digestive system

Chapter 26 Lecture Notes: Digestive System

Overview of the Digestive System

• Digestive System
  • Provides means to break down ingested nutrients
  • Nutrients required for survival are unusable in original form
  • Provides means to absorb them
  • Consists of organs that:
  • Ingest food
  • Mix and propel food
  • Add secretions
  • Expel waste products
  • Includes:
  • Upper and lower gastrointestinal tract
  • Accessory organs

General Functions of the Digestive System

• Six Main Functions:
  1. Ingestion
  • Introduction of solid and liquid nutrients into the oral cavity; first step in the digestion and absorption process.
  1. Motility
  • Voluntary and involuntary muscular contractions for mixing and moving materials through the gastrointestinal (GI) tract.
  1. Secretion
  • Production and release of fluid products that facilitate digestion (e.g., digestive enzymes, acid, bile).
  1. Digestion
  • Breakdown of ingested food into smaller structures:
    • Mechanical Digestion: Physical breakdown (chewing, mixing).
    • Chemical Digestion: Involves specific enzymes to break chemical bonds; transforms large complex molecules into smaller ones.
  1. Absorption
  • Transport of digested molecules, electrolytes, vitamins, and water from the GI tract into blood or lymph.
  1. Elimination
  • Expulsion of indigestible components that are not absorbed.

Introduction to Digestive Enzymes

• Enzymes
  • Catalysts:
  • Proteins that speed up chemical reactions.
  • Not changed or used up in reactions.

Movement of Materials Through the Digestive Tract

• Muscularis (Externa)
  • Functions to propel contents within the GI tract.
  • Peristalsis: Alternating contraction sequence of inner and outer layers; propels ingested materials.
  • Mixing (Segmentation): "Back-and-forward" motion that blends ingested materials with secretions, lacking directional movement.
  • Mass Movements: Powerful peristaltic-like contractions; force fecal matter from transverse colon through the rest of the large intestine.

Regulation of Digestive System Processes

• Receptors
  • Monitor changes associated with the GI tract and contents; embedded throughout the mucosa and submucosa of the GI tract.
  • Mechanoreceptors: Detect stretch or pressure.
  • Chemoreceptors: Detect specific chemicals in the lumen.
• Nervous Control
  • Sensory input is relayed to the central nervous system (CNS); autonomic motor output relayed through facial, glossopharyngeal, and vagus nerves to effectors (e.g., salivary glands, pancreas, muscularis).
  • Long Reflexes: Coordinated secretory and smooth muscle contractions.
  • Short Reflexes: Local reflexes of the enteric nervous system without CNS involvement.
• Hormonal Control
  • Three primary hormones in regulation of digestion:
  • Gastrin: From the stomach
  • Secretin: From the small intestine
  • Cholecystokinin (CCK): From the small intestine.

Upper Gastrointestinal Tract: Overview of the Organs

• Upper GI Tract Organs
  • Oral Cavity and Salivary Glands:
  • Mechanical digestion begins here.
  • Saliva is secreted in response to food and contains salivary amylase (enzyme initiating carbohydrate digestion).
  • Materials form a bolus to be swallowed.
  • Pharynx:
  • Bolus moved here; swallowing occurs.
  • Mucus is secreted to facilitate swallowing.
  • Esophagus:
  • Transports bolus from pharynx into the stomach; lubricated by mucus.
  • Stomach:
  • Bolus mixed with gastric secretions by smooth muscle contraction.
  • Chyme formed from mixing (acidic mixture of partially digested food).

Upper Gastrointestinal Tract: Oral Cavity and Salivary Glands

• The Oral Cavity

  • Tongue:
  • Numerous projections called papillae involved in taste.
  • Responsible for sensory analysis of food (temperature, touch); manipulates and mixes materials during chewing.
  • Important for functions in swallowing and speech.
  • Secretions:
  • Mucins: Form mucus layer.
  • Lingual Lipase: Enzyme that breaks down lipids.

