The Digestive System

• Digestive System Divisions

  • Alimentary canal (GI tract)
  • Organs: oral cavity (mouth), pharynx (throat), esophagus, stomach, small intestine, large intestine, anus
  • Accessory digestive organs: Teeth, tongue, gallbladder, Salivary glands, liver, pancreas
• Learning Objectives

  • List and define the major processes occurring during digestive system activity
  • Describe the general processes of digestion and absorption
  • List the enzymes involved in digestion and name the foodstuffs on which they act
  • List the end products of protein, fat, and carbohydrate digestion
  • Describe the processes by which breakdown products of foodstuffs are absorbed in the small intestine
• GI hormones


  Maximizing digestion and absorption
• Absorption

  Takes monomers into the bloodstream to be used by the cells
• Digestive (alimentary) tract

  • Open at both ends and continuous with the environment
  • Considered "outside" the body
  • Materials that cannot be digested (cellulose) never actually "enter" the body
  • One-way transport allows for specialization of function along the tract
• Regulation of Digestive Function

  • Autonomous smooth muscle function
  • Intrinsic nerve plexuses: Enteric nervous system
  • Extrinsic nerves: Parasympathetic and Sympathetic system
  • GI hormones
• Digestive Tract Functions

  • Motility: movement of food through the tract
  • Secretion: Exocrine and Endocrine
  • Digestion: breaking food down into smaller units, both physically and chemically
  • Absorption: passing broken-down food into blood or lymph
  • Storage and elimination: temporary storage and subsequent elimination of undigested food molecules
  • Immune barrier
• Sympathetic system

  Inhibits digestion
• Lumen

  Inside of digestive tract
• Digestion

  Breaks polymers (carbohydrates, fats, and proteins) into monomer building blocks via hydrolysis reactions
• Parasympathetic system

  Enhances digestive processes
• Mechanical Digestion

  • Provides surface area
  • Provides lubrication
• Sympathetic system

  Enhances digestive processes
• Esophagus

  • About 10 inches long
  • Mouth, pharynx, and upper esophagus lined with skeletal muscles innervated by somatic motor neurons
  • Lower esophagus lined with smooth muscle controlled by autonomic nervous system
  • Peristalsis – wavelike muscular contractions
  • Lower esophageal (gastroesophageal) sphincter opens to allow food to pass into stomach. It stays closed to prevent regurgitation
• Gastric Secretions

  1. Protects stomach lining

  2. Alkaline solution

  3. Secretes action in

  4. Stimulates parietal, Chief, ECL Cells

  5. Formation of HCl by Parietal Cells
• Mouth (Oral Cavity)

  1. Mastication (chewing): teeth mechanically break food down into smaller pieces for deglutition (swallowing) and mixes it with saliva

  2. Saliva contains mucus, an antimicrobial agent (lysozyme), and salivary amylase to start chemical digestion of starch
• The Swallowing Reflex

  1. Swallowing (deglutition) moves food from mouth to stomach

  2. Oral: voluntary; muscles of mouth and tongue mix food with saliva to form a bolus

  3. Pharyngeal: involuntary - Uvula (soft palate) lifts to cover nasopharynx, and the epiglottis covers vocal cords - Upper esophageal sphincter relaxes

  4. Esophageal: autonomic; controlled by the swallowing center of brain stem; bolus is moved down esophagus to stomach via peristalsis - Most complex reflex in body occurs in less than 1 sec
• GI hormones

  • Maximizing digestion and absorption
  • Regulates muscle contraction
  • Pacemaker cell has its own nervous system
  • Gut Brain occurs in
  • Comes from autonomic nervous system, rest & digest
  • Dominates in "fight or flight"
• Salivary Secretion

  1. Continuous spontaneous basal rate of 0.5mL/min

  2. Important in keeping mouth and throat moist at all times

  3. Can be increased by reflex

  4. Simple and conditioned (acquired) salivary reflexes

  5. Secretes 1-2 liters of Saliva per day

  6. In Afferent Control Center Pathway Efferent C Learned response based on experiences

  7. Starts before Simple reflex
• Stomach

  1. Stores food

  2. Churns food to mix with gastric secretions

  3. Begins protein digestion

  4. Kills bacteria in the food (acid)

  5. Moves food into small intestine in the form of a pasty material called chyme

  6. Food is delivered from the esophagus to the cardiac region

  7. Storage – upper stomach

  8. Digestion – lower stomach

  9. Lining has folds called rugae

  10. Regulates the food that enters duodenum (pepsin is used to break down proteins)
• Secretion of HCl

  Stimulated by gastrin, histamine, and ACh
• Zollinger-Ellison syndrome – ulcers due to high amounts of gastrin produced by a tumor in the duodenum or pancreas
• Helicobacter pylori: bacterium that reduces mucosal barriers to acid; most common cause of peptic and duodenal ulcers
• Small Intestine completes digestion of carbohydrates, proteins, and fats and absorbs nutrients
• Villi contain simple columnar epithelium (enterocytes) with goblet cells, lymphocytes, blood capillaries, and a lymphatic vessel called a lacteal
• Prostaglandins are needed to stimulate protective alkaline mucus production. NSAIDs inhibit prostaglandin activity
• Secretion of bicarbonate

  Parietal cells secrete Clо as well as H+ into gastric juice while secreting bicarbonate into the blood
• Peptic ulcers: erosions of the mucosa of the stomach or duodenum produced by HCl and/or pepsin
• Microvilli (brush border) are folding of the apical surface of each epithelial cell of the villus
• Small Intestine has villi and microvilli that increase surface area for absorption
• Small Intestine has three sections: Duodenum, Jejunum, Ileum
• Treatment for ulcers combines K+/H+ pump inhibitors and 2 different antibiotics
• Most common causes of peptic ulcers are nonsteroidal anti-inflammatory drugs (NSAIDs) and Helicobacter pylori
• Gastroesophageal reflux disease (GERD) – reflux of the acidic gastric chyme produces symptoms including heartburn, cough, and sore throat
• Capillaries absorb monosaccharides and amino acids, and lacteals absorb fats
• Sugars, lipids, amino acids, calcium, and iron are absorbed in the duodenum and jejunum, while bile salts, vitamin B12, water, and electrolytes are absorbed in the ileum
• Capillaries and lacteals absorption

  Capillaries absorb monosaccharides and amino acids, and lacteals absorb fats
• Functions of the Large Intestine
• Intestinal contractions and motility
• Microbiota colonization and early childhood