Digestive System

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Chapter 24

• Digestive System

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Digestive System Anatomy

• Digestive tract
• Alimentary tract or canal
• GI tract
• Accessory organs
• Primarily glands
• Regions
• Mouth or oral cavity
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Anus

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Functions

• Ingestion Introduction of food into stomach
• Mastication Chewing
• Propulsion
• Deglutition Swallowing
• Peristalsis Moves material through digestive
  tract

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Functions

• Mixing Segmental contraction that occurs in
  small intestine
• Secretion Lubricate, liquefy, digest
• Digestion Mechanical and chemical
• Absorption Movement from tract into circulation
  or lymph
• Elimination Waste products removed from body

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Digestive Tract Histology
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Digestive System Regulation

• Nervous regulation
• Involves enteric nervous system
• Types of neurons sensory, motor, interneurons
• Coordinates peristalsis and regulates local
  reflexes

• Chemical regulation
• Production of hormones
• Gastrin, secretin
• Production of paracrine chemicals
• Histamine
• Help local reflexes in ENS control digestive
  environments as pH levels

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Peritoneum and Mesenteries

• Peritoneum
• Visceral Covers organs
• Parietal Covers interior surface of body wall
• Retroperitoneal Behind peritoneum as kidneys,
  pancreas, duodenum
• Mesenteries
• Routes which vessels and nerves pass from body
  wall to organs
• Greater omentum
• Lesser omentum

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Oral Cavity

• Mouth or oral cavity
• Vestibule Space between lips or cheeks and
  alveolar processes
• Oral cavity proper
• Lips (labia) and cheeks
• Palate Oral cavity roof
• Hard and soft
• Palatine tonsils
• Tongue Involved in speech, taste, mastication,
  swallowing

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Teeth

• Two sets
• Primary, deciduous, milk Childhood
• Permanent or secondary Adult (32)
• Types
• Incisors, canine, premolar and molars

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Teeth
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Salivary Glands

• Produce saliva
• Prevents bacterial infection
• Lubrication
• Contains salivary amylase
• Breaks down starch
• Three pairs
• Parotid Largest
• Submandibular
• Sublingual Smallest

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Pharynx and Esophagus

• Esophagus
• Transports food from pharynx to stomach
• Passes through esophageal hiatus (opening) of
  diaphragm and ends at stomach
• Hiatal hernia
• Sphincters
• Upper
• Lower

• Pharynx
• Nasopharynx
• Oropharynx Transmits food normally
• Laryngopharynx Transmits food normally

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Deglutition (Swallowing)

• Three phases
• Voluntary
• Bolus of food moved by tongue from oral cavity to
  pharynx
• Pharyngeal
• Reflex Upper esophageal sphincter relaxes,
  elevated pharynx opens the esophagus, food pushed
  into esophagus
• Esophageal
• Reflex Epiglottis is tipped posteriorly, larynx
  elevated to prevent food from passing into larynx

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Phases of Deglutition (Swallowing)
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Stomach Anatomy

• Openings
• Gastroesophageal To esophagus
• Pyloric To duodenum
• Regions
• Cardiac
• Fundus
• Body
• Pyloric

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Stomach Histology

• Layers
• Serosa or visceral peritoneum Outermost
• Muscularis Three layers
• Outer longitudinal
• Middle circular
• Inner oblique
• Submucosa
• Mucosa

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Stomach Histology

• Rugae Folds in stomach when empty
• Gastric pits Openings for gastric glands
• Contain cells
• Surface mucous Mucus
• Mucous neck Mucus
• Parietal Hydrochloric acid and intrinsic factor
• Chief Pepsinogen
• Endocrine Regulatory hormones

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Hydrochloric Acid Production
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Phases of Gastric Secretion
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Movements in Stomach
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Small Intestine

• Site of greatest amount of digestion and
  absorption
• Divisions
• Duodenum
• Jejunum
• Ileum Peyers patches or lymph nodules
• Modifications
• Circular folds or plicae circulares, villi,
  lacteal, microvilli
• Cells of mucosa
• Absorptive, goblet, granular, endocrine

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Small Intestine Secretions

• Mucus
• Protects against digestive enzymes and stomach
  acids
• Digestive enzymes
• Disaccharidases Break down disaccharides to
  monosaccharides
• Peptidases Hydrolyze peptide bonds
• Nucleases Break down nucleic acids
• Duodenal glands
• Stimulated by vagus nerve, secretin, chemical or
  tactile irritation of duodenal mucosa

