Digestive System

I. Overview Diagram

A. Alimentary Canal

• Also called gastrointestinal tract
• Composed of mouth, pharynx, esophagus, stomach, small intestine, and large intestine
• Functions in the digestion and absorption of food
  

B. Accessory Organs

• Include teeth, gallbladder, salivary glands, liver, and pancreas
• Produce secretions to aid in the process of digestion
• Examples: saliva, bile, and digestive enzymes

C. Digestive Process

1. Ingestion
2. Propulsion (swallowing and peristalsis)
3. Mechanical Digestion (chewing, churning, segmentation)
4. Chemical Digestion (saliva, acid and enzymes)
5. Absorption (lymph and blood vessels)
6. Defecation

D. Regulation of the Digestive and Absorptive Process Diagram

i. Digestive activity is provoked by a range of mechanical and chemical stimuli

• Mechanoreceptors and chemoreceptors embedded in the lining of the GI tract
• Receptors respond to stimuli (stretching of the organ, osmolarity and pH of the contents, and the presence of substrates and end products of digestion)
• When stimulated, receptors initiate reflexes
  • activate or inhibit glands that secrete digestive juices into the lumen or hormones into the blood
  • mix lumen contents and move them the length of the tract

ii. Controls of digestive activity are both extrinsic and intrinsic

• Intrinsic - nerve plexuses and local hormone producing cells
• Extrinsic - CNS centers and autonomic nerves

II. Peritoneum, Blood Supply, Tunics, and Nerves

A. Peritoneum Diagram

• Peritoneum is the serous membrane lining the abdominopelvic cavity
• Visceral peritoneum covers the external surfaces of most digestive organs and is continuous with the parietal peritoneum that lines the body wall
• Between the two peritoneums is the peritoneal cavity
• Mesentery is a double layer peritoneum; provides routes for BV, lymphatics, nerves
• Alimentary canal organs are classiefied as
  • Retroperitoneal - no mesentery and organs lies posterior to the peritoneum
  • Intraperitoneal - mesentery and organs lies within the peritoneal cavity
    

B. Blood Supply

• Splanchnic circulation - includes those arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation
• Arterial supply includes hepatic, splenic, and left gastric branches of the celiac trunk that serve the spleen, liver, and stomach, and the mesenteric arteries (superior and inferior) that serve the small and larger intestines

C. Tunics (inner to outer) Diagram

• Mucosa - composed of
  • simple columnar epithelium rich in mucus secreting goblet cells
  • lamina propria that nourish epithelial cells and absorbs nutrients
  • muscularis mucosae containing smooth muscle involved in local motion
• Submucosa - contains blood and lymph
• Muscularis externa - circular and longitudinal muscles involved in segmentation and peristalsis
• Serosa - visceral peritoneum

D. Nerves

• Submucosal nerve plexus - regulates the activity of glands and smooth muscle
• Mesenteric nerve plexus - regulates motility

III. Mouth, Pharynx (oropharynx), and Esophagus

A. Mouth Diagram

• Mucosa lined cavity (oral/buccal cavity)
• Anterior
  • Lips (labia)
  • Orbicularis oris
  • Red margin with nonkeratinized skin and lacks sweat or sebaceous glands
  • Superior/ inferior labial frenulum
  • Alveolar margin with alveolar sockets and periodontal ligaments (gomphoses)
  • Teeth: anatomy Diagram and organization Diagram
    
• Lateral
  • Cheeks (buccinators)
  • Between cheek/lips and gums = vestibule
• Superior
  • Hard and soft palate with uvula
  • Arches = palatoglossal and palatopharyngeal
  • Between arches = fauces or area with palatine tonsils
• Inferior
  • Tongue - intrinsic muscle within tongue and extrinsic muscle extends outward from bone
  • Lingual frenulum
  • Papillae - filiform (smallest, increases friction) and fungiform and circumvallate (taste)
  • Sulcus terminalis divides anterior 2/3 of tongue and posterior 1/3 tongue with lingual tonsil deep to mucosa

B. Pharynx and Salivary Glands

i. Pharynx - nasopharynx superior portion; oropharynx is the posterior portion of oral cavity; leads into the laryngopharynx Diagram

ii. Salivary Glands Diagram

• Functions: (1) cleans mouth, (2) moisten and dissolve food, and (3) contains enzymes
• Saliva produced by intrinsic salivary (buccal) glands scattered throughout oral cavity mucosa and three extrinsic salivary glands: anterior - parotid, medially - submandibular, and under tongue - sublingual
• Composed of two types of cells: serous and mucus
• Composition of Saliva:
  • 97-99.5% water
  • pH 6.75-7.0
  • Sodium, potassium, chloride, phosphate, and bicarbonate
  • Mucin
  • Salivary amylase

C. Esophagus Diagram

• Muscular tube; food routed into esophagus as the epiglottis closes of the larynx
• Runs vertically through the mediastinum of thorax, pierces the diaphragm at the esophageal hiatus, and joins the stomach at the cardiac orifice which is surrounded by cardiac (gastroesophageal) sphincter
• Contains nonkeratinized stratified squamal epithelium until stomach then simple columnar
• When empty, mucosa/submucosa are longitudinal folds but flatten out when food is in transit
• Submucosa contains esophageal glands that secrete lubricating mucus

