101 - Gastrointestinal Tract Flashcards
Functions of the GIT 1 2 3 4
1) Digestion
2) Absorption
3) Movement
4) Protection
Layers of GIT wall 1 2 3 4
– Mucosa
– Submucosa
– Muscularis externa
– Serosa
Length of GIT
~9M
Layers of muscularis externa
Inner circular layer
Outer longitudinal layer
Mucosa 1 2 3 4
1) Innermost layer, closest to the lumen
2) Columnar or stratified squamous epithelium, depending on location
3) Supported by a connective tissue layer (lamina propria) that contains nerves, vessels
4) Thin layer of smooth muscle (muscularis mucosae) forms boundary of mucosa
Structure of mucosa
Can be thick due to folding of the surface or presence of glands
Blood vessels in lamina propria
Blood vessels in lamina propria are small (capillaries and lymphatics)
Composition of submucosa
Dense, fibrous connective tissue (rich in collagen, elastin)
Sometimes glands present.
Blood vessels, nerves present.
Role of submucosa
Provides strength and elasticity
Thickness of muscularis externa
Usually two layers of smooth muscle (circumferential and longitudinal layers)
What is responsible for peristalsis?
Muscularis externa
What co-ordiantes movement of muscularis externa?
Autonomic nervous system
ROle of serosa
Provides non-stick surface of gut
What is the serosa?
Layer of connective tissue binding GIT to surrounding structures or an epithelial structure in the free gut (peritoneal cavity)
Cell type of serosa
Simple squamous
Components of autonomic NS
Sympathetic, parasympathetic, enteric
Locations of enteric NS plexuses
1
1
1) Myenteric plexus lies between longitudinal and circular muscle layers
2) Submucous lies in the submucosa
Epithelium in oesophagus
Stratified squamous (non-keratinising) Has submucosal glands for lubrication
Unusual aspect of composition of oesophagus muscularis externa
Half smooth muscle and half skeletal muscle
Muscle distribution in the oesophagus (of muscularis externa)
Top third is skeletal muscle.
Middle third is mixed.
Lower third is smooth muscle.
Macronutrients that stomach is best at breaking down
Proteins (not as good at digesting carbohydrates or fats)
Name for material partially digested by stomach
Chyme
Epithelium of etomach
Simple columnar
Structure of stomach wall 1 2 3 4
1) Simple columnar epithelium
2) Prominent muscularis mucosae
3) Thick submucosa
4) Thick muscularis externa (with a third, oblique layer in parts)
Role of rugae
To allow expansion of stomach
Surface of stomach wall
Contains many holes (gastric pits) which lead to gastric glands.
Structure of gastric pits
Each gastric pit splits into 3-5 gastric glands.
These are simple tubular glands that penetrate into lamina propria.
Organisation varies across stomach
Structure of gastric glands (top to bottom) 1 2 3 4 5
Made up of many cell type, arranged in particular areas from top to bottom.
1) Top cells are gastric surface cells and neck cells (produce mucus)
2) Regenerative (stem) cells in neck of gland.
3) Mucous neck cells
4) Parietal cell produce hydrochloric acid
5) Chief cells release pepsinogen at the base of the gland
Pepsinogen
Inert form of pepsin, which breaks down proteins
Pepsin activation
1
2
1) Pepsinogen is released from chief cells.
2) Acidic environment activates some pepsinogen to pepsin, which then cleaves and activates more pepsinogen
Protection of the stomach from pepsin and low pH
1-2mm layer of mucus released constantly from gastric glands
Structure of epithelium of small intestine
Simple columnar epithelium (with microvili)
Vili form prominent extensions of epithelium
Vili are present on transverse folds (plica circulares)
Base of vili give rise to tubular glands (Crypts of Lieberkuhn)
Microvili distribution
All columnar epithelial cells in small intestine have microvili except goblet cells.
Form brush border
Arrangement of vili to maximise surface area
Plica circulares increase surface area by 2-3.
Vili increase surface area by a factor of 10.
Crypts of Liberkuhn 1 2 3 4
Penetrate into lamina propria from base of vili.
Secrete fluid and mucus
Mucus secreted from goblet cells on vili (in both vili and crypts)
Contain stem cells for renewal of epithelium
Where do many of the cancers of the GIT form?
From stem cells in the crypts (intestinal cancers), gastric pits (gastric cancers)
Arrangement of lamina propria in a vilus
Extends into the core of each vilus
Contents of lamina propria in the small intestine
Contain smooth muscle (muscularis mucosa), blood vessels, