3-1 The Digestive System

The Digestive System

• Digestive Function
• Ingestion
• Digestion
• Mechanical – Physical breakdown of food (big pieces to small pieces but same chemical formulae)
• Chemical – Chemical breakdown of food (breaking of chemical bonds in food)
• Absorption
• Compaction
• Defecation

Subdivisions of the Digestive System

• Digestive tract (GI tract)
• 30’ long tube extending from �mouth to anus
• Accessory organs
• teeth, tongue, liver, gallbladder, �pancreas, salivary glands

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Overview of GI tract Functions

• Mouth
• bite, chew, swallow
• Pharynx and esophagus
• transport
• Stomach
• mechanical disruption; absorption of water & alcohol
• Small intestine
• chemical & mechanical digestion & absorption
• Large intestine
• absorb electrolytes & vitamins (B and K)
• Compaction
• Rectum and anus
• Compaction and defecation

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Layers of the GI Tract

1. Mucosal layer

2. Submucosal layer

3. Muscularis layer

4. Serosa layer

Mucosa

• Epithelium
• Stratified squamous
• Mouth, esophagus, & anus)
• Tough
• Simple columnar in the rest
• Secretes enzymes and absorbs nutrients
• Specialized cells (goblet) secrete mucous onto cell surfaces
• Enteroendocrine cells secrete hormones controlling organ function
• Lamina propria
• Thin layer of loose connective tissue
• Contains vessels & lymphatic tissue (MALT)
• Muscularis mucosae
• Thin layer of smooth muscle
• Causes folds to form in mucosal layer
• Increases local movements increasing absorption with exposure to “new” nutrients

Submucosa

• Loose connective tissue
• Contains vessels, glands and lymphatic tissue (MALT)
• Submucosal plexus
• Contributions from parasympathetic post-ganglionic neurons
• Controls movements by muscularis mucosa and glandular secretion
• Long reflexes
• Autonomic nerve fibers communicate between the gut and brain to coordinate digestive activities

Muscularis Externa

• Skeletal muscle = voluntary control
• Mouth, pharynx , upper esophagus and anus
• *Potential* control over swallowing and defecation
• Smooth muscle = involuntary control
• Inner circular fibers & outer longitudinal fibers
• Mixes, compacts, & propels food along by peristalsis and segmentation
• Myenteric plexus
• both parasympathetic & sympathetic innervation of circular and longitudinal smooth muscle layers
• Short reflexes
• Stretching and chemical stimuli in the GI tract stimulate contractions nearby regions of the muscularis externa

Serosa (Peritoneum)

• An example of a serous membrane
• Covers all organs and walls of cavities not open to the outside of the body
• Secretes slippery fluid
• Consists of connective tissue covered with simple squamous epithelium

Peritoneum

• Peritoneum
• visceral layer covers organs
• parietal layer lines the walls of body cavity
• Peritoneal cavity
• potential space containing a bit of serous fluid

Parts of the Peritoneum

• Mesentery
• Mesocolon
• Lesser omentum
• Greater omentum
• Peritonitis (inflammation)
• trauma
• rupture of GI tract
• appendicitis
• perforated ulcer

Stages of Digestion

• Mechanical digestion is physical breakdown of food into smaller particles
• Teeth & churning action of stomach & intestines
• Chemical digestion is series of hydrolysis reactions that break macromolecules into their monomers
• Enzymes from saliva, stomach, pancreas, and intestines
• Polysaccharides 🡪 oligo-, di-, and monosaccharides
• Proteins 🡪 oligo-, tri-, dipepeptides, and amino acids
• Fats 🡪 glycerol and fatty acids

3-2 The Mouth, Pharynx, and Esophagus

Mouth

• Lips and cheeks
• Contains buccinator muscle that keeps food between upper & lower teeth
• Vestibule
• Area between cheeks and teeth
• Oral cavity proper
• Roof = hard, soft palate and uvula
• Floor = the tongue

Tongue

• The tongue, together with associated muscle, forms the floor of the oral cavity
• Composed of skeletal muscle covered with stratified squamous mucous membrane
• Functions:
• Sensation of taste, texture, temperature
• Manipulates food (bolus) for mastication and swallowing
• Secretes enzymes and mucus
• Initiates swallowing
• Assists in speech
• Upper and lateral surfaces of the tongue are covered with papillae, some of which contain taste buds

