L34 - Digestive System

What is the general path of food through the GIT?

1. Oral cavity

2. Esophagus

3. Stomach

4. Small intestine

5. Cecum

6. Large intestine

7. Rectum

8. Anus

What is the main function of the esophagus?

Transport the food bolus from the oral cavity to the stomach

What are the three major anatomical regions of the stomach?

Cardia → fundus/body → pylorus

What are the three regions of the small intestine?

Duodenum → jejunum → ileum

What is the major overall function of the small intestine?

Complete digestion and absorption

What is the major function of the cecum?

Bacterial fermentation

• It marks the transition from small to large intestine

What are the three regions of the colon?

1. Ascending

2. Transverse

3. Descending colon

What are the four basic layers of the GI tract wall?

1. Tunica mucosa

2. Submucosa

3. Muscularis

4. Serosa

What are the three layers of the tunica mucosa?

1. Lamina epithelialis

2. Lamina propria

3. Lamina muscularis mucosae

What is the main role of the lamina epithelialis?

Lines the GI lumen and contains absorptive and secretory cells

What does the lamina propria contain?

• Blood vessels

• Nerves

• Immune cells

What does the tunica submucosa contain?

• Loose CT,

• Larger blood vessels

• Lymphatics

• Nerve plexus

• Ganglion cells

Which enteric nerve plexus is found in the submucosa?

Submucosal plexus

What are the two smooth muscle layers of the tunica muscularis?

1. Circular smooth muscle

2. Longitudinal smooth muscle

Where is the myenteric plexus located?

Between the circular and longitudinal muscle layers

What is the tunica serosa?

The outermost connective tissue layer of the GIT

Why do GI histologic features change along the tract?

Each region is structurally specialized to perform a different function

What are the three major regulators of GI function?

1. Endocrine

2. Paracrine

3. Neural

What is endocrine regulation in the GIT?

A hormone travels through the blood to a distant target cell

What is paracrine regulation in the GIT?

Local communication with neighboring cells

What are enteroendocrine cells (EECs)?

Cells in the lamina epithelialis that produce gut hormones

What do enteroendocrine cells respond to?

Changes in the luminal environment, such as stretching

What defines a gut hormone?

Made by EECs, released after a stimulus, and travels in blood to a distant target

What are the two nervous systems involved in neural GI regulation?

1. Extrinsic nervous system (ANS)

2. Intrinsic nervous system (ENS)

Where are ANS vs. ENS cell bodies located?

• ANS = outside the gut wall

• ENS = within the gut wall

What are the two major plexuses of the ENS?

1. Submucosal plexuses

2. Myenteric plexuses

What changes do GI sensory neurons monitor?

• Luminal distention

• Chemistry

• Mechanical stimulation

What major functions does the ENS regulate?

• Motor activity

• Secretory activity

• Vascular tone

Can the ENS function independently of the CNS?

Yes

• It can act as a complete reflex circuit.

How does the ENS produce peristalsis around a bolus?

Contracts smooth muscle proximal to the bolus and relaxes muscle distal to it

What epithelium lines the esophagus?

Stratified squamous epithelium

How does esophageal epithelium support its function?

Multiple cell layers provide protection during food transport

What is the function of esophageal submucosal mucous glands?

Secrete mucus to help move the food bolus

What is the function of the lower esophageal sphincter (LES)?

Coordinates bolus entry into the stomach and prevents gastric reflux

What are the major functions of the stomach?

• Storage

• Secretion

• Protection

• Mixing

• Controlled delivery of chyme

What are rugae?

Folds of the mucosa + submucosa that allow the stomach to stretch

What type of epithelium lines the stomach?

Simple columnar epithelium.

What are the major cell types of gastric glands?

1. Mucous

2. Parietal

3. Chief

4. Enteroendocrine

5. Sem cells

What do gastric mucous cells secrete?

Mucus

What do parietal cells secrete?

Acid (H⁺) and intrinsic factor

What do chief cells secrete?

• Pepsinoge

• Lipase

What do gastric G cells secrete?

Gastrin

Where are gastric stem cells located?

Neck region of gastric glands

What is the major function of the duodenum?

Bicarbonate secretion and coordination of digestion through hormones/reflexes

What is the major function of the jejunum?

Digestion and absorption of ions, nutrients, and water

What is the major function of the ileum?

Absorption of residual nutrients, vitamin B12, and bile acids

What is the function of intestinal villi?

Increase absorptive surface area

How do villi change moving distally through the small intestine?

They decrease in number and height

How do goblet cells and lymphatic tissue change distally?

They increase

What are Peyer’s patches?

Lymphatic nodules in the ileum that help protect against invasive microbes

Where do new intestinal epithelial cells originate?

Stem cells at the base of intestinal crypts

What are the two absorption routes within intestinal villi?

1. Vascular capillaries

2. Lymphatic central lacteals

What are the major functions of the large intestine?

• Water/electrolyte absorption

• Bacterial fermentation

• Waste storage/elimination

Does the colon contain villi?

No

• It lacks villi

How does the colon increase surface area without villi?

Crypts and microvilli

What cell type becomes more abundant in the large intestine?

Goblet cells

GIT Regions + Function SUMMARY

GIT Region	Main Function
Oral cavity	Food intake
Esophagus	Transport bolus to stomach
Stomach	Storage, mechanical digestion, secretion & mixing
Duodenum	Bicarbonate secretion; coordinates digestion
Jejunum	Digestion + absorption of nutrients, ions & water
Ileum	Absorbs residual nutrients, B12 & bile acids
Cecum	Bacterial fermentation
Large intestine	Water/electrolyte absorption, fermentation, waste storage
Rectum → anus	Storage/elimination of waste

Four Layers of the GIT Wall SUMMARY

Layer	Major Components / Features	Key Role
Tunica mucosa	Epithelium + lamina propria + muscularis mucosae	Absorption, secretion, protection
↳ Lamina epithelialis	Absorptive + secretory cells	Interfaces with lumen
↳ Lamina propria	Blood vessels, nerves, immune cells	Support
↳ Muscularis mucosae	Muscle	Part of mucosa
Tunica submucosa	Loose CT, vessels, lymphatics, submucosal plexus	Support + neural regulation
Tunica muscularis	Circular + longitudinal smooth muscle; myenteric plexus between them	GI motility
Tunica serosa	Outermost CT layer	Outer covering

GI Regulation SUMMARY

Regulation	Major Components / Features	Key Role
Endocrine	Hormone → blood → distant target	EECs produce gut hormones
Paracrine	Signal acts on neighboring cells	Local communication
Neural	ENS + extrinsic ANS	Controls GI responses

Intrinsic vs. Extrinsic Neural Regulation SUMMARY

Feature	Intrinsic — ENS	Extrinsic — ANS
Cell bodies	Within gut wall	Outside gut wall
Can act independently?	Yes	Provides external influence
Major ENS plexuses	Submucosal + myenteric	—
ENS controls	Motor activity, secretion, vascular tone	Influences ENS/GI activity

Regional Histology → Function SUMMARY

Region	Key Histologic/Anatomic Feature	How It Supports Function
Esophagus	Stratified squamous epithelium	Multiple layers → protection
	Submucosal mucous glands	Lubricate → bolus movement
Stomach	Rugae	Allow expansion/storage
	Gastric glands	Specialized secretion
Small intestine	Villi	↑ surface area → absorption
Distal SI	↓ villi, ↑ goblet cells & lymphatic tissue	Increasing protection distally
Ileum	Peyer’s patches	Immune protection
Large intestine	No villi; crypts + microvilli	Supports absorption
	↑ goblet cells	Mucus production

Gastric Gland Cells SUMMARY

Cell	Secretion
Mucous cell	Mucus
Parietal cell	H⁺ + intrinsic factor
Chief cell	Pepsinogen + lipase
G cell	Gastrin
Stem cell	Replaces cells; located in neck