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Duodenum and Pancreas
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Duodenum Anatomy and Histology
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Liver

• Lobes
• Major Left and right
• Minor Caudate and quadrate
• Ducts
• Common hepatic
• Cystic
• From gallbladder
• Common bile
• Joins pancreatic duct at hepatopancreatic ampulla

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Functions of the Liver

• Bile production
• Salts emulsify fats, contain pigments as
  bilirubin
• Storage
• Glycogen, fat, vitamins, copper and iron
• Nutrient interconversion
• Detoxification
• Hepatocytes remove ammonia and convert to urea
• Phagocytosis
• Kupffer cells phagocytize worn-out and dying red
  and white blood cells, some bacteria
• Synthesis
• Albumins, fibrinogen, globulins, heparin,
  clotting factors

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Blood and Bile Flow
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Duct System
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Gallbladder

• Bile is stored and concentrated
• Stimulated by cholecystokinin and vegal
  stimulation
• Dumps into small intestine
• Production of gallstones possible
• Drastic dieting with rapid weight loss

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Pancreas

• Anatomy
• Endocrine
• Pancreatic islets produce insulin and glucagon
• Exocrine
• Acini produce digestive enzymes
• Regions Head, body, tail

• Secretions
• Pancreatic juice (exocrine)
• Trypsin
• Chymotrypsin
• Carboxypeptidase
• Pancreatic amylase
• Pancreatic lipases
• Enzymes that reduce DNA and ribonucleic acid

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Bicarbonate Ion Production
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Pancreatic Secretion Control
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Large Intestine

• Extends from ileocecal junction to anus
• Consists of cecum, colon, rectum, anal canal
• Movements sluggish (18-24 hours)

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Large Intestine

• Cecum
• Blind sac, vermiform appendix attached
• Colon
• Ascending, transverse, descending, sigmoid
• Rectum
• Straight muscular tube
• Anal canal
• Internal anal sphincter (smooth muscle)
• External anal sphincter (skeletal muscle)
• Hemorrhoids Vein enlargement or inflammation

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Secretions of Large Intestine

• Mucus provides protection
• Parasympathetic stimulation increases rate of
  goblet cell secretion
• Pumps
• Exchange of bicarbonate ions for chloride ions
• Exchange of sodium ions for hydrogen ions
• Bacterial actions produce gases called flatus

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Histology of Large Intestine
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Movement in Large Intestine

• Mass movements
• Common after meals
• Local reflexes in enteric plexus
• Gastrocolic Initiated by stomach
• Duodenocolic Initiated by duodenum
• Defecation reflex
• Distension of the rectal wall by feces
• Defecation
• Usually accompanied by voluntary movements to
  expel feces through abdominal cavity pressure
  caused by inspiration

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Reflexes in Colon and Rectum
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Digestion, Absorption, Transport

• Digestion
• Breakdown of food molecules for absorption into
  circulation
• Mechanical Breaks large food particles to small
• Chemical Breaking of covalent bonds by digestive
  enzymes
• Absorption and transport
• Molecules are moved out of digestive tract and
  into circulation for distribution throughout body

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Carbohydrates

• Consist of starches, glycogen, sucrose, lactose,
  glucose, fructose
• Polysaccharides broken down to monosaccharides
• Monosaccharides taken up by active transport or
  facilitated diffusion and carried to liver
• Glucose is transported to cells requiring energy
• Insulin influences rate of transport

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Lipids

• Include triglycerides, phospholipids, steroids,
  fat-soluble vitamins
• Emulsification breaks down large lipid droplets
  to small

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Lipoproteins

• Types
• Chylomicrons
• Enter lymph
• VLDL
• LDL
• Transports cholesterol to cells
• HDL
• Transports cholesterol from cells to liver

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Proteins

• Pepsin breaks proteins into smaller polypeptide
  chains
• Proteolytic enzymes produce small peptide chains
• Dipeptides, tripeptides, amino acids

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Water and Ions

• Water
• Can move in either direction across wall of small
  intestine depending on osmotic gradients
• Ions
• Sodium, potassium, calcium, magnesium, phosphate
  are actively transported

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Effects of Aging

• Decrease in mucus layer, connective tissue,
  muscles and secretions
• Increased susceptibility to infections and toxic
  agents
• Ulcerations and cancers