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Saliva

• 1-1.5 L produced daily
• Functions of saliva
• Moistens and cleanses mouth
• Dissolves food for taste and digestion
• Begin starch & fat digestion
• Binds food together as a bolus
• Hypotonic solutions of >97% water and solutes:
• Amylase = begins starch digestion
• Lingual lipase = digests fat after reaches the stomach
• Mucus = aids in swallowing by binding and lubricating bolus
• Lysozyme = enzyme that kills bacteria
• Immunoglobulin A = inhibits bacterial growth
• Electrolytes = Na⁺, K⁺, Cl^-, phosphate & bicarbonate
• Bicarbonate ions buffer acidic foods, vomiting hurts enamel
• pH of 6.8-7.0
• Serous cells secrete a watery, enzyme-rich fluid
• Mucous cells (pale staining) secrete a slimy, mucus secretion

Salivary Glands

• Parotid Glands (20%)
• Below your ear and over the masseter
• Mostly serous cells
• Mumps: Viral infection of these glands
• Submandibular Glands (70%)
• Under lower edge of mandible
• 50/50 serous and mucus cells
• Sublingual Glands (5%)
• Deep to the tongue in floor of mouth
• Mostly mucus cells
• All have ducts that empty into the oral cavity
• Remaining 5% comes from oral submucosal glands
• Lingual glands secrete lipolytic enzyme that activates in low pH of stomach

Mumps

• Myxovirus that attacks the parotid gland
• Symptoms
• Inflammation and enlargement of the parotid
• Fever, malaise & sore throat (especially swallowing sour foods)
• Swelling on one or both sides
• Sterility (rarely) possible in males with testicular involvement (only one side involved)
• Vaccine available since 1967

Dentition

• Masticate food (PD)
• Primary or baby teeth
• 20 teeth that start erupting at 6 months
• ~1 new pair of teeth per month
• Permanent teeth
• 32 teeth that erupt between 6-25 years of age
• Differing structures indicate function
• incisors for biting
• canines or cuspids for tearing
• premolars & molars for crushing and grinding food

A Treatise on the Method of Curing the Gout, Scurvy, Leprosy, Elephantiasis. Francis Spilsbury, 1774

Composition of Teeth

• Crown, neck, and root
• Enamel
• Hardest substance in body
• Calcium phosphate/carbonate
• Non-repairable
• Dentin and Cementum
• Made by odontocytes and cementocytes
• Calcified, living, bone-like matrix
• Repairable
• Pulp Cavity & Root Canal
• Space within crown, neck, and root
• Contains pulp (CT, vessels, and nerves)
• Apical foramen allows passage of vessels through tooth wall
• Gingiva form ‘gasket’ for alveolar socket
• Scurvy especially affects gingiva
• May be the single greatest cause of death in 16^th century armies
• Also why France had a 19^th century tradition of eating horse

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Digestion in the Mouth

• Mechanical digestion (mastication or chewing)
• Breaks into pieces
• Mixes with saliva so it forms a bolus
• Chemical digestion
• Amylase
• Begins starch digestion at pH of 6.5-7.0 found in mouth
• May digest up to 40% of polysaccharides
• when bolus & enzyme hit the pH 2.5 gastric juices hydrolysis ceases
• Lingual lipase
• secreted by glands in tongue
• begins breakdown of triglycerides into fatty acids and glycerol

Pharynx

• Funnel-shaped tube extending from internal nares to the esophagus (posteriorly) and larynx (anteriorly)
• Skeletal muscle lined by mucous membrane
• Deglutition or swallowing is facilitated by saliva and mucus
• starts when bolus is pushed into the oropharynx
• sensory nerves send signals to deglutition center in brainstem
• soft palate is lifted to close nasopharynx
• larynx is lifted as epiglottis is bent to cover glottis

