mesenteries of the Abdominal Digestive Organs Figure 23.34d Mesenteries of the abdominal digestive organs. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy Like the small intestine, the large intestine is lined with simple columnar epithelium (except anal canal has stratified squamous to withstand abrasion) Large intestine (in contrast to the small): – Does not contain circular folds, villi, or a brush border – Contains thicker mucosa with deeper crypts that have more goblet cells ▪ Mucus lubricates passageway (to move feces) and protects wall from irritation – Anal sinuses, recesses between anal columns, release mucus when compressed by feces (aids in emptying anal canal) – Inferior margins of anal sinuses form wavy line Two superficial venous plexuses of anal canal form hemorrhoids if dilated and inflamed Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 23.12 Clostridium difficile, an anaerobic bacterium, is most common cause of antibiotic-associated diarrhea (accounts for 14,000 deaths per year) In some, C. difficile makes up tiny fraction of their bacterial flora; in others it enters via fecal-oral route (poor handwashing) When other bacteria destroyed by antibiotics, C. difficile may flourish and cause colon inflammation (pseudomembranous colitis) that leads to bowel perforation and sepsis – C. difficile infections are resistant to antibiotics and difficult to treat (often reoccur) – Fecal transplant: restores competitive bacteria by transferring fecal bacteria from uninfected donor to patient ▪ Cures C. difficile infections 90–100% of the time Copyright © 2025 Pearson Education, Inc. All Rights Reserved Digestive Processes in the Large Intestine Food residue remains in large intestine 12–24 hours – No further digestion (except for bacterial) – Salts (e.g., NaCl, NaHCO3 ) and water, and vitamins released from bacterial metabolism, are absorbed Major function of large intestine is propulsion of feces to anus for defecation Motility of the large intestine – Haustral contractions: slow segmenting movements, mostly in ascending and transverse colon; last 1 min and occur every 30 min ▪ Distension of haustrum triggers its contraction (to mix contents) – Mass movements (mass peristalsis): long, slow-moving, powerful contractile waves that move feces toward rectum; occur about 3– 4 per day ▪ Descending colon and sigmoid colon store feces (and finish dehydrating it) until moved into rectum by mass movements ▪ Usually occur during/after eating (via gastrocolic reflex) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 23.13 Large intestine is important for our comfort, but is not essential for life If colon removed, terminal ileum can be brought to abdominal wall in a procedure called an ileostomy – Food residues eliminated into sac attached to stoma in abdominal wall Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 23.14 Low-fiber diet leads to narrowing of colon and stronger contractions that increase pressure on walls – Can result in diverticulosis, a condition where diverticula (herniations of mucosa through intestinal wall) develops in colon, usually sigmoid colon ▪ Affects > 50% of people over 70 Sometimes, diverticulosis progresses to diverticulitis (inflamed diverticula) – May rupture and leak into peritoneal cavity (potentially life threatening) Irritable bowel syndrome (IBS): recurring abdominal pain relieved by defecation; functional disorder not explained by anatomical or biochemical abnormalities – May have changes in consistency and frequency of stools, bloating, flatulence, nausea, depression – Stress is a precipitating factor, so stress management is important in treatment Copyright © 2025 Pearson Education, Inc. All Rights Reserved Digestive Processes in the Large Intestine Defecation – Mass movements force feces into rectum – Distension initiates defecation reflex (parasympathetic spinal reflex) ▪ Triggers contraction of sigmoid colon and rectum ▪ Relaxes internal anal sphincter ▪ Voluntary control allows contraction/relaxation of external anal sphincter – During reflex, rectal muscles contract to expel feces, which is assisted by: ▪ Valsalva’s maneuver—closing of glottis and contraction of diaphragm and abdominal wall muscles to increase intra-abdominal pressure ▪ Contract levator ani muscle, causing anal canal to be lifted superiorly Copyright © 2025 Pearson Education, Inc. All Rights Reserved Defecation Reflex (3 of 3) Figure 23.35 Defecation reflex. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Clinical—Homeostatic Imbalance 23.15 Diarrhea, or watery stools, results when large intestine does not have sufficient time to absorb remaining water – Causes include irritation of colon by bacteria or jostling of digestive viscera (occurs in marathon runners) – Prolonged diarrhea may result in dehydration and electrolyte imbalance (acidosis and loss of potassium) Constipation occurs when residue remains in colon too long and too much water is absorbed, so stool becomes difficult to pass – May result from insufficient fiber or fluid intake, repeatedly delaying defecation, lack of exercise, or laxative abuse Copyright © 2025 Pearson Education, Inc. All Rights Reserved Part 3—Physiology of Digestion and Absorption Copyright © 2025 Pearson Education, Inc. All Rights Reserved 23.10 Digestion Hydrolyzes Food into Nutrients That Are Absorbed Across the Gut Epithelium Digestion breaks down ingested foods into their chemical building blocks, molecules that are small enough to be absorbed across the small intestine wall Copyright © 2025 Pearson Education, Inc. All Rights Reserved Mechanism of Digestion: Enzymatic Hydrolysis Digestion: catabolic process that breaks macromolecules (polymers) down into molecular building blocks (monomers) small enough for absorption; mostly in small intestine – Via enzymes secreted into GI tract from intrinsic and accessory glands, and intestinal brush border enzymes – Enzymes carry out