[4] Digestive System

Length of the digestive canal

5-7 meters

Digestive Canal/GI Tract Organs

Esophagus, stomach, small and large intestine, anal canal

Functions of Digestive Canal/GI Tract Organs

• Actual tube the food passes through

• Muscular contractions physically break down food by churning + propelling + enzymes

Accessory Digestive Organs

Teeth, tongue, salivary glands, pharynx, liver, gall bladder, pancreas

Layers of the GI tract

Mucosa

Submucosa

Muscular Layer

[Mucosa] Present in the mouth, pharynx, esophagus, and anal canal

Non-keratenized Stratified Squamous Epithelium

[Mucosa] Present in the stomach and intestines and has a faster rate of absorption

Simple Columnar Epithelium

[Mucosa]

• Areolar CT + lymphatic vessels (for absorption of fats) and blood vessels

• Supports the epithelium and binds it to the muscularis mucosae

• Route of absorption

• Contains MALT

Lamina Propria

[Mucosa]

• Layer of smooth muscle fibers

• Creates many folds to increase surface area

Muscularis Mucosae

More areolar CT, lymphatic vessels, and blood vessels

Submucosa

[Muscular Layer]

Mouth, pharynx, and superior/middle esophagus are organs responsible for _____

Voluntary swallowing

[Muscular Layer]

External anal sphincter is responsible for ____

Voluntary control of defecation

[Muscular Layer]

Responsible for involuntary contractions (i.e. food breakdown, mixing with enzymes, and propelling food)

Smooth muscle

• Areolar CT + simple squamous epithelium

• Allows organ to be freely mobile

Serosa

True or false: Serosa is not present in the esophagus

True: Esophagus is fixed/anchored in place

• “Brain of the gut”

• Consists of 100M neurons

• Branching network of motor neurons, interneurons (connects the myenteric and submucosal neural plexus), sensory neurons

Enteric Nervous System

• Location: between circular and longitudinal smooth muscles of muscle layer

• Contains motor neurons (GI motility) → frequency and strength of contraction

Myenteric neural plexus/Plexus of Auerbach

• Location: within the submucosa

• Contains motor (controls secretions) and sensory neurons (detects stimuli in lumen)

Submucosal neural plexus/Plexus of meissner

Parasympathetic Innervation

Mostly supplied by Vagus nerve + sacral parasympathetic fibers

Sympathetic Innervation

From thoracic and lumbar spinal cord

Motilin receptors

Stimulates ACh release

Examples of motilin agonists

Erythromycin and azithromycin

5-HT_{1A, 2} receptors

• Peristalsis (contraction to push food)

• Indirect effect from ACh from 5-HT₄

M₃ ACh receptors

Smooth muscle contraction

Largest serous membrane in the body; consists of a layer of simple squamous epithelium + areolar CT.

Peritoneum

[Peritoneum] Lines the wall of the abdominal cavity

Parietal peritoneum

[Peritoneum] Lines the organs

Visceral peritoneum (serosa of organs)

[Peritoneum] Space in between parietal and visceral peritoneum

Peritoneal cavity

Covered by visceral peritoneum → freely mobile

Intraperitoneal Organs

Examples of intraperitoneal organs

Stomach, proximal duodenum, small intestine, transverse colon, liver

the fat that covers the organs

Omentum

Found behind peritoneal cavity

Retroperitoneal organs

Examples of retroperitoneal organs

Kidneys, ascending and descending colons, later parts of the duodenum, pancreas

Formed by the lips, cheeks, hard and soft palate, oral cavity, salivary glands and tongue

Mouth

Covers alveolar processes

Gums/Gingivae

Anchors tooth to dental alveolus of mandible/maxillary’s alveolar process

Periodontium

• Majority of the tooth

• Calcified CT

Dentin

• Made of calcium salts: Ca₃(PO₄)₂ + CaCO₃

• Protects the dentin from wear and tear

Enamel

Space enclosed by dentin

Pulp Cavity

Extension of pulp cavity (nerves, blood vessels)

Root canal

• Mucous membrane lining the inside of the cheeks

• Upper layer contains the membrane-coating granules

• Permeable area of the oral cavity

Buccal mucosa

Thickness of the buccal mucosa

40-50 cell layers (~400-700μm)

True or false: Oral absorption has no first pass effect

True: straight to systemic circulation

Minor salivary glands

Lips, cheek, palate, and tongue

Major salivary glands

Parotid, sublingual, and submandibular

Components of Saliva

Water (99.5%) and solutes

[Saliva - Solutes] Types of solutes in the saliva

Ions, gases, urea and uric acid, mucus, IgA, and enzymes

[Saliva - Solutes] Amylase activator ion

Cl^-

[Saliva - Solutes] Buffer ions

HCO₃^–, PO₄^3-, urea, proteins

[Saliva - Solutes] True or false: There are more mucus in the sublingual region

True

[Saliva - Solutes] A solute responsible for the prevention of microbe attachment to the epithelium

IgA

[Saliva - Solutes] Enzymes in the saliva

Lysozyme and salivary amylase

Contains glycosylated proteins, enzymes, and electro viscoelastic 3D network

Mucin

Mucin facilitates _____

Mucoadhesion (adhesion of material to the mucus via interfacial forces of attraction)

First stage of mucoadhesion

Contact: initial physical attraction

Second stage of mucoadhesion

Consolidation: Activation and hardening of adhesive bond

True or false: The secretion of the saliva is controlled by the somatic nervous system

False: autonomic nervous system

Effect of parasympathetic stimulation to salivation

• Continuous secretion of saliva

• ACh binds to M1 and M3 receptors

Excess salivation

Sialorrhea

Effect of sympathetic stimulation on salivation

• Dryness of mouth (xerostomia) —> dominates during stress

• Combined effect of NE on ANS a2 and a1 receptors

• Also known as beke

• Caused by mumps virus

• Inflammation and enlargement of parotid glands

Mumps

True or false: Mumps have more effect on females than males

False: it can cause orchitis on males, which leads to impotence

Treatment of mumps

MMR vaccine

• “Aphthous stomatitis/singaw”

• Multifactorial but self-limiting

Canker sores

• Tooth decay/breaking down of enamel

• Mediated by: dental biofilm, dietary carbohydrates, oral hygiene problems, and low fluoride

Dental caries

• Tongue manipulation and teeth grinding + saliva

• Bolus formation (chewed food)

Mechanical digestion

• Salivary amylase (results to simpler sugars)

• Triglycerides are digested by lingual lipase (can only be activated by acidic pH)

Chemical Digestion

• 8-12 cell layers (~100-200μm)

• More space for saliva pooling, but less surface area

Sublingual mucosa

Serves as a common pathway for both food and air

Pharynx

A region of the pharynx that is a respiratory passageway only

Nasopharynx

A region of the pharynx that is a passageway for both food and air

Oropharynx

A region of the pharynx that is a passageway for both food and air and directs the bolus towards the esophagus

Laryngopharynx

Tongue moves upwards, backward to push bolus.

Voluntary phase of swallowing

• Bolus stimulates oropharynx receptors 

• Impulses travel to deglutition center (medulla, brainstem lower pons)

• Impulses go back → soft palate and uvula move up to close nasopharynx, epiglottis closes larynx

Involuntary Phase

• Aspiration of oropharyngeal/gastric contents.

• A lung infection caused by inhaling food, liquid, saliva, or stomach contents into the lower airways.

Aspiration pneumonia

Regulates the passage of the bolus from the pharynx into the esophagus.

Upper Esophageal Sphincter (UES)

Regulates the passage of food from the esophagus into the stomach and helps prevent gastroesophageal reflux.

Lower esophageal sphincter (LES)

• J-shaped enlargement of the digestive canal.

• The most distensible organ of the digestive tract and can accommodate a large quantity of food.

• Serves as a mixing chamber and holding reservoir

Stomach

[Stomach] Lines the surface of the mucosa, also called surface mucous cells, that secrete protective mucus.

Simple columnar epithelium

[Stomach] Tubular glands extending into the lamina propria from gastric pits; contain specialized cells that secrete gastric juices and mucus.

Gastric Glands

Peristalsis occurs every ____

15-25 seconds

• Waves of contraction that push food toward the pyloric sphincter.

• Food mixes with gastric juice to form chyme.

Propulsion

• Pyloric sphincter → too narrow for bolus to enter

• The process through which the bolus is forced back into the body of the stomach.

Retropulsion

The gradual passage of chyme through the pyloric sphincter into the duodenum.

Gastric emptying

Secretes mucus and HCO₃^– to protect the gastric lining from acid and digestive enzymes.

Mucous Neck Cells

• Produces intrinsic factor for vit B12 absorption and transport

• Secrete H⁺ and Cl^- that activates lingual lipase

• Catalyzes formation of carbonic acid through carbonic anhydrase

Parietal Cells

• Secretes pepsinogen (converted by HCl to pepsin, which breaks down protein into smaller peptides)

• Secretes gastric lipase —> TG to FAs and monoglycerides

Chief/Zymogenic Cells

Secretes gastrin that stimulates gastric acid secretion

G Cells

An open sore that develops on the inner lining of the stomach, the upper part of the small intestine (the duodenum), or occasionally the lower esophagus.

Peptic ulcer

Causative agents of peptic ulcer

• H. pylori - damages the protective mucosal barrier and causes inflammation

• NSAIDs - Inhibits PG synthesis, reducing mucus and bicarbonate secretion and weakening mucosal protection

Aggressive factors of peptic ulcer

Gastric acid (HCl) and pepsin, which can damage the mucosa.

Protective factors of peptic ulcer

Mucus-bicarbonate barrier, mucosal blood flow, and epithelial repair.

• A chronic digestive disorder where stomach acid frequently flows back into the esophagus.

• Condition in which reflux of stomach contents causes troublesome symptoms and/or complications.

Gastroesophageal Reflux Disease (GERD)

Causative agent of GERD

Mainly caused by a weakened or improperly relaxing lower esophageal sphincter (LES).

[GERD] Gastric factors

Acid pocket forms near the gastroesophageal junction

[GERD] Impaired anti-reflux barrier

Dysfunction of the lower esophageal sphincter, crural diaphragm, and flap valve, allowing gastric contents to flow back into the esophagus.

[GERD] Temporary relaxation of the lower esophageal sphincter that allows gastric contents to reflux into the esophagus.

Transient lower esophageal sphincter relaxation