Digestive System: Organs and Functions

• The digestive system is an elongated tube from mouth to rectum with accessory organs aiding digestion.
• Organs: mouth, pharynx, esophagus, stomach, intestines.
• Accessory organs: teeth, salivary glands, liver, gallbladder, pancreas.

Functions of the Digestive System

• Movement of food.
• Secretion of digestive juices and mucous.
• Digestion: breaking down food into absorbable molecules.
• Absorption into blood or lymph.
• Elimination of indigestible material and microflora.

Major Organs: The Mouth (pH 7)

• Initial part of the digestive system and food entry point.
• Teeth cut, tear, crush, and grind food.
• Salivary glands secrete saliva with enzymes.

Saliva

• Moistens food and contains ptyalin (salivary amylase).
• Initiates starch digestion into smaller polysaccharides.
• Mechanical digestion increases surface area for faster chemical digestion.

The Esophagus

• Tube connecting the mouth to the stomach, located behind the trachea and through the thoracic cavity.
• The epiglottis prevents food from entering the trachea during swallowing.
• Peristalsis: involuntary rhythmic contractions moving food down.
• The lining secretes mucus, supporting food movement.
• Esophageal sphincter (cardiac sphincter) allows bolus into the stomach and prevents acid reflux.

Stomach

• J-shaped muscular sac with inner folds (rugae) to increase surface area.
• Churns bolus with gastric juices for mechanical digestion.
• Gastric juices include HCL, salts, enzymes, water, and mucus.
• HCL aids in food breakdown and kills bacteria creating a pH of 2.
• Chyme refers to the bolus mixed with gastric juices.
• Pepsin initiates protein breakdown into polypeptides.
• Pyloric sphincter controls chyme movement from the stomach to the small intestine.

Stomach Protection Mechanisms

• Secretes gastric juices only when food is present.
• Mucus coating protects the stomach lining.
• Pepsin secreted as inactive pepsinogen.

Small Intestine

• Complete digestion of macromolecules and absorption of component molecules (glucose, glycerol, fatty acids, amino acids).
• Three parts: duodenum, jejunum, ileum.

Duodenum

First part of the small intestine, approximately 30cm long and U-shaped. Most digestion occurs here with enzymes from the pancreas and gallbladder.

• Villi and microvilli increase surface area for absorption.

Jejunum

• The jejunum is approximately 2 meters long with more villi and microvilli for nutrient absorption.

Ileum

• Approximately 3 meters long with fewer villi. Responsible for pushing waste into the large intestine, while still absorbing.
• Main function is absorption of nutrients and minerals.

Large Intestine

• Composed of: cecum, colon (ascending, transverse, descending, sigmoid), rectum.

Cecum

• First section of the large intestine.
• Pouch-like structure, 2 inches long.
• Receives digested material from the small intestine and passes it to the colon.

Colon

• Major section of the large intestine.
• Principal site for water and salt absorption and has four parts.

Ascending Colon

• First part of the colon, pushes undigested debris up from the cecum.

Transverse Colon

• Second portion of the colon, moves food from left to right.

Descending Colon

• Third portion, pushes contents down to the lower left side of the abdomen.

Sigmoid Colon

• Final, S-shaped stage emptying into the rectum.

Rectum

• Final section measuring 1-1.6 inches.
• Collects waste and empties through the anus.

Accessory Organs

• Support the digestive system but are not part of the digestive tract.
• Connected by ducts and secrete fluids into the digestive tract
• Liver: largest, produces bile.
• Gallbladder: stores bile, which emulsifies fats in the small intestine.
• Pancreas: secretes enzymes to digest carbohydrates, lipids, and proteins.

Defense Mechanisms in the Digestive Tract

• Mucin: coats bacteria and prevents attachment.
• Mouth: saliva contains antibodies and lysozyme.
• Stomach: mucus lining and hydrochloric acid (pH 2-3).
• Small intestine: bile salts, enzymes, Peyer’s patches (GALT).
• Liver: detoxifies toxins, contains phagocytic cells.
• Large intestine: resident microflora competes with pathogens.
• Infections occur when defenses are overwhelmed and vomiting/diarrhea help expel pathogens.

Normal Microflora

• Mouth: >400 species, ~12 billion organisms.
• Esophagus: no permanent microflora.
• Stomach: acid pH prevents colonization.
• First 2/3 of small intestine: few microorganisms.
• Last 1/3 of small intestine: some colonization by Enterobacteriaceae (E. coli) and Bacteroides.
• Large intestine: food remains for ~60 hours, feces are 25-50% bacteria.
• Bacteroides and Bifidobacteria make up 99% of fecal organisms.

Gut Microflora Interaction

• Gut microflora protects against infection.
• Altered microflora (e.g., by antibiotics) allows pathogens like C. difficile to thrive.
• Some flora produce antimicrobial substances and stimulate the immune system.

Pathogenesis of Diarrheal Disease

• Mouth/digestive tract can be a portal of entry for pathogens.
• Diarrhea results from stopping water absorption or increased fluid secretion into the gut.

Virulence Factors of Bacteria

• Toxin production (enterotoxins, cytotoxins, neurotoxins, endotoxins).
• Enterotoxins alter electrolyte and water flow (e.g., Cholera toxin).
• Ability to evade host mechanisms.
• Adhesion to mucosal surfaces (adhesins).
• Invasion of host cells (e.g., Salmonella) causing inflammation and ulceration.

Agents of Foodborne Disease

• Clostridium and Bacillus species.
• Staphylococcus aureus.
• Salmonellosis and Shigellosis.
• E. coli O157:H7.
• Listeriosis.
• Campylobacteriosis.
• Viral infections.
• Protozoan and worm infections.
• Algal and fungal food poisoning (mycotoxins).

Clostridium Botulinum

• Gram-positive, spore-forming, anaerobic bacteria.
• Produces toxin causing botulism, destroyed by boiling.
• Seven neurotoxic subtypes (A-G) causing flaccid paralysis.
• Botulism is a neuroparalytic disease where the toxin alters acetylcholine release.
• Three major types: infant, food-borne, and wound.
• Potential bio-weapon.

Clostridium Botulinum Toxin

• C. botulinum produces potent neurotoxins.
• Seven types: A, B, C1, D, E, F, and G.
• Types A, B, and E cause illness in humans (A has higher mortality).
• A single strain of C. botulinum will generally only produce one type.
• 1 g of toxin can kill 1 million people.
• 10-8g of toxin can kill a human adult.

Botulism

• Often-fatal disease of the nervous system.
• Caused by neurotoxic proteins, one-millionth of a gram can kill a man.
• Botulin produced by Clostridium botulinum in low-oxygen, low-acidity conditions.
• Inhibits acetylcholine production, causing paralysis in 12-36 hours.

Epidemiology of Botulism

• In U.S., average 110 cases annually.
• Approximately 25% foodborne, 72% infant form.
• Largest outbreak in 1977 from jalapeno peppers.
• Recent outbreaks from canned fish and hazelnut yogurts.
• Weaponized by several nations.

Botulism Transmission

• Ingestion of organism, spores, or neurotoxin.
• Wound contamination.
• Inhalation.

Botulism Pathogenesis

• Incubation period varies.
• Toxin enters bloodstream and binds to nerve endings, inhibiting acetylcholine release.
• Symmetrical, descending paralysis occurs.

Clostridium Botulinum Toxin

• Binds to nerve endings at the nerve-muscle junction, blocking acetylcholine release.
• Toxin is heat sensitive and destroyed at 80°C after 10 minutes.

Foodborne Botulism

• Caused by ingesting toxin from contaminated food, commonly home-canned foods.
• Symptoms occur 12-36 hours after ingestion.

Infant Botulism

• Most common form, affecting babies from 1 week to 2 years.
• Caused by ingesting contaminated food or household dust containing spores.
• Symptoms: poor feeding, weak gag/cry, decreased movement, lethargy, blunted facial expression.

Wound Botulism

• Organism enters wound and develops under anaerobic conditions.
• Associated with black-tar heroin addicts.

Diagnosis of Botulism

• Clinical signs.
• Toxin in serum, stool, gastric aspirate, suspected food.
• Culture of stool or gastric aspirate (5-7 days).
• Electromyography.
• Mouse neutralization test (48 hours).

Prevention of Botulism

• The botulinum toxin is generally not produced in foods below pH 4.7.
• Hygiene during home canning and high-temperature boiling.
• Do not feed honey to children under 24 months.

Treatment of Botulism

• Ventilator support.
• Antitoxin treatment.
• Remove contaminated food from the gut.

Botox®

• Type A toxin produced by Allergan, Co. Mayo.
• Approved for eye muscle disorders in 1989, cervical dystonia in 2000, and frown lines in 2002.
• Reduces frown lines for up to 120 days.
• Other uses: dystonia's, cerebral palsy, severe underarm sweating and cosmetic surgery