hydrolysis, whereby water is added to break chemical bonds In general, pancreatic (also salivary and gastric) enzymes break down larger polymers into smaller polymers that are eventually broken down into absorbable monomers by (intestinal) brush border enzymes Copyright © 2025 Pearson Education, Inc. All Rights Reserved  Mechanisms of Absorption Absorption is process of moving substances from lumen of gut into body Tight junctions force most substances to pass through epithelial cells (rather than between them) – Enter through apical membrane (lumen side) and exit through basolateral membrane (interstitial fluid side) – Once in IF, substances (except lipids) diffuse into capillaries (lipids enter lacteals) Small nonpolar lipid (lipophilic) molecules absorbed passively via simple diffusion All other substances need a membrane transport mechanism (mostly active transport) Most nutrients are absorbed before chyme reaches ileum Copyright © 2025 Pearson Education, Inc. All Rights Reserved Mechanisms of Absorption Copyright © 2025 Pearson Education, Inc. All Rights Reserved 23.11 How Is Each Type of Nutrient Processed? Copyright © 2025 Pearson Education, Inc. All Rights Reserved Carbohydrate Digestion and Absorption in the Small Intestine (4 of 4) Figure 23.37 Carbohydrate digestion and absorption in the small intestine. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Protein Digestion and Absorption in the Small Intestine (4 of 4) Figure 23.38 Protein digestion and absorption in the small intestine. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Lipids Steps of lipid (triglyceride) digestion and absorption in small intestine 1. Emulsification: breaking up large fat globules into tiny droplets (via bile salts) that remain suspended in chyme, increasing surface area for digestion of lipids ▪ Bile salts have nonpolar and polar regions to interact with both lipids and water 2. Digestion: pancreatic lipase breaks down triglyceride into monoglyceride and two free fatty acids 3. Micelle formation: end products of digestion become associated with bile salts and lecithin (phospholipid), forming micelles 4. Diffusion: lipids leave micelles and cross apical membrane via simple diffusion 5. Chylomicron formation: lipids converted back into triglycerides and packaged with phospholipids, cholesterol, proteins; forming lipoprotein called chylomicron 6. Chylomicron transport: chylomicrons exit basolateral membrane via exocytosis; once in the IF they diffuse into lacteals (lymphatic vessels) (too large for capillary) ▪ Eventually emptied into venous blood at thoracic duct Short-chain fatty acids can diffuse directly into blood Copyright © 2025 Pearson Education, Inc. All Rights Reserved Emulsification, Digestion, and Absorption of Fats (6 of 6) Figure 23.39 Emulsification, digestion, and absorption of fats. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Flowchart of Digestion and Absorption of Nucleic Acids Figure 23.36 Flowchart of digestion and absorption of foodstuffs. Copyright © 2025 Pearson Education, Inc. All Rights Reserved Absorption of Vitamins, Electrolytes, and Water Vitamin absorption – Small intestine absorbs dietary vitamins ▪ Fat-soluble vitamins (A, D, E, and K) are incorporated into micelles with other lipophilic solutes; diffuse into absorptive cells by simple diffusion ▪ Most water-soluble (B and C) vitamins are absorbed by passive or active transporters – Vitamin B12 (large, charged molecule) binds with intrinsic factor and is absorbed by endocytosis – Large intestine absorbs vitamin K and B vitamins from bacterial metabolism Copyright © 2025 Pearson Education, Inc. All Rights Reserved Absorption of Vitamins, Electrolytes, and Water Electrolyte absorption – Most ions are transported actively along length of small intestine; iron and calcium absorbed in duodenum (amount depends on needs) – Sodium, chloride, and bicarbonate ▪ Na absorption is active (Na  K  pumps), creating electrical gradient for anions (e.g., Cl ) to passively follow – Potassium ▪ K  transported by facilitated diffusion; gradient created as water absorbed ▪ Lack of water absorption (e.g., diarrhea) reduces K  absorption, and can even “pull” K  out of IF into gut lumen Copyright © 2025 Pearson Education, Inc. All Rights Reserved Absorption of Vitamins, Electrolytes, and Water – Iron ▪ Ionic iron actively transported into mucosal cells where most binds to ferritin until needed, then transported in blood by transferrin – Calcium ▪ Calcium absorption is regulated (increased) by active vitamin D (calcitriol) and parathyroid hormone (PTH) Copyright © 2025 Pearson Education, Inc. All Rights Reserved Absorption of Vitamins, Electrolytes, and Water Water absorption – 9L water (mostly from secretions) enters small intestine daily ▪ 95% absorbed by osmosis ▪ More absorbed in large intestine, leaving 100 ml in feces – Concentration gradients established by active transport of solutes (mostly Na ) provide net osmosis out of lumen Copyright © 2025 Pearson Education, Inc. All Rights Reserved IP Animation: Enzymatic Digestion and Absorption Click here to view ADA compliant Animation: IP Animation: Enzymatic Digestion and Absorption https://mediaplayer.pearsoncmg.com/assets/secs-enzymatic-digestion-and-absorption Copyright © 2025 Pearson Education, Inc. All Rights Reserved Developmental Aspects of the Digestive System Primitive gut: epithelial lining is formed from endoderm; rest of GI tract from mesoderm Oral membrane: becomes mouth; formed when foregut fuses with stomodeum Cloacal membrane: becomes anus; formed when hindgut fuses with proctodeum By week 5, alimentary canal is a continuous tube from mouth to anus Shortly after, accessory organs bud from mucosa Copyright © 2025 Pearson Education, Inc. All Rights Reserved Embryonic Development of the Digestive System Figure 23.40 Embryonic development of the digestive system. Copyright © 2025 Pearson Education, Inc. All Rights Reserved