• Saliva

  • Produced by three pairs of salivary glands:
  • Parotid Glands: 25% of saliva production.
  • Submandibular Glands: 70% of saliva production.
  • Sublingual Glands: 5% of saliva production.
  • Components:
  • 1.0-1.5 L produced daily, mostly during mealtime.
  • 99.5% water and a mixture of solutes including salivary amylase, mucin, and lysozyme.
  • Functions:
  • Moistens food, forming bolus.
  • Dissolves chemicals stimulating taste buds.
  • Cleanses oral cavity and contains antibacterial substances (lysozyme, antibodies) that inhibit bacterial growth.
  • Initiates digestion (salivary amylase acts on carbohydrates, lingual lipase acts on lipids).

• Regulation of Salivary Secretions

  • Regulated by the autonomic nervous system (ANS).
  • Increased by:
  • Thought, smell, or sight of food.
  • Entry of food into the oral cavity and stomach (particularly acidic foods).
  • Irritating or unpleasant stimuli, nausea.

• Mechanical Digestion: Mastication

  • Chewing reduces bulk to facilitate swallowing and increases surface area for enzyme exposure.
  • Promotes salivation; requires coordinated activities of teeth, lips, tongue, cheeks, jaws; controlled by nuclei in medulla and pons (mastication center).
  • Note: No nutrients are absorbed in the oral cavity.

Upper Gastrointestinal Tract: Pharynx

• Pharynx:
  • Shared passage for food and air; connects nasal and oral passages to respiratory and digestive systems.
  • Lymphoid tissue (tonsils) contributes to the body’s defense system.
  • Muscles cooperate with the oral cavity and esophagus to initiate deglutition (swallowing).

Upper Gastrointestinal Tract: Esophagus

• Primary Function:
  • Carries solid food and liquids to the stomach from the oral cavity and pharynx.
  • Bolus is pushed through the esophagus by peristaltic waves under CNS control.
  • Approach of the bolus triggers the lower esophageal sphincter to open, allowing the bolus to enter the stomach.
  • Transit Time through the Esophagus:
  • Liquids: 2 seconds.
  • Bolus: 9 seconds.
  • Dry or poorly lubricated bolus moves slower, may require a second peristaltic wave for propulsion into the stomach.

Upper Gastrointestinal Tract: Pharynx and Esophagus

• Motility: The Swallowing Process
  • Swallowing (Deglutition): Moves ingested materials from the oral cavity to the stomach.
  • Three Phases:
  1. Voluntary Phase:
     • Controlled by the cerebral cortex; bolus formed from ingested materials and saliva; directed posteriorly toward the oropharynx.
  2. Pharyngeal Phase:
     • Involuntary reflex initiated by tactile sensory receptors around the fauces, sending input to the swallowing center in the medulla oblongata; signals relayed to effectors for bolus passage through the pharynx to the esophagus (sequential contractions of pharyngeal muscles).
     • Effector responses:
       • Entry of bolus into the oropharynx.
       • Elevation of soft palate and uvula to block oropharynx.
       • Elevation of larynx by hyoid muscles moves the epiglottis to cover the laryngeal opening, inhibiting the respiratory center to prevent inhalation.
  3. Esophageal Phase:
     • Involuntary phase during which bolus passes through the esophagus, stimulating sequential waves of muscular contraction, propelling bolus toward the stomach.
     • Superior and inferior esophageal sphincters remain closed at rest and relax when bolus is swallowed; they contract afterward to prevent reflux of materials.

Clinical View: Reflux Esophagitis

• Reflux Esophagitis:
  • Inflammation of the esophagus due to acidic chyme refluxing into the esophagus.
  • Symptoms include pain posterior to the sternum (heartburn).
  • More frequently seen in overweight individuals, smokers, and after large meals; also associated with hiatal hernias (part of stomach protruding through diaphragm).
  • Recommended management includes lifestyle changes and head elevation.

Clinical View: Gastroesophageal Reflux Disease (GERD)

• Gastroesophageal Reflux Disease (GERD):
  • Results from chronic reflux esophagitis.
  • Causes erosion of esophageal tissue, may lead to scar tissue formation which narrows the lumen.
  • May change from stratified squamous to columnar secretory epithelium, increasing cancer risk.
  • Treatment may involve medications including proton pump inhibitors.

Upper Gastrointestinal Tract: Stomach

• Stomach:
  • Food is saturated with gastric juices, exposed to stomach acids, and enzyme effects of pepsin.
  • Continues chemical and mechanical digestion, primarily of proteins and fats, spending 2-6 hours in the stomach.
  • Absorption is limited to small, nonpolar substances.
• Histology:
  • Lined by simple columnar epithelium supported by lamina propria; lining is indented by depressions (gastric pits).
  • Gastric Glands: Extend deep into the mucosa from the base of each gastric pit.
  • Muscularis Layer: Comprises three smooth muscle layers:
  • Inner oblique
  • Middle circular
  • Outer longitudinal
  • Thickness increases from the body to the pylorus.

Gastric Secretions

• Produced by Five Types of Secretory Cells:
  • Surface Mucous Cells: Secrete alkaline mucus to prevent ulceration of the stomach lining.
  • Mucous Neck Cells: Produce acidic mucin to maintain acidic conditions.
  • Parietal Cells: Produce hydrochloric acid (HCl) and intrinsic factor:
  • HCl forms from H+ and Cl- secreted across the cell surface; responsible for stomach's low pH, activates pepsinogen to pepsin, kills microorganisms, breaks down connective tissue.
  • Chief Cells: Most numerous; secrete pepsinogen activated by HCl and other pepsin molecules. Also secretes gastric lipase, plays a limited role in fat digestion.
  • G-Cells: Enteroendocrine cells that secrete gastrin, stimulating stomach secretions and motility.

Stomach Motility

• Gastric Mixing:
  • Form of mechanical digestion; churns and mixes food leading to reduced size of swallowed particles, converting semidigested bolus into chyme.
• Gastric Emptying:
  • Movement of acidic chyme from the stomach into the duodenum.

Regulation of Digestive Processes in the Stomach

• General Function of the Stomach: Acts as a holding bag for partially digested food.
• Pacemaker Cells: Spontaneously depolarize and establish the basic rhythm of muscular contractions; signals spread through the smooth muscle cells in the muscularis layer.
• Force of Contraction and Gastric Gland Secretion regulated by nervous reflexes and hormones, organized in three phases:
  1. Cephalic Phase:
  • Initiated by the thought or sensation of food; nerve signals from higher brain regions sent to hypothalamus, relaying signals to the medulla oblongata, increasing vagal stimulation of the stomach and causing increased motility and secretory activity.
  1. Gastric Phase: Process following the bolus reaching the stomach; regulated via gastric reflex and release of gastrin hormone:
  • Reflex initiated as baroreceptors detect stomach stretch and chemoreceptors detect protein and increased pH.
  • Nerve signals are relayed to the medulla oblongata, resulting in increased stomach motility and activity of secretory glands.
  • Release of gastrin from the presence of food further stimulates secretory activity and contraction of peristaltic movements in the stomach.
  1. Intestinal Phase: Processes occurring after chyme reaches the small intestine; characterized by:
  • Intestinal reflex that opposes cephalic and gastric reflexes; chyme in the duodenum signals to the medulla oblongata to decrease stomach motility and activity.
  • CCK and Secretin both decrease stomach motility and secretory activity, slowing gastric emptying.

Clinical View: Peptic Ulcers

• Peptic Ulcers:
  • Solitary erosions of portions of the stomach or duodenum (gastric ulcers vs. duodenal ulcers).
  • Symptoms include gnawing and burning pain in the epigastric region, nausea, vomiting, and potential bleeding.
  • Can erode and perforate, constituting a medical emergency; commonly associated with Helicobacter pylori, contributing to gastric lining erosion.
  • Treatment is similar to that of gastric reflux.

Clinical View: Gastric Content Expulsion by Vomiting

• Vomiting:
  • Rapid expulsion of gastric contents through the oral cavity; controlled by the vomiting center in the medulla oblongata, responding to head injury, motion sickness, infection, toxicity, or food irritation.
  • Involves:
  • Closure of nasal passages and the glottis.
  • Skeletal muscle contractions increase intragastric pressure, forcing contents into and through the esophagus.
  • Aspiration is a danger to consider.

