NPB101: Digestive Tract

Hamada lectures 42-45 SP2025

Where does the digestive tract begin and end

From the mouth to anus

What are the 4 major tissue layers of the digestive tract?

1. lumen

2. mucosa

3. submucosa

4. serosa

Also includes muscular external (smooth muscle) and enteric nervous system

What is significant about the digestive lumen’s relationship to the external environment?

• It is continuous with the external environment.

• It is exposed to:

  • pH changes

  • Harsh enzymes

  • Microorganisms

What are the 4 basic digestive processes?

1. Motility

2. Secretion

3. Digestion

4. Absorption

Motility

1. Propulsive:

   • Move contents forward through the digestive tract

2. Mixing movements:

   • aid digestion by mixing food with digestive juices

   • facilitate absorption by exposing food to absorbing surfaces

Secretion

Digestive juices into the lumen by exocrine glands

Digestion

biochemical breakdown of food

Absorption

Primarily in the small intestine; transfer of small absorbable units along water, vitamins, electrolytes from the digestive lumen into the blood or lymph

What anatomical structures are involved in the mouth and salivary gland function?

• Lip: contain food in mouth; guide food during chewing and swallowing

• Tongue: same as teeth

• Teeth: begin mechanical breakdown by chewing of food

• Palate: roof of oral cavity that separates oral cavity from nasal passage, allows chewing and breathing to occur simultaneously

• Uvula: soft tissue that hands from the rear of the mouth and seals off nasal passage during swallowing

Cephalic Phase Response (CPR)

Important to prepare food digestion and nutrient absorption efficiently

Secrete saliva in response to ____ stimulation

autonomic

What does saliva do?

1. mucus moisten food and lubricate

2. lysozymes lyse bacteria

3. Bicarbonate buffer neutralize acids

4. amylase (begins chemical digestion of carbohydrates)

What are the types types of salivary secretion reflexes?

1. Simple reflex

2. Conditional reflex (cephalic phase response)

What is the function of the pharynx in digestion?

• Located at the rear of the mouth

• A common passageway for both digestive and respiratory systems

What is the esophagus and what does it connect?

A muscular tube connecting the pharynx and stomach

What are the two sphincters of the esophagus and their functions?

• Upper sphincter (pharyngoesophageal): normally closed to prevent air entering the esophagus

• Lower sphincter (gastroesophageal): normally closed to prevent acid reflux from the stomach

What is swallowing and how is it controlled?

• An all-or-none reflex

• Triggered by pressure of bolus in pharynx → activates pressure receptors → sends signal to swallowing center in the medulla (brain stem

What are the stages of swallowing?

• Oropharyngeal stage

• Esophageal stage – involves propulsive movements

What are the main regions of the stomach and their features?

• Fundus: above gastroesophageal sphincter

• Body: middle portion

• Antrum: bottom portion; thick smooth muscle; connects to small intestine via pyloric sphincter

What are the primary functions of the stomach?

• Store ingested food and deliver it to small intestine at proper rate

• Secrete HCl and enzymes to start protein digestion

• Produce chyme: a thick liquid of food + gastric secretions

What region of the stomach contains oxyntic mucosa, and what structures does it include?

• Found in the body and fundus

• Located in gastric pits lining the mucosal wall

What are the 4 types of cells associated with gastric pits?

1. Mucous neck cells (secrete thin, watery mucus)

2. Chief cells (secrete pepsinogen - precursor of pepsin)

3. Parietal (oxyntic) cells (secrete HCl, intrinsic factor)

4. Surface epithelial cells (secrete thick, alkaline mucus)

What do chief cells secrete, and what is the role of pepsinogen and pepsin?

• Chief cells: secrete pepsinogen (inactive)

• Pepsinogen is cleaved by HCl to form pepsin

• Pepsin:

  • Activates more pepsinogen (autocatalytic)

  • Begins protein digestion by splitting amino acid linkages

  • Works best in acidic environment

What do parietal (oxyntic) cells secrete?

• H+ and Cl- are actively secreted by separate pumps HCl-:

  • Activates pepsinogen

  • Aids breakdown of connective tissue

  • Kills ingested microorganisms

What are parietal (oxyntic) cells function?

• Intrinsic factor:

  • Needed for vitamin B12 absorption

  • B12 is essential for normal RBC function

What do mucous neck cells and surface epithelial cells secrete?

• Mucous neck cells: secrete thin, watery mucus

• Surface epithelial cells: secrete thick, alkaline mucus

What do mucous neck cells and surface epithelial cells do?

• Protects against mechanical injury

• Prevents self-digestion

• Shields against acid injury

What secretions are produced in the pyloric gland area (antrum)?

• Mucus

• Pepsinogen

• Gastrin (hormone) → enters bloodstream → stimulates parietal and chief cells to secrete HCl and pepsinogen

Describe how gastric mixing and emptying works

• Strong antral peristaltic contractions propel chyme toward pyloric sphincter

• A small amount enters duodenum, rest is pushed back into the antrum (tumbling effect)

What hormone stimulates gastric mixing and secretions?

Gastrin → stimulates parietal and chief cells to produce HCl and pepsinogen

What factors control gastric mixing and emptying? Stomach

1. Volume of chyme

2. Fluidity of chyme

What factors control gastric mixing and emptying? Duodenum

1. Fat

2. Acid

3. Hypertonicity

4. Distention

What is the primary site of digestion and absorption?

• Small intestine

• No digestion occurs after the small intestine

What is segmentation in the small intestine?

• Type of oscillating, ringlike contraction

• Mixes and propels chyme

• Not peristalsis

• Chyme moves forward because contraction frequency decreases along the intestine

What enzymes aid digestion in the small intestine?

• lumen is accomplished by pancreatic enzymes

• Bile secretions enhance fat digestion

What does the exocrine pancreas secrete and which cells are involved?

• Acinar cells: release 3 types of enzymes

  1. Proteolytic enzymes (trypsinogen)

     • Activated by enterokinase in the duodenum → becomes trypsin (active form)

     • Trypsin autocatalyzes more trypsinogen

     • Trypsin inhibitor prevents premature activation

  2. Pancreatic amylase (carbohydrate digestion)

  3. Pancreatic lipase (fats) → only fat-digesting enzyme in GI tract

• Duct cells: release sodium bicarbonate (NaHCO₃) to create alkaline environment

What hormones regulate pancreatic secretions and what triggers them?

• Secretin: released in response to acidic chyme

• Cholecystokinin (CCK): triggered by fat and protein in chyme

• All 3 enzymes (proteolytic, amylase, lipase) are packaged into common vesicles → fat/protein triggers release of all 3

• Carbohydrates do not trigger enzyme release

What are the major components and functions associated with bile?

• Bile salts: aid digestion and absorption of fats

• Gallbladder: stores bile

• Sphincter of Oddi: controls bile entry into duodenum

• Bile salts are recycled

What anatomical regions are bypassed in Roux-en-Y gastric bypass surgery?

• Antrum

• Duodenum

What completes digestion in the small intestine?

• Pancreatic enzymes in lumen

• Bile enhances fat digestion

What is the brush border and what enzymes does it contain?

• microvilli on intestinal epithelial cells

• Contains 3 enzymes:

  1. Enterokinase

  2. Disaccharidases

  3. Aminopeptidases

Where is digestion of different macromolecules completed?

• Fat digestion: completed extracellularly

• Protein and carbohydrate digestion: completed intracellularly

How are carbohydrates digested and absorbed?

• In lumen:

  • Polysaccharides → disaccharides (via salivary and pancreatic amylase)

• At brush border:

  • Disaccharides → monosaccharides (via disaccharidases)

• Absorption: Monosaccharides → capillaries (blood)

How do bile salts aid fat digestion?

1. Act as a detergent: emulsify fat

2. Help form micelles for absorption

How do bile salts aid fat digestion by acting as a detergent (emulsifying fat)?

• Convert large fat globules → lipid emulsion of small droplets

• Increases surface area for pancreatic lipase

How do bile salts help form micelles for absorption?

• Micelles = tiny structures (smaller than fat droplet)

• Composed of: bile salts, cholesterol, and lecithin

• Micelles have hydrophobic core and hydrophilic shell

How is fat digested and absorbed?

• Emulsified by bile salts

• Digested by pancreatic lipase → monoglycerides + fatty acids

• These products are packed into micelles

• Micelles diffuse to intestinal cells → release monoglycerides and fatty acids

• Re-synthesized into triglycerides

• Triglycerides → enter lymphatic system

• Fatty acids also enter blood

How does content flow from the small to large intestine?

• One-way flow from ileum → cecum

• Prevents colonic bacteria from entering small intestine

What structures regulate this one-way flow?

• Ileocecal valve

• Ileocecal sphincter

What effects does gastrin have on intestinal flow?

• Enhances gastric motility

• Stimulates ileal motility

• Relaxes ileocecal sphincter

What are the two types of movements in the large intestine?

• Haustral contractions: slow shuffling contents of large intestine to aid absorption (mainly water and salts)

• Mass movements: large, strong contractions in ascending and transverse colon

  • Occur 3–4 times/day

  • Triggered by gastrin (after meals)

What initiates the defecation reflex?

• Mass movement pushes feces into rectum

• Rectal wall distension activates stretch receptors → initiate reflex

• Reflex:

  • Internal anal sphincter (smooth muscle) → relaxes

  • Rectum and sigmoid colon → contract

  • External anal sphincter (skeletal muscle) must also relax for defecation under voluntary control

What is constipation and what causes it?

• Occurs when defecation is delayed

• Too much water is reabsorbed → feces become dry and hard

What is appendicitis and how can it occur?

• Caused by hardened feces lodged in appendix

• Blocks normal circulation and mucus secretion

What are the 4 main mechanisms that regulate digestive function?

• Autonomous smooth muscle (e.g., Interstitial Cells of Cajal)

• Intrinsic nerve plexuses (Enteric Nervous System)

• Extrinsic nerves (Autonomic Nervous System: Sympathetic & Parasympathetic)

• Gastrointestinal hormones (e.g., Sympathetic Nervous System → Gastrin, Secretin, CCK)

What are Interstitial Cells of Cajal and what do they do?

• Non-contractile smooth muscle cells

• Act as pacesetters with slow-wave potentials

• Digestive tract smooth muscle cells are connected via gap junctions
  → act as a functional syncytium

• Regulate:

  • Rate of contraction

  • Intensity of contraction