Module 4: GI system

Function of the GI system

Obtains water and nutrients from the external environment and brings them into the bodys internal environment

Four major functions of the GI system

Motility, Secretion, Digestion, Absorption

Motility

The movement of food and luminal contents through the Gi tract

Secretion

The synthesis and release of substances into the GI lumen including digestive enzymes, mucus, serous fluid

Digestion

The breakdown of nutrients into smaller components that can be absorbed

Two types of digestion

Mechanical digestion and chemical digestion

Mechanical digestion

Physically breaks food into smaller pieces

Chemical digestion

Breaks chemical bonds in nutrients

Absorption

Movement of nutrients and water from the GI lumen into the bodys internal environment

Structure of GI system

Long tube with outgrowths extending from the mouth to the anus

Sphincters

Rings of muscle surrounding the GI tube

Sphincters contracting

Close the lumen

Sphincters relaxing

Open the lumen

Epithelium in GI tract

Lines the entire GI tract and performs different functions depending on location

Stratified squamous epithelium location

Lines the mouth/oral cavity, esophagus, anus

Function of stratified squamous epithelium

Provide a protective, multilayered barrier that can withstand mechanical abrasion

Simple columnar epithelium location

Lines the stomach, small intestine, large intestine

Function of simple columnar epithelium

The single layer of columnar cells provides an epithelial surface specialised for secretion and absorption

Goblet cells

Unicellular glands found among columnar epithelial cells

Functions of mucus

Lubricate food/luminal contents and protect the epithelial surface

Multicellular glands

Glands formed when the GI epithelium invaginates into the underlying tissue

Simple gland

Gland with one duct

Compound gland

Gland with two or more ducts

Four basic layers of the GI tract

Mucosa, submucosa, muscularis, adventitia

Mucosa

The innermost layer of the Gi tract directly surrounding the lumen

Components of mucosa

Epithelium, basement membrane, lamina propria, muscularis mucosa, (glands)

Lamina propria

Fibrous connective tissue within the mucosa containing blood vessels, lymphatics, (glands)

Muscularis mucosae

A thin layer of smooth muscle at the base of the mucosa producing small, localised movements of the mucosa

Submucosa

The layer underneath the mucosa consisting mainly of fibrous connective tissue

Components of the submucosa

Blood vessels, lymphatics, glands, submucosal plexus

Submucosal plexus

A network of nerves belonging to the enteric nervous system located in the submucosa which regulate secretion

Muscularis

A smooth muscle layer responsible for moving and mixing GI contents

Components of the muscularis

Inner circular muscle, outer longitudinal muscle

Inner circular muscle

Muscle fibres wrap around the tube

Outer longitudinal muscle

Muscle fibres run along the length of the tube

Myenteric plexus

A network of nerves located between the inner circular and outer longitudinal muscle layers which regulates motility

Adventitia

The outermost basic layer which consists of fibrous connective tissue

Major functions of the mouth

-mechanical digestion through chewing
-chemical digestion through salivary enzymes
-helps form and move the food bolus towards the pharynx

Chemical digestion

Salivary glands release enzymes particularly amylase which begins carbohydrate digestion

Lubrication in the mouth

Mucus and saliva lubricate food helping it form a food bolus and move smoothly towards the pharynx and esophagus

Pathway from oral cavity → esophagus

Oral cavity → fauces → oropharynx → laryngopharynx → esophagus

Pairs of major salivary glands

Parotid, sublingual, submandibular

Parotid gland

Secretes serous fluid containing amylase

Amylase

Begins carbohydrate digestion

Parotid glands location

Around the ear region

Sublingual glands

Secretes primarily mucus for lubrication

Submandibular gland

Mixed gland producing serous fluid containing amylase and mucus

Acinus

A cluster of secretory cells within a salivary gland

Acinar cells

Cells within the acinus which can secrete serous fluid, digestive enzymes, mucus

Duct cells of salivary glands

Secretes bicarbonate which contributes to pH buffering

Duct system

Transports secretion to the oral cavity

Esophagus

Transports the food bolus from the pharynx to the stomach

Esophagus length

25 cm long

Epiglottis

Helps ensure food enters the esophagus rather than trachea

Mucosa and submucosa of the esophagus

Highly folded allowing the esophagus to expand to accommodate a food bolus

Muscularis of the esophagus

-first third: skeletal muscle
-middle third: skeletal and smooth muscle
-last third: smooth muscle

Mucus from esophageal glands → surface

Mucous gland → duct → epithelial surface → mucus coats lumen