Lower Gastrointestinal Tract: Overview of the Organs

• Lower GI Tract Organs:
  • Continue digestion and absorption processes.
  • Eliminate undigested material.
• Small Intestine:
  • Divided into three continuous regions: duodenum, jejunum, ileum (with duodenum considered part of the upper GI tract).
  • Receives chyme from the stomach mixed with accessory organ secretions; most chemical digestion and absorption occur here.
• Accessory Organs:
  • Include bile and pancreatic juices. Bile is produced by the liver (stored, concentrated, and released by the gallbladder). Pancreatic juice secreted from the pancreas includes digestive enzymes.
• Large Intestine:
  • Continues absorption of water, electrolytes, and vitamins; feces produced and eliminated through the anus.

Lower Gastrointestinal Tract: Small Intestine

• Small Intestine:
  • Long tube inferior to the stomach; nutrient-containing digesta spends at least 12 hours here.
  • Chemical digestion is completed here, with most enzymes originating from accessory structures (liver, gallbladder, pancreas).
  • Absorbs most nutrients, as well as a large percentage of water and electrolytes; vitamins are also absorbed. Consists of:
  • Duodenum: Mixing bowl; receives chyme and bile.
  • Jejunum: Main area where nutrients are absorbed.
  • Ileum: Completes absorption processes.

Nutritional Absorption

• Nutrient Absorption:
  • Small intestine absorbs approximately 90% of nutrients; remainder absorbed in the large intestine.
  • The epithelial surface has adaptations for absorption:
  • Increased surface area 600X through:
    • Plicae (transverse folds)
    • Villi with microvilli
  • Extensive capillary networks within villi cover its surface, leading to the hepatic portal circulation, which adjusts nutrient concentrations in blood before systemic circulation.

Small Intestine Secretions

• Secretions:
  • Intestinal Glands: Invaginations of the mucosa between intestinal villi that secrete intestinal juice.
  • Goblet Cells: Produce mucin-forming mucus; number increases from duodenum to ileum.
  • Unicellular Gland Cells: Synthesize enteropeptidase (an enzyme converting trypsinogen to trypsin).
  • Enteroendocrine Cells: Release hormones such as CCK (Cholecystokinin) and GIP (Gastric Inhibitory Polypeptide).
  • Submucosal Glands: Produce alkaline mucus secretion protecting the duodenum from chyme.

Motility of the Small Intestine

• Smooth Muscle Movements:
  • Mixes chyme with gland secretions via segmentation (back-and-forward motion) to expose new areas of brush border for absorption.
  • Propels contents through the small intestine via peristalsis and gastroileal reflex, moving contents from the ileum to cecum in response to food in the stomach, with the ileocecal sphincter usually constricted.

Lower Gastrointestinal Tract: Accessory Organs and Ducts

• Liver:

  • Accessory digestive organ; main function is the production of bile

• Bile:

  • Secreted by the liver; contains:
  • Water
  • Bicarbonate ions
  • Bile salts and pigments
  • Cholesterol, lecithin, mucin
  • Bile salts and lecithin aid in the mechanical digestion of lipids.

Clinical View: Cirrhosis of the Liver

• Cirrhosis:

  • Results when hepatocytes are replaced by fibrous scar tissue, compressing blood vessels leading to high blood pressure in the hepatic portal venous system.
  • Caused by chronic injury from chronic alcoholism, liver disease, drugs, or toxins, frequently associated with viral hepatitis B or C.
  • Symptoms include fatigue, weight loss, nausea, and pain in the right upper quadrant; if advanced, jaundice, edema, ascites, itching, toxin accumulation, dilated veins of the esophagus may develop.

• Gallbladder:

  • Sac-like organ attached to the liver's inferior surface; stores, concentrates, and releases bile produced by the liver.

• Pancreas:

  • Functions:
  • Endocrine: Producing and secreting insulin and glucagon.
  • Exocrine: Producing pancreatic juice to assist in digestion; contains acinar cells producing and releasing hydrolytic digestive enzymes, with small ducts leading from the acinus into larger ducts leading to the duodenum. Duct cells secrete alkaline HCO3- fluid.