Deglutition

• Series of muscular contractions coordinated by swallowing center in medulla & pons
• Motor signals from cranial nerves V, VII, IX and XII
• Oral phase
• Tongue pushes bolus to back of oral cavity
• Pharyngeal stage
• Breathing stops & airways are closed
• Soft palate & uvula are lifted to close off nasopharynx
• Vocal and vestibular folds close
• Epiglottis is bent over airway as larynx is lifted and tongue depressed

Esophagus

• Collapsed muscular tube
• Extensive glands in submucosa
• In front of vertebrae
• Posterior to trachea & heart
• Pierces the diaphragm at hiatus
• hiatal hernia or diaphragmatic hernia
• Upper and lower esophageal sphincters regulate passage of solids, liquids, and gases

Deglutition

• Esophageal Phase
• Peristalsis drive bolus inferiorly
• Lower esophageal sphincter relaxes
• LES constricts following passage of bolus

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3-3 The Stomach

Anatomy of Stomach

• Which side is it on?
• Size when empty:
• Large sausage
• Stretches due to rugae
• Parts of stomach
• Cardia
• Fundus (air in x-ray)
• Body
• Pylorus
• Empties as small squirts of chyme leave the stomach through the pyloric valve

Functions of Stomach

• Mixes saliva, food, and gastric juice to create chyme
• Reservoir for food before release in small intestine
• Secretion
• Gastric juice
• HCl
• Kills bacteria
• Denatures proteins
• Pepsin
• Catabolyzes proteins
• Lipase
• Lingual and gastric
• Catabolizes fats
• Gastrin
• Increases gastric juice production, increases stomach motility, constricts lower esophageal sphincter, relaxes pyloric sphincter
• Intrinsic factor
• Allows absorption of Vitamin B12

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Mechanical digestion

Chemical digestion

(a little)

Histology of the Stomach

• Gastric mucosa covered in gastric pits leading to gastric glands
• Base of pits demarcated by muscularis mucosa
• Simple columnar epithelium
• Thick mucosal layer
• Normal-ish submucosa
• Thick muscularis externa with 3 layers

Mucosa & Gastric Glands

• Mucus Cell
• Mostly found from surface to neck of gastric glands
• Regenerative Cell
• Found in base and neck of gastric glands
• Parietal Cell
• HCl
• Intrinsic factor
• absorption of vitamin B12 for RBC production
• Ghrelin (stimulates hunger)
• Chief Cell
• Pepsinogen
• Gastric lipase
• Enteroendocrine Cell
• Secrete many hormones
• Gastrin = “get it out of here”
• More later

Gastric Secretions – Parietal Cells

• 2-3L per day
• Hydrochloric Acid
• Parietal cells pump Cl^- into- and exchange H⁺ for K⁺ in the lumen
• HCO⁺ is exchange for Cl^- in the blood (alkaline tide)
• As low as pH 0.8
• Functions:
• Activates pepsin and lipase
• Breaks up CT and cell walls
• Converts Fe³⁺ into absorbable Fe²⁺
• Destroys many pathogens
• Intrinsic Factor
• Necessary for absorption of Vitamin B₁₂
• Autoimmune disease can lead to pernicious anemia
• Ghrelin
• Stimulates feeding center of hypothalamus
• Also released by enteroendocrine cells in small intestines

CAH

Gastric Secretions – Chief Cells

• Pepsin
• Only works below pH of ~3.5
• Produced as pepsinogen (a zymogen)
• Pepsinogen + ↓pH (<5) 🡪 Pepsin
• Pepsinogen + Pepsin 🡪 More Pepsin
• Autocatalysis
• Uses hydrolysis to catabolize larger poly/oligopeptides into smaller peptides
• Gastric Lipase
• With lingual lipase digest 10-15% of dietary fats
• Reminder digested in small intestines

Gastric Secretions – Chemical Messengers

• Released from Enteroendocrine Cells (found in other regions too)

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Digestion and Absorption

• “Big” digestive events occur here to prepare for more complete digestive reactions in small intestine
• Both mechanical and chemical
• Absoprtion
• Lipid soluble compounds
• Aspirin
• Water
• Mostly absorbed in small intestine
• More on empty stomach
• Less on full stomach
• 10-20% of ethanol