IV. Digestion (Chewing and Swallowing) Diagram

• Mastication (chewing) - cheeks and closed lips hold the food between the teeth, the tongue mixes the food with saliva to soften it, and the teeth cut and grin solid food
• Deglutition (swallowing) - food compacted by tongue into bolus
• Buccal phase - tip of tongue placed against hard palate, then tongue contracts forcing bolus into oropharynx
• Pharyngeal-esophageal phase - the tongue blocks off the mouth, the soft palate rises to close off the nasopharynx, and larynx rises so that the epiglottis covers its opening into the respiratory passageways

V. Stomach (temporary storage tank and converts bolus into chyme)

A. Macroscopic Anatomy Diagram Diagram

• Greater/lesser curvature serosa (external)
• Cardiac region (cardia) muscularis externa (longitudinal, circular, oblique muscles)
• Fundus submucosa
• Rugae mucosa
• Body
• Pyloric region (pyloric antrum, pyloric canal, pylorus, pyloric sphincter valve)
• Lesser omentum (liver to lesser curvature)
• Greater omentum (greater curvature to cover small intestine)

B. Microscopic Anatomy Diagram Diagram

• Simple columnar epithelium (goblet cells in between)
• Gastric pits with gastric glands
• Glands:
  • Cells of cardia - mucus secreting
  • Cells of pyloric antrum - hormone secreting (gastrin)
  • Cells of body - most other secretions
• Secretory cells:
  • Mucus neck cells -("neck" of gland) produces different type of mucus than surface
  • Parietal (oxyntic) cells - HCl and intrinsic factor - HCl secretion
  • Chief cells (zymogenic) - pepsinogen
  • Enteroendocrine cells - produces:
    • gastrin - cause gastric glands to increase secretory activity (hydrochloric acid)
    • histamine - activates parietal cells to release hydrochloric acid
    • serotonin - causes contraction of stomach
    • cholecystokinin - potentiates secretin's action on liver/pancreas (increase bile and pancreas "juice")
    • somatostatin - inhibits gastric secretion of all products and inhibits gastric motility and emptying

C. Digestive Process (Regulation of Gastric Secretion) Diagram

• Cephalic Phase
  • Triggered by aroma, taste, sight, or thought of food
  • Olfactory and taste buds relay inputs to cerebral cortex and hypothalamus
  • Hypothalamus stimulates the vagal nuclei of the medulla oblongata
  • Motor impulses are transmitted via the vagal nerve to the enteric ganglia which send neurons out to stimulate the stomach glands
• Gastric Phase
  • Triggered by distension, peptides, and low acidity
  • Stomach distension activates stretch receptors, causing acetylcholine (ACh) release, which in turn stimulates the release of gastric juices by secretory cells
  • Peptides, caffeine, and low acidity activate G cells in stomach to release gastrin
• Inestinal Phase
  • Excitatory event - presence of low pH and partially digested foods in duodenum stimulates intestinal mucosa cells to release intestinal (enteric) gastrin
  • Inhibitory event- distension of the duodenum causes
    • enterogastric reflex - inhibition of vagal nuclei of medulla, inhibition local reflexes, ativation of sympatheic fibers causing pyloric shpincter to tighten
    • enterogastrone release - release of of several intestinal hormones which inhibit gastric secretion

VI. Small Intestine (site of absorption) and Accessory Structures

A. Small Intestines

i. Macroscopic Anatomy Diagram

• Pyloric sphincter to ileocecal valve
• Three subdivisions: ileum, jejunum, and duodenum
• Duodenum contains: Diagram
  
  • hepatopancreatic ampulla (liver and pancreatic ducts meet)
  • major duodenal papilla (ampulla opening)
  • hepatopancreatic sphincter (of Oddi)

ii. Microscopic Anatomy

• Huge surface area (increased by structural modifications)
• Plicae circulares - Diagram permanent folds of mucosa/submucosa
• Villi - Diagram fingerlike projections of mucosa, contains blood capillaries and lacteals
• Microvilli - tiny projections of the plasma membrane of absorptive (brush border) cells that contain enzymes (brush border enzymes) used to complete digestion
• Mucosa -containing absorptive cells, goblet cells, enterondocrine cells
• Between villi mucosa is studded with pits that lead into tubular intestinal glands called intestinal crypts or crypts of Lieberkuhn (with cells secreting intestinal juice)
• Also in intestinal crypts are Paneth cells that produce lysozyme (antibacterial enzyme)

B. Liver and Gallbladder Diagram

• Liver - largest gland/organ in the body Diagram
  
• Composed of four lobes:
  • anteriolateral - right and left lobes (separated by falciform ligament)
  • posterior - caudate (superior) and quadrate (inferior)
• Superiormost portion of liver fused with diaphragm
• Lesser omentum anchors liver to lesser curvature of stomach
• Hepatic artery and hepatic portal vein (which enter liver at the porta hepatis) and the common bile duct travel through lesser omentum to reach their destinations
• Right and left hepatic ducts merge to form the common hepatic duct
• The common heptatic duct merges with the cystic duct to form the bile duct
• Liver composed of hexagonal shaped liver lobules containing plates of hepatocytes radiating outward from a central vein
• Portal triads Diagram surround lobule and contain a hepatic artery, a hepatic vein, and a bile duct
• Between hepatocyte plates are capillaries (liver sinusoids) that receive blood from triads and transport blood into central vein; sinusoids contain Kupfer cells (macrophages)
• Hepatic cells:
  • process blood-borne nutrients
  • store fat-soluble vitamins
  • detoxify substances
• Secreted bile flows through tiny canals called bile canaliculi that run between adjacent hepatocytes toward the bile duct branches in portal triads
• Bile (cholesterol and derivatives) emulsify fats
• Mechanisms promoting the secretion of bile:

1. acidic, fatty chyme enters duodenum and causes release of cholecystokinin (CCK) and secretin from duodenum wall enteroendocrine cells
2. CCK and secretin enter bloodstream
3. bile salts and secretin transported via bloodstream stimulate liver to produce bile
4. vagal stimulation causes weak contractions of gallbladder
5. CCK causes gallbladder to contract and hepatopancreatic sphincter to relax: bile enters duodenum
6. bile salts reabsorbed into blood

C. Pancreas

• Extends across the abdomen from its tail (abutting the spleen) to its head (encircled by C- shaped duodenum)
• Contains endocrine cells (alpha and beta) and exocrine cells which produce pancreatic juice
• Pancreatic juice drains from centrally located main pancreatic duct that fuses with the bile duct
• Smaller accessory pancreatic duct dumps into directly into duodenum
• Gland contains acinar cells with zymogen granules (contain digestive enzymes)
• Pancreatic juice contains alkaline secretions and inactive and active enzymes
• Regulation of pancreatic juice secretion similar to bile secretion

VII. Large Intestines    Diagram Diagram

• Frames small intestine on three sides and extends from ileocecal valve to the anus
• Functions to absorb water from indigestible foods and eliminate feces
• Contains bacterium that aid in the synthesis of vitamin K
• Macroscopic Anatomy
  • Teniae coli - three bands of smooth muscle in longitudinal muscularis
  • Haustra - pocketlike sacs
  • Epiploic appendages - fat-filled pouches of visceral peritoneum
• Subdivisions of large intestine: cecum, appendix, colon, rectum, and anal canal
  
  • Ileocecal valve
  • Cecum
  • Vermiform appendix
  • Ascending, Transverse, and Descending colon
  • Right colic (hepatic) and left colic (splenic) flexure
  • Sigmoid colon
  • Rectum
  • Anal canal (rectal valve, levator ani muscle, external/internal sphincters)
    
• Microscopic Anatomy
  • Columnar epithelium
  • No plicae circularis, no villi, no cells secreting digestive enzymes
  • Mucosa is thicker
  • Abundant and deeper crypts
  • Goblet cells produce lubricating mucus
    
• Digestive Process (large intestines)
  
  • Motility:
    1. Haustral contractions - smooth muscle contractions within haustra
    2. Mass movements - powerful contractile waves
  • Reflex:
    1. Gastroileal reflex - from small intestines into large
    2. Gastrocolic reflex - from large intestine into colon
    3. Defecation reflex - from colon into anal canal outward

VIII. Chemical Digestion

• Most food substance(s) chemically hydrolyzed by enzymes

A. Carbohydrates

• Salivary amylase in mouth
• Pancreatic amylase in small intestine
• Brush border enzymes in small intestine (dextrinase, glucoamylase, lactase, maltase, sucrase)

B. Proteins

• Pepsin in stomach - cleaves peptide bonds
• Pancreatic enzymes (trypsin, chymotrypsin, elastase, carboxypeptidases) in small intesine
• Brush border enzymes (aminopeptidases, carboxypeptidases, and dipeptidases) in small intestine

C. Fats

• Bile salts (bile) in small intestine
• pancreatic lipase in small intestine

D. Nucleic acids

• Pancreatic ribonuclease and deoxyribonuclease in small intestine
• Brush border enzymes (nucleosidases and phosphatases) in small intestine

Clinical Terms:

• Peptic ulcers - gastric and duodenal, caused by Helicobacter pylori, NSAIDS, Hcl hypersecretion
• Colorectal cancer
• Cirrohsis - scarred liver due to chronic inflammation
• Hepatitis - A,B,C,D, and E
• Biliary calculi - gall stones - crystals of cholesterol in bile
• Anorexia/bulemia
• Borborygmus - rumbling noise caused by gas through intestines
• Cholecystitis - inflammation of gall bladder (autoimmune)
• Colitis - inflammation of colon (autoimmune)
• Dysphagia - difficulty in swallowing
• Enteritis - inflammation of the intestines
• Flatuation/erucation