Pancreatic Juice

• Composition:
  • Alkaline fluid formed from secretions of acinar cells and duct cells, consists mostly of water, HCO3-, and hydrolytic enzymes:
  • Pancreatic Amylase: Digests starch.
  • Pancreatic Lipase: Digests fats.
  • Inactive proteases that digest proteins when activated.
  • Nucleases: For the digestion of nucleic acids.

Cholecystokinin (CCK) and Secretin

• Cholecystokinin (CCK):

  • Hormone released from the small intestine in response to fatty chyme; stimulates gallbladder contraction to release bile and pancreas to release pancreatic juice while relaxing smooth muscle in the hepatopancreatic ampulla, allowing bile and pancreatic juice entry into the small intestine while inhibiting stomach motility and gastric secretions.

• Secretin:

  • Released in response to increased chyme acidity; prompts release of alkaline solutions containing HCO3- from the liver and duct cells of the pancreas (neutralizing acidic chyme), while inhibiting gastric secretions and motility.

Lower Gastrointestinal Tract: Large Intestine

• Large Intestine (Large Bowel):
  • A relatively wide tube shorter than the small intestine located in the abdominal pelvic cavity.
  • Functions:
  • Absorbs water and electrolytes from remaining digested material.
  • Compacts watery chyme into feces.
  • Stores feces until eliminated through defecation.
  • Extends from the ileocecal junction to the anus.

Histology and Functions of the Large Intestine

• Histology:
  • Mucosa lined by simple columnar epithelium with numerous goblet cells, lacking villi; has intestinal glands extending inward toward muscularis mucosae that secrete mucin to lubricate undigested material, with many lymphatic nodules in the lamina propria.
  • Muscularis of Cecum and Colon:
  • Outer longitudinal layer is discontinuous; forms teniae coli.

Bacterial Action in the Large Intestine

• Indigenous Microbiota:
  • Normal bacterial flora in the large intestine; break down carbohydrates, proteins, and lipids remaining in chyme; produce carbon dioxide and other substances.
  • Produce B vitamins and vitamin K which are absorbed into the blood.

Motility and Regulation in the Large Intestine

• Peristaltic Movements: Generally weak and sluggish.
• Haustral Churning:
  • Relaxed haustra fill with material, with distension stimulating reflex contractions in muscularis to increase churning and move material distally.
• Mass Movements:
  • Strong contractions involving teniae coli; propel fecal material towards the rectum, occurring 2-3 times a day, often after a meal.
• Gastrocolic Reflex:
  • Initiated by stomach distension; causes a mass movement.
• Defecation Reflex:
  • Filling of the rectum initiates the urge to defecate via signaling receptors to the spinal cord, increasing parasympathetic output to the sigmoid colon and rectum and decreasing output to the internal (involuntary) anal sphincter.

Clinical View: Colorectal Cancer

• Colorectal Cancer:
  • The second most common type of cancer in the U.S.; can occur anywhere in the large intestine or rectum.
  • Most often arises from polyps (outgrowth from the colon mucosa).
  • Risk factors include a low fiber diet, family history, age, and a history of ulcerative colitis.
  • Symptoms may include rectal bleeding, abdominal pain, and weight loss.
  • Surgical removal is necessary; screened via fecal occult blood tests, sigmoidoscopy, and colonoscopy.

Clinical View: Diverticulosis and Diverticulitis

• Diverticulosis:
  • Presence of small bulges (diverticula) in the intestinal lining, typically formed when the colon narrows due to low fiber.
• Diverticulitis:
  • Inflammation of diverticula, which can become life-threatening if they rupture and spill intestinal contents.

Clinical View: Constipation and Diarrhea

• Constipation:
  • Usually temporary; impaired ability to defecate; compacted feces become difficult to eliminate due to low fiber, dehydration, lack of exercise, poor bowel habits, or anesthesia.
• Diarrhea:
  • May result from disruption of normal intestinal water absorption, for example, from osmotically active solutes, causing large volumes of water to move into the colon.

Nutrients and Their Digestion: Carbohydrate Digestion

• Carbohydrates:

  • Types:
  • Monosaccharides (e.g., glucose, fructose).
  • Disaccharides (e.g., sucrose, maltose).
  • Polysaccharides (e.g., starch and cellulose).

