Chapter 23 - Digestive System

Overview

• The alimentary canal or gastrointestinal (GI) tract digests and absorbs food
• Alimentary canal - mouth, pharynx, esophagus, stomach, small intestine, and large intestine
• Accessory digestive organs - teeth, tongue, gallbladder, salivary glands, liver, and pancreas
• Digestive Process - GI tract is a "disassembly" line
  • Nutrients become more available to the body in each step
  • There are six essential activities:
    • Ingestion, propulsion, and mechanical digestion
    • Chemical digestion, absorption, and defecation
• Gastrointestinal Tract Activities
  • Ingestion - taking food into the digestive tract
  • Propulsion - swallowing and peristalsis
  • Peristalsis - waves of contraction and relaxation of muscles in the organ walls
  • Mechanical digestion - chewing, mixing, and churning food
  • Chemical digestion - catabolic breakdown of food
  • Absorption - movement of nutrients from the GI tract to the blood or lymph
  • Defecation - elimination of indigestible solid wastes

Histology of the Alimentary Canal

• From esophagus to anal canal the GI tract walls have the same 4 tunics
• From the lumen outward they are the mucosa, submucosa, muscularis externa, and serosa
• Mucosa
  • Moist epithelial layer that lines the lumen of the alimentary canal
  • Its three major functions are:
    • Secretion of mucus
    • Absorption of the end products of digestion
    • Protection against infectious disease
  • Consists of three layers:
    • Epithelium - simple columnar epithelium and goblet cells
      • The mucus secretions:
        • Protect digestive organs from digesting themselves
        • Ease food along the tract
      • Stomach and small intestine mucosa contain:
        • Enzyme-secreting cells
        • Hormone-secreting cells (endocrine and digestive organs)
    • Lamina Propria - Loose areolar and reticular connective tissue
      • Nourishes the epithelium and absorbs nutrients
      • Contains lymph nodes (part of MALT) important in defense against bacteria
    • Muscularis mucosae - smooth muscle cells that produce local movements of mucosa
• Submucosa- nourish, transport; nerve supply for secretions
  • Dense connective tissue containing elastic fibers, blood and lymphatic vessels, lymph nodes
  • Enteric nerve supply - regulates glands and mucosal smooth muscle
    • Submucosal plexus (parasympathetic)
• Muscularis externa - responsible for segmentation and peristalsis
  • Enteric Nerve supply - myenteric plexus control sympathetic & parasympathetic
  • Two layers of muscle - circular and longitudinal
    • Segmentation and peristalsis are largely automatic involving local reflex arcs
    • Linked to the CNS via long autonomic reflex arc
• Serosa - the protective visceral peritoneum
  • Replaced by the fibrous adventitia in the esophagus

Mouth - Oral or buccal cavity

• Is bounded by lips, cheeks, palate, and tongue
• Is continuous with the oropharynx posteriorly
• To withstand abrasions:
  • The mouth is lined with stratified squamous epithelium
  • Gums, hard palate, and dorsum of the tongue are slightly keratinized
• Lips and Cheeks
  • Vestibule - bounded by the lips and cheeks externally, and teeth and gums internally
  • Oral cavity proper - area that lies within the teeth and gums
  • Labial frenulum - median fold that joins the internal aspect of each lip to the gum
• Palate
  • Hard palate - underlain by palatine bones and palatine processes of the maxillae
    • Assists the tongue in chewing
    • Slightly corrugated on either side of the raphe
  • Soft palate - mobile fold formed mostly of skeletal muscle
    • Closes off the nasopharynx during swallowing
    • Uvula projects downward from its free edge
• Tongue
  • Functions include:
    • Gripping and repositioning food during chewing
    • Mixing food with saliva and forming the bolus
    • Initiation of swallowing, and speech
  • Lingual frenulum secures the tongue to the floor of the mouth
  • Superior surface bears three types of papillae
    • Filiform - give the tongue roughness and provide friction
    • Fungiform - scattered over the tongue and give it a reddish hue
    • Circumvallate - V-shaped row in back of tongue

Salivary Glands

• Intrinsic salivary glands (buccal glands) - scattered throughout the oral mucosa
• Three pairs of extrinsic glands - parotid, submandibular, and sublingual
  • Parotid - lies anterior to the ear between the masseter muscle and skin
    • Parotid duct - opens into vestibule next to the second upper molar
  • Submandibular - lies along the medial aspect of the mandibular body
    • Its ducts open at the base of the lingual frenulum
  • Sublingual - lies anterior to the submandibular gland under the tongue
    • It opens via 10-12 ducts into the floor of the mouth
• Produce and secrete saliva that:
  • Cleanses the mouth; moistens and dissolves food chemicals
  • Aids in bolus formation
  • Contains enzymes that break down starch
• Source and composition
  • Secreted from serous and mucous cells of salivary glands
    • 97-99.5% water, hypo-osmotic, slightly acidic solution containing
    • Electrolytes - Na+, K+, Cl-, PO42-, HCO3-
    • Digestive enzyme - salivary amylase
    • Proteins - mucin, lysozyme, defensins, and IgA
• Control of salivation
  • Intrinsic glands keep the mouth moist
  • Extrinsic salivary glands secrete serous, enzyme-rich saliva in response to:
    • Ingested food which stimulates chemoreceptors and pressoreceptors
    • The thought of food
  • Strong sympathetic stimulation inhibits salivation and results in dry mouth

Teeth

• Primary and permanent dentitions have formed by age 21
• Primary - 20 deciduous teeth that erupt at intervals between 6 and 24 months
• Permanent - enlarge and develop causing the root of deciduous teeth to be resorbed and fall out between the ages of 6 and 12 years
  • All but the third molars have erupted by the end of adolescence
  • Usually 32 permanent teeth
• Teeth Classification -shape & function
  • Incisors - chisel-shaped teeth adapted for cutting or nipping
  • Canines - conical or fanglike teeth that tear or pierce
  • Premolars (bicuspids) and molars - have broad crowns with rounded tips and are best suited for grinding or crushing
• Tooth Strucutre
  • Crown - exposed part of the tooth above the gingiva (gum)
  • Enamel - acellular, brittle material composed of calcium salts and hydroxyapatite crystals
  • Root - portion of tooth embedded in the jawbone
  • Cementum - calcified connective tissue
    • Attaches to periodontal ligament
  • Periodontal ligament
    • Anchors tooth in the alveolus
    • Fibrous joint called a gomaphosis
  • Dentin - bonelike material deep to the enamel cap that forms bulk of the tooth
  • Pulp cavity - cavity surrounded by dentin that contains pulp
  • Pulp - connective tissue, blood vessels, and nerves
  • Root canal - portion of the pulp cavity that extends into the root
• Tooth and Gum Disease
  • Dental caries - gradual demineralization of enamel and dentin by bacterial action
    • Dental plaque, a film of sugar, bacteria, and mouth debris, adheres to teeth
    • Acid produced by the bacteria in the plaque dissolves calcium salts
    • Without these salts, organic matter is digested by proteolytic enzymes
    • Daily flossing and brushing help prevent caries by removing forming plaque
  • Gingivitis -as plaque accumulates, it calcifies and forms calculus, or tartar
    • Accumulation of calculus:
      • Disrupts the seal between the gingivae and the teeth
      • Puts the gums at risk for infection
  • Periodontitis - serious gum disease resulting from an immune response
    • Immune system attacks intruders as well as body tissues, carving pockets around the teeth and dissolving bone

To the Stomach

• Pharynx
  • From the mouth, the oro- and laryngopharynx allow passage of:
  • Food and fluids to the esophagus
  • Air to the trachea
  • Lined with stratified squamous epithelium and mucus glands
• Esophagus
  • Muscular tube going from the laryngopharynx to the stomach
  • Travels through the mediastinum and pierces the diaphragm
  • Joins the stomach at the cardiac orifice
• Deglutition (swallowing)
  • Involves the coordinated activity of the tongue, soft palate, pharynx, esophagus and 22 separate muscle groups
  • Buccal phase - bolus is forced into the oropharynx
  • Pharyngeal-esophageal phase - controlled by the medulla and lower pons - all routes except into the digestive tract are sealed off
  • Peristalsis moves food through the pharynx to the esophagus

Stomach

• Chemical breakdown of proteins begins and food is converted to chyme
• Gross Anatomy
  • Cardiac region - surrounds the cardiac orifice
  • Fundus - dome-shaped region beneath the diaphragm
  • Body - midportion of the stomach
  • Pyloric region -made up of antrum and canal which terminates at pylorus
  • The pylorus is continuous with duodenum through the pyloric sphincter
  • Greater curvature - entire extent of the convex lateral surface
  • Lesser curvature - concave medial surface
  • Lesser omentum - runs from the liver to the lesser curvature
  • Greater omentum -drapes from greater curvature to small intestine
• Microscopic Anatomy
  • Muscularis - has an additional oblique layer that:
    • Allows the stomach to churn, mix, and pummel food physically
    • Breaks down food into smaller fragments
  • Epithelial lining is composed of:
    • Goblet cells that produce a coat of alkaline mucus
    • The mucous surface layer traps a bicarbonate-rich fluid beneath it
  • Gastric pits contain gastric glands that secrete gastric juice, mucus, gastrin
• Glands and Digestion in the Stomach
  • Chief cells - produce pepsinogen
    • Pepsinogen is activated to pepsin by HCl(from the parietal cellS)in the stomach
  • Enteroendocrine cells - secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria
  • Stomach Lining
    • The stomach is exposed to harshest conditions in the digestive tract
    • To keep from digesting itself, the stomach has a mucosal barrier with:
      • A thick coat of bicarbonate-rich mucus on the stomach wall
      • Epithelial cells that are joined by tight junctions
      • Gastric glands that have cells impermeable to HCl
  • Digestion in the stomach
    • Holds ingested food
    • Degrades this food both physically and chemically
    • Delivers chyme to the small intestine
    • Enzymatically digests proteins with pepsin
    • Secretes intrinsic factor required for absorption of vitamin B₁₂
• Regulation of Gastrin Secretion
  • Neural and hormonal mechanisms regulate the release of gastric juice
  • Stimulatory and inhibitory events occur in three phases
    • Cephalic (reflex) phase: prior to food entry
      • Excitatory events include: sight, taste, smell or thought of food
      • Inhibitory events include: loss of appetite or depression
    • Gastric phase: once food enters the stomach
      • Excitatory events include: Release of gastrin to the blood
        • Activation of stretch receptors (neural activation)
        • Activation of chemoreceptors by peptides, caffeine, and rising pH
      • Inhibitory events include: A pH lower than 2; emotional upset that overrides the parasympathetic division
    • Intestinal phase: as partially digested food enters the duodenum
      • Excitatory phase - low pH; partially digested food enters the duodenum and encourages gastric gland activity
      • Inhibitory phase - distension of duodenum, presence of fatty, acidic, or hypertonic chyme, and/or irritants in the duodenum
        • Initiates inhibition of local reflexes and vagal nuclei
        • Closes the pyloric sphincter
• Response of Stomach to Filling
  • Stomach pressure remains constant until about 1L of food is ingested
  • Relative unchanging pressure results from reflex-mediated relaxation and plasticity
    • Receptive relaxation - as food travels in the esophagus, stomach muscles relax
    • Adaptive relaxation - the stomach dilates in response to gastric filling
    • Plasticity - intrinsic ability of smooth muscle to exhibit the stress-relaxation response
• Gastric Contractile Activity
  • Peristaltic waves move toward the pylorus at the rate of 3 per minute
  • This basic electrical rhythm is initiated by pacemaker cells (cells of Cajal)
  • Most vigorous peristalsis and mixing occurs near the pylorus
  • Chyme is either:
    • Delivered in small amounts to the duodenum or
    • Forced backward into the stomach for further mixing
• Regulation of Gastric Emptying
  • Gastric emptying is regulated by:
    • The neural enterogastric reflex
    • Hormonal (enterogastrone) mechanisms
  • These mechanisms inhibit gastric secretion and duodenal filling
  • Carbohydrate-rich chyme quickly moves through the duodenum
  • Fat-laden chyme is digested more slowly causing food to remain in the stomach longer

Small Intestine

• Gross Anatomy
  • Runs from pyloric sphincter to the ileocecal valve
  • Has three subdivisions: duodenum, jejunum, and ileum
  • The bile duct and main pancreatic duct:
    • Join the duodenum at the hepatopancreatic ampulla
    • Are controlled by the sphincter of Oddi
  • The jejunum extends from the duodenum to the ileum
  • The ileum joins the large intestine at the ileocecal valve
• Microscopic Anatomy - Modifications
  • Plicae circulares: deep circular folds of the mucosa and submucosa
  • Villi - fingerlike extensions of the mucosa
  • Microvilli - tiny projections of absorptive mucosal cell plasma membranes
• Histology of the Wall
  • The epithelium of the mucosa is made up of:
    • Absorptive cells and goblet cells
    • Enteroendocrine cells
    • Interspersed T cells called intraepithelial lymphocytes (IELs)
  • Cells of intestinal crypts secrete intestinal juice
    • Slightly alkaline and isotonic with blood plasma
    • Largely water, enzyme-poor, but contains mucus
  • Peyer's patches are found in the submucosa
  • Brunner's glands in the duodenum secrete alkaline mucus

Liver

• The largest gland in the body
• Superficially has four lobes - right, left, caudate, and quadrate
• Falciform ligament:
  • Separates the right and left lobes anteriorly
  • Suspends the liver from the diaphragm and anterior abdominal wall
• The gallbladder rests in a recess on the inferior surface of the right lobe
  • Hepatic bile ducts carry bile to the common hepatic duct
    • The common hepatic duct, which fuses with the cystic duct
    • These two ducts form the common bile duct
• Microscopic Anatomy
• Hexagonal-shaped liver lobules are the structural and functional units of the liver
• Portal triads consist
  • Bile duct
  • Hepatic artery - supplies oxygen-rich blood to the liver
  • Hepatic portal vein - carries venous blood with nutrients from digestive viscera
• Liver sinusoids - enlarged, leaky capillaries located between hepatic plates
• Kupffer cells - hepatic macrophages found in liver sinusoids
• Hepatocytes' functions include:
  • Production of bile
  • Processing bloodborne nutrients
  • Storage of fat-soluble vitamins
  • Detoxification
• Secreted bile flows between hepatocytes toward bile ducts in portal triads

The Gallbladder

• Thin-walled, green muscular sac on the ventral surface of the liver
• Stores and concentrates bile by absorbing its water and ions
• Releases bile via the cystic duct, which flows into the bile duct
• Composition of Bile
  • A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes
  • Bile salts are cholesterol derivatives that:
    • Emulsify fat
    • Facilitate fat and cholesterol absorption
    • Help solubilize cholesterol
  • Enterohepatic circulation recycles bile salts
  • The chief bile pigment is bilirubin, a waste product of heme
• Regulation of Bile Release
  • Acidic, fatty chyme causes the duodenum to release:
    • Cholecystokinin(CCK) and secretin into the bloodstream
  • Bile salts and secretin transported in blood stimulate the liver to produce bile
  • Vagal stimulation causes weak contractions of the gallbladder
  • Cholecystokinin causes:
    • The gallbladder to contract
    • The hepatopancreatic sphincter to relax
  • As a result, bile enters the duodenum

Pancreas

• Location
  • Lies deep to the greater curvature of the stomach
• Exocrine function - Secretes pancreatic juice which breaks down all categories of foodstuff
  • Water solution of enzymes and electrolytes (primarily HCO3-)
    • Neutralizes acid chyme
    • Provides optimal environment for pancreatic enzymes
  • Enzymes are released in inactive form and activated in the duodenum
    • Examples include
      • Trypsinogen is activated to trypsin
      • Procarboxypeptidase is activated to carboxypeptidase
  • Active enzymes secreted
    • Amylase, lipases, and nucleases
    • These enzymes require ions or bile for optimal activity
  • The pancreas also has an endocrine function - release of insulin and glucagon
• Regulation of Pancreatic Secretion
  • Secretin and CCK are released when fatty or acidic chyme enters the duodenum
  • CCK and secretin enter the bloodstream
  • Upon reaching the pancreas:
    • CCK induces the secretion of enzyme-rich pancreatic juice
    • Secretin causes secretion of bicarbonate-rich pancreatic juice
  • Vagal stimulation also causes release of pancreatic juice

Large Intestine

• Gross Anatomy
  • Has three unique features:
    • Teniae coli - three bands of longitudinal smooth muscle in its muscularis
    • Haustra - pocketlike sacs caused by the tone of the teniae coli
      • Sequentially contract in slow segmental contractions as they are stimulated by distension
    • Epiploic appendages - fat-filled pouches of visceral peritoneum
  • Is subdivided into the cecum, appendix, colon, rectum, and anal canal
  • The saclike cecum:
    • Lies below the ileocecal valve in the right iliac fossa
    • Contains a wormlike vermiform appendix
  • Has distinct regions: ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, and sigmoid colon
  • The transverse and sigmoid portions are anchored via mesenteries called mesocolons
  • The sigmoid colon joins the rectum
  • The anal canal, the last segment of the large intestine, opens to the exterior at the anus
• Microscopic Anatomy
  • Colon mucosa is simple columnar epithelium except in the anal canal
  • Has numerous deep crypts lined with goblet cells
  • Anal canal mucosa is stratified squamous epithelium
  • Anal sinuses exude mucus and compress feces
  • Superficial venous plexuses are associated with the anal canal - Inflammation of these veins results in itchy varicosities called hemorrhoids
  • The bacterial flora of the large intestine consist of:
    • Bacteria surviving the small intestine that enter the cecum and
    • Those entering via the anus
  • These bacteria: Colonize the colon
    • Ferment indigestible carbohydrates
    • Release irritating acids and gases (flatus)
    • Synthesize B complex vitamins and vitamin K
  • Functions of - essential for comfort, the colon is not essential for life
    • Vitamins, water, and electrolytes are reclaimed
    • Its major function is propulsion of fecal material toward the anus
• Valves and Sphincters of the Rectum and Anus
  • Three valves of the rectum stop feces from being passed with gas
  • The anus has two sphincters:
    • Internal anal sphincter composed of smooth muscle
    • External anal sphincter composed of skeletal muscle
  • These sphincters are closed except during defecation

Defecation

• Presence of food in the stomach:
  • Activates the gastrocolic reflex
  • Initiates peristalsis that forces contents toward the rectum
• Distension of rectal walls caused by feces:
  • Stimulates contraction of the rectal walls
  • Relaxes the internal anal sphincter
• Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs

Chemical Digestion

• Carbohydrates
  • Absorption: via cotransport with Na+, and facilitated diffusion
    • Enter the capillary bed in the villi
    • Transported to the liver via the hepatic portal vein
  • Enzymes: salivary amylase, pancreatic amylase, and brush border enzymes
• Proteins
  • Absorption: similar to carbohydrates
  • Enzymes used: pepsin in the stomach
  • Enzymes acting in the small intestine
    • Pancreatic enzymes - trypsin, chymotrypsin, carboxypeptidase
    • Brush border enzymes - aminopeptidases, carboxypeptidases, and dipeptidases
• Nucleic Acids
  • Absorption: active transport via membrane carriers
  • Absorbed in villi and transported to liver via hepatic portal vein
  • Enzymes used: pancreatic ribonucleases and deoxyribonuclease in the small intestines
• Electrolyte Absorption
  • Most ions are actively absorbed along the length of small intestine
    • Na+ is coupled with absorption of glucose and amino acids
    • Ionic iron is transported into mucosal cells where it binds to ferritin
  • Anions passively follow the electrical potential established by Na+
  • K+ diffuses across intestinal mucosa in response to osmotic gradients
  • Ca2+ absorption:
    • Is related to blood levels of ionic calcium
    • Is regulated by vitamin D and parathyroid hormone (PTH)
• Water Absorption
  • 95% of water is absorbed in the small intestines by osmosis
  • Water moves in both directions across intestinal mucosa
  • Net osmosis occurs whenever a concentration gradient is established by active transport of solutes into the mucosal cells
  • Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients
• Fats
  • Absorption: Diffusion into intestinal cells where they:
    • Combine with proteins and extrude chylomicrons
    • Enter lacteals and are transported to systemic circulation via lymph
  • Glycerol and short chain fatty acids are:
    • Absorbed into the capillary blood in villi
    • Transported via the hepatic portal vein
  • Enzymes/chemicals used: bile salts and pancreatic lipase
• Fatty Acid Absorption
  • Fatty acids and monoglycerides enter intestinal cells via diffusion
  • They are combined with proteins within the cells
  • Resulting chylomicrons are extruded
  • They enter lacteals and are transported to the circulation via lymph

Malabsorption of Nutrients

• Results from anything that interferes with delivery of bile or pancreatic juice
• Factors that damage the intestinal mucosa (e.g., bacterial infection)
• Gluten enteropathy (adult celiac disease) - gluten damages the intestinal villi and reduces the length of microvilli
• Treated by eliminating gluten from diet (all grains but rice and corn)

Cancer

• Stomach and colon cancers rarely have early signs or symptoms
• Metastasized colon cancers frequently cause secondary liver cancer
• Prevention is by regular dental and medical examinations
• Colon cancer is the 2nd largest cause of cancer deaths in males (lung cancer is 1st)
• Forms from benign mucosal tumors called polyps whose formation increases with age
• Regular colon examination should be done for all those over 50

References

• GERD Santrus
• Cell Adhesion Molecule Inhibition BioLucid
• Irritable Bile Syndrome John Hopkins
• Releasing a New Wave in PPI Therapy BioLucid
• GI Drug Delivery - Ulcerative Colitis BioLucid
• Crohns Disease Hybrid Medical

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