Submucosa

Muscularis

• Three layers of smooth muscle
• Outer longitudinal
• Circular
• Inner oblique
• Coordinated by enteric pacemaker cells
• Permits greater churning & mixing of food with gastric juice
• In pylorus, ensure only small particles make it to duodenum in small 3 mL boli of chyme
• Takes about 4 hours to fully empty, less with liquids, more with fats

Serosa

• Simple squamous epithelium over a bit of connective tissue
• Also known as visceral peritoneum

GI reflexes

• Short reflex
• Reflex arc where stimulus, integration and effector are all located in GI system
• Long reflex (i.e. vagovagal)
• Reflex arc where integration occurs in the brain or spinal cord

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Regulation of Gastric Secretion and Motility

• Cephalic phase
• Gastric phase
• Intestinal phase

1. Cephalic Phase = “Stomach Getting Ready”

• Can be responsible for almost half of secretory activity
• Cerebral cortex through sight, smell, taste & thought
• stimulate parasympathetic nervous system through hypothalamus
• Motor pathway through Vagus nerve
• increases stomach muscle and glandular activity
• Pavlovian conditioning
• See supplemental video for this week

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2. Gastric Phase = “Stomach Working”

• Ingested food stimulates gastric activity
• Nervous control keeps stomach active
• Stretch receptors & chemoreceptors provide information
• ↓ pH or partially digested proteins
• Vigorous peristalsis and glandular secretions continue due to Ach and GRP secretions from parasympathetic innervation
• Chyme is released into the duodenum
• Endocrine influences over stomach activity
• Distention and presence of caffeine and amino acids ↑G cell secretion of gastrin into bloodstream
• Increases stomach glandular secretion
• Increases stomach churning and sphincter relaxation
• Histamine increase chief and parietal cell secretion
• As pH drops below 2, it starts to inhibit G cell activity

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3. Intestinal Phase = “Stomach Emptying”

• Begins as chyme reaches duodenum and helps regulate gastric release of chyme
• Initially duodenal stretch and amino acids trigger release of Gastrin from intestinal G cells
• Acid and fats in duodenum trigger enterogastric reflex that ↓vagal activity and ↑sympathetic activity
• Secretin and Cholecystokinin which suppress gastric activity
• Secrete glucose-dependent insulinotropic peptide (GIP) to ↑insulin production

Vomiting (emesis)

• Forceful expulsion of contents of stomach & duodenum through the mouth
• Cause
• irritation or excessive distension of stomach
• unpleasant sights, general anesthesia, dizziness & certain drugs
• Sensory input from medulla cause stomach contraction & complete sphincter relaxation
• Contents of stomach squeezed between abdominal muscles and diaphragm and forced through open mouth
• Can be serious because loss of acidic gastric juice can lead to alkalosis

4-1 The Liver, Gallbladder, and Pancreas

Anatomy of the Liver and Gallbladder

• Liver
• About 3 lbs, inferior to diaphragm on right side
• 4 Lobes: Right, Left, Quadrate (anterior), and Caudate (posterior)
• Right lobe larger
• Size causes right kidney to be lower than left
• Falciform ligament separates R from L lobes
• Round ligament is remains of umbilicus
• Gallbladder
• Inferior surface of right lobe
• Stores and concentrates bile

Histology of the Liver

• Hepatocytes arranged horizontally in lobules with vertical “plates”
• Absorb glucose, amino acids, iron, vitamins, etc. for metabolism or storage
• Secrete glucose, bile, albumin, fibrinogen, angiotensinogen, clotting factors, etc.
• Remove and metabolize toxins, drugs, hormones, bile pigments
• Synthesize and catabolize glycogen
• Sinusoids in between “plates” are blood-filled spaces
• Lined with sinusoidal and fenestrated capillaries
• Stellate macrophages phagocytize microbes & foreign matter

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Histology of Liver -- Hepatic Triad

• 3 structures found in corner between lobules
• hepatic portal vein and hepatic artery bring blood to the liver
• bile duct collects bile from bile canaliculi between sheets of hepatocytes to be secreted from liver in hepatic ducts

Pathway of Bile Secretion

• Bile capillaries
• Hepatic ducts connect to form common hepatic duct
• Cystic duct from gallbladder & common hepatic duct join to form common bile duct
• Common bile duct & pancreatic duct empty into duodenum

Bile Production

• One quart of bile/day is secreted by the liver
• Yellow-green in color
• pH 7.6 to 8.6
• Gallbladder stores & concentrates bile
• Bile backs up into gallbladder from a filled bile duct
• Concentrated up to 20-fold between meals
• Components
• Water
• Cholesterol
• Bile salts = Na & K salts of bile acids created from cholesterol
• Micelles
• Bile pigments (bilirubin) from hemoglobin molecule
• Bilirubin 🡪 urobilinogen (in lumen) 🡪 50% reabsorbed in SmI 🡪 kidney converts to urobilin OR GI to stercobilin

Anatomy of the Pancreas

• About the size of a cigar
• Produces enzymes that digest carbs, fats, nucleic acids, and proteins
• Produces sodium bicarbonate
• Empties into duodenum
• Islets of Langerhans
• 1% of gland
• Produce insulin, glucagon, somatostatin, PP

Composition and Functions of Pancreatic Juice

• 1.5 L/day at pH of 7.1 to 8.2
• Contains water, enzymes & sodium bicarbonate
• Digestive enzymes/zymogens
• Pancreatic amylase
• Polysaccharides
• Pancreatic lipase
• Fats
• Deoxyribo/Ribonucleases
• Proteases
• Trypsinogen
• activated by enteropeptidase (a brush border enzyme)
• Trypsin inhibitor
• combines with any trypsin produced inside pancreas
• Chymotrypsinogen
• activated by trypsin
• Procarboxypeptidase
• activated by trypsin
• Proelastase
• activated by trypsin

Regulation of Pancreatic Secretions

• Acetylcholine
• Stimulates secretion of pancreatic juice into pancreatic acini (especially during cephalic phase of digestion)
• Cholecystokinin (CCK)
• Secreted by duodenum and jejunum in response to fats
• Gallbladder
• Stimulates contractions of GB and relaxation of hepatopancreatic sphincter
• Pancreas
• Stimulates acini to produce pancreatic juice
• Secretin
• Main role is to regulate pH in duodenum
• Secretion stimulated by increased acidity in intestine causes increased sodium bicarbonate release
• Increases liver and pancreatic NaHCO₃-rich secretions

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• GIP
• fatty acids & sugar causes increased insulin release
• Gastrin
• Weak synergist of CCK

Regulation of Bile and Pancreatic Secretions

Liver Function

• Carbohydrate metabolism
• Converts fatty and amino acids into glucose
• Converts glucose into glycogen
• Lipid metabolism
• Synthesize cholesterol, lipoproteins (LDL and HDL)
• Protein metabolism
• Deamination (remove NH2) to use amino acids as energy sourse
• Converts ammonia into urea
• Synthesizes plasma proteins
• Converts AA’s into different AA’s
• Other functions:
• Detoxifies some drugs
• Synthesizes bile salts
• Bilirubin excretion
• Vitamin D activation

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4-2 The Small Intestine

Anatomy of the Small Intestine

• 20’ long ~1” diameter
• Large surface area for majority of absorption
• 3 parts
• Duodenum 10”
• Begins at pyloric valve
• Jejunum 8’
• Ileum 12’
• Ends at ileocecal valve

Structures That Increase Surface Area in Small Intestine

• Circular folds
• Permanent 1 cm tall folds that contain mucosal and submucosal layer
• Not found in distal ileum
• Cannot stretch out like rugae in stomach
• Also slow movement of chyme and increase turbulence
• Villi
• 1 Millimeter tall
• Lined with enterocytes and goblet cells
• Core is lamina propria of mucosal layer
• Contains vascular capillaries and lacteals (lymphatic capillaries)
• Microvilli
• Cell surface feature known as brush border
• Absorption and digestion
• Digestive enzymes found at cell surface on microvilli
• Brush border enzymes (contact digestion)
• Significant cell division within intestinal glands produces new cells that move up
• Once out of the way---rupturing and releasing their digestive enzymes & proteins

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Histology of Small Intestine

Cells of Intestinal Glands

• Absorptive cell
• Goblet cell
• Unicellular glands that are part of simple columnar epithelium
• Enteroendocrine
• Secretin
• Cholecystokinin
• Glucose-dependent insulinotropic peptide
• Paneth cells
• secretes lysozyme

Roles of Intestinal Juice & Brush-Border Enzymes

• Submucosal layer has duodenal glands
• Secretes alkaline mucus
• Mucosal layer contains intestinal glands
• Secretes intestinal juice
• 1-2 qt./day------ at pH 7.6
• Brush border enzymes
• Paneth cells secrete lysozyme kills bacteria

4-3 Digestion and Absorption

Mechanical Digestion in the Small Intestine

• Peristalsis
• Weak peristalsis in comparison to the stomach
• Chyme remains for 3 to 5 hours
• Segmentation
• Local mixing of chyme with intestinal juices
• Sloshing back & forth
• Motilin
• Secreted by duodenum when near empty
• Increases intestinal motility via migrating motor complexes
• Gastroileal reflex
• Gastrin and/or stretching of stomach trigger relaxation of ileocecal sphincter

Digestion of Carbohydrates

• Mouth
• Salivary amylase
• Stomach
• Nothing happens
• Duodenum
• Pancreatic amylase
• Brush border enzymes (maltase, sucrase & lactase) act on disaccharides
• Produces monosaccharides
• Fructose, glucose & galactose

Carbohydrate Digestion in Small Intestine

• Salivary amylase stops working in acidic stomach (if < 4.5)
• 50% of dietary starch digested before it reaches small intestine
• Pancreatic amylase completes first step in 10 minutes
• Brush border enzymes
• Dextrinase & Glucoamylase
• Break oligosaccharides into disaccharides
• Maltase
• Maltose 🡪 2Glucose
• Sucrase
• Sucrose 🡪 Glucose + Fructose
• Lactase
• Lactose 🡪 Glucose + Galactose

Lactose Intolerance

• Mucosal cells of small intestine fail to produce lactase
• Essential for digestion of lactose sugar in milk
• Mostly absent after 4 years unless you have MCM6 mutation
• Undigested lactose retains fluid in the feces leading to diarrhea
• Bacterial fermentation produces gases
• Symptoms
• Diarrhea
• Gas
• Bloating
• Abdominal cramps
• Dietary supplements are helpful

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Carbohydrate Absorption

Digestion of Proteins

• Mouth
• None
• Stomach
• HCl denatures or unfolds proteins
• Pepsin turns polypeptides into olig0-/tri-/di-peptides

Digestion and Absorption of Proteins

• Lumen of small intestine
• Facilitated by Pancreatic enzymes
• Breakdown of polypeptides into oligo-/tri-/di-peptides
• Brush border of small intestine
• HCl denatures or unfolds proteins
• Pepsin turns polypeptides into olig0-/tri-/di-peptides

Digestion of Lipids

• Mouth
• Lingual lipase
• Not active until in low pH stomach environment
• Stomach
• Gastric lipase (mostly for milk)
• Small intestine
• Emulsification by amphiphilic bile
• Pancreatic lipase
• Splits into triglycerides into fatty acids & monoglyceride
• No enzymes in brush border

Lipid Digestion

• Emulsification
• Fat globule is broken up and coated by lecithin and bile acids

Lipid Digestion

Lipid Absorption

Digestion of Nucleic Acids

• Pancreatic juice contains 2 nucleases
• ribonuclease which digests RNA
• deoxyribonuclease which digests DNA
• Nucleotides produced are further digested by brush border enzymes (nucleosidease and phosphatase)
• pentose, phosphate & nitrogenous bases
• Absorbed by active transport

Absorption in Small Intestine

Absorption of Electrolytes

• Sources of electrolytes
• GI secretions & ingested foods and liquids
• Enter epithelial cells by diffusion & secondary active transport
• Intestinal Ca+ absorption requires vitamin D & parathyroid hormone

Absorption of Vitamins

• Fat-soluble vitamins
• travel in micelles & are absorbed by simple diffusion
• Water-soluble vitamins
• absorbed by diffusion
• B12 combines with intrinsic factor before it is transported into the cells
• receptor mediated endocytosis

Absorption of Water

• 9 liters of fluid added to GI tract each day
• Small intestine reabsorbs 8 liters
• Large intestine reabsorbs 0.9 L of final liter
• Absorption is by osmosis through cell walls into vascular capillaries inside villi
• Diarrhea occurs when too little water is absorbed
• feces pass through too quickly if irritated
• feces contains high concentrations of a solute (lactose)

4-4 The Colon

Anatomy of Large Intestine

• 5 feet long by 2½ inches in diameter
• Ascending & descending colon are retroperitoneal
• Rectum
• Last 8” of GI tract
• Anterior to the sacrum & coccyx
• Simple columnar epithelium
• Anal canal
• Last 1 inch of GI tract
• Stratified squamous epithelium
• Internal anal sphincter
• Smooth muscle
• Involuntary
• External anal sphincter
• Skeletal muscle
• Voluntary

Histology of Large Intestine

• Mucosa
• Smooth tube
• No villi or circular folds
• Intestinal glands fill the the mucosa
• Simple columnar cells absorb water
• Goblet cells secrete mucus
• Submucosal & mucosa contain lymphatic nodules

Histology of Large Intestine

• Muscular layer
• Internal circular layer is normal
• Outer longitudinal muscle
• Taeniae coli = 3 longitudinal bands
• Haustra (pouches)
• Omental appendices/Epiploic appendages
• Serosa = visceral peritoneum
• Appendix
• contains large amounts of lymphatic tissue�

Appendicitis

• Inflammation of the appendix due to blockage of the lumen by chyme, foreign body, carcinoma, stenosis, or kinking
• Symptoms
• High fever
• Elevated WBC count
• Especially neutrophil count above 75%
• Referred pain, anorexia, nausea and vomiting
• Pain localizes in right lower quadrant
• Infection may progress to gangrene and perforation within 24 to 36 hours

Mechanical Digestion in Large Intestine

• Transit time is 12 to 24 hours
• reabsorbs water and electrolytes
• Feces consist of 75% water & 25% solids (30% bacteria, 30% undigested fiber, remainder fat & sloughed epithelial cells
• Haustral Churning
• Peristaltic waves
• 3 to 12 contractions per minute, every 30 minutes
• Relaxed pouches are filled from below by muscular contractions (elevator)
• Mass Movement
• 1-3 times per day
• Move feces several centimeters
• Often triggered by:
• Gastrocolic reflex
• When stomach fills, a strong peristaltic wave moves contents of transverse colon into rectum
• Duodenocolic reflex
• Similar to gastrocolic reflex

Chemical Digestion in Large Intestine

• No enzymes are secreted, only mucous
• Bacterial fermentation
• Undigested carbohydrates into carbon dioxide & methane gas
• Undigested proteins into simpler substances (indoles & skatoles)
• Convert bilirubin into simpler substances that produce color (stercobilin)
• Bacteria produce vitamin K and B in colon
• Flatus (gas)
• average person produces 500 mL per day
• most is swallowed air but it can contain methane, hydrogen sulfide, indole & skatole that produce the odor

Anatomy of Anal Canal

• Anal canal is 3 cm total length
• Anal columns are longitudinal ridges separated by mucus secreting anal sinuses
• Hemorrhoids are permanently distended veins

Defecation

1. Feces stretch rectum and stimulate stretch receptors, sending afferent signals up spinal cord
2. Spinal reflex stimulates contraction of rectum
3. Spinal reflex also relaxes internal anal sphincter
4. Cerebral cortex signals prevent or allow defecation via contraction of external anal sphincter
• Abdominal contractions increase abdominal pressure as levator ani lifts anal canal upwards and feces will fall away

Dietary Fiber

• Insoluble fiber
• woody parts of plants (wheat bran, vegie skins)
• speeds up transit time & reduces colon cancer
• Soluble fiber
• gel-like consistency = beans, oats, citrus white parts, apples
• lowers blood cholesterol by preventing reabsorption of bile salts so liver has to use cholesterol to make more

Aging and the Digestive System

• Changes that occur
• decreased secretory mechanisms
• decreased motility
• loss of strength & tone of muscular tissue
• changes in neurosensory feedback
• diminished response to pain & internal stimuli
• Cancer of the colon or rectum is common

Fin