• Carbohydrate Breakdown Process:

  • Starch is broken down into individual glucose molecules; disaccharides into individual monosaccharides.
  • Main sites for carbohydrate digestion are in the oral cavity and small intestine.

Carbohydrate Breakdown in the Oral Cavity

• Process:
  • Catalyzed by salivary amylase from salivary glands, which breaks bonds between glucose molecules within starch; inactivated by low pH of the stomach.
  • Occurs within 15 to 20 minutes after ingestion, with no new carbohydrate enzymes present in the stomach.

Pancreatic Enzymes in Carbohydrate Digestion

• Pancreatic Amylase:
  • Synthesized and released by the pancreas as a component of pancreatic juice; continues starch digestion into shorter strands of glucose, maltose, and individual glucose.

Carbohydrate Breakdown in the Small Intestine

• Brush Border Enzymes:
  • Complete starch breakdown, including dextrinase, glucoamylase, and maltase to digest disaccharides (e.g., lactase digests lactose to glucose and galactose).
• Absorption of Monosaccharides:
  • Absorbed across small intestinal epithelial lining; venous blood transports fructose and galactose to the liver, where they are converted into glucose.
  • Glucose is utilized for energy, converted to glycogen, or stored as fat.
• Cellulose:
  • Part of the plant cell wall; not digestible due to the lack of enzymes, thus contributes dietary fiber (bulk) to lumen content.

Nutrients and Their Digestion: Protein Digestion

• Proteins:
  • Digestion releases individual amino acids; broken down by enzymes targeting peptide bonds between amino acids.
  • Released as inactive enzymes to prevent breakdown of proteins in the cells.

Protein Breakdown Process in the Stomach

• Initial Breakdown:
  • Begins in the gastric lumen with pepsin, formed from inactive precursor pepsinogen;
  • Stomach's low pH activates pepsinogen to pepsin and denatures proteins, facilitating chemical breakdown.

Protein Breakdown in the Small Intestine

• Enzymes Released from Pancreas:
  • Continue protein digestion:
  • Trypsinogen, chymotrypsinogen, procarboxypeptidase; released in inactive form.
• Activation:
  • Enteropeptidase (synthesized in the small intestine) activates trypsinogen to trypsin, which further activates more enzymes for protein breakdown.
  • Enzymes help produce smaller strands of peptides.
• Dipeptidase:
  • Breaks final bonds between two amino acids.
• Free Amino Acids:
  • Absorbed across the small intestinal epithelial lining; utilized as building blocks for new proteins or converted into glucose.

Nutrients and Their Digestion: Lipid Digestion

• Lipids:
  • Non-water-soluble, highly variable structures.
  • Triglycerides: Composed of glycerol and three fatty acids; require enzymes for bond breakdown.
  • Cholesterol: Does not need to be broken down for absorption.

Lipid Breakdown Process in the Stomach

• Enzymes:
  • Lingual Lipase: Component of saliva; activated in the stomach.
  • Gastric Lipase: Produced by chief cells in the stomach; together, these digest approximately 30% of triglycerides into diglycerides and fatty acids.

Pancreatic Lipase in LIdid Digestion

• Pancreatic Lipase:
  • Produced by the pancreas and released into the duodenum; digests triglycerides into monoglycerides and two fatty acids. Requires separation of large lipid droplets into smaller droplets, termed emulsification.

Lipid Breakdown in the Small Intestine

• Emulsification:
  • Occurs through action of bile salts (amphipathic molecules), allowing pancreatic lipase greater access to triglycerides.
• Micelles Formation:
  • Digested triglycerides, cholesterol, other lipids, and fat-soluble vitamins are encapsulated by bile salts into micelles for absorption.

Lipid Absorption

• Transport to Epithelial Cells:
  • Lipids enter cells via micelles; bile salts are reused, remaining in the lumen.
• Inside Epithelial Cells:
  • Triglycerides are reformed and combined with cholesterol and other lipids, wrapped in protein to form chylomicrons, which are released via exocytosis.
  • Chylomicrons enter the lacteals (lymphatic capillaries), continue into the bloodstream, and deliver lipids to the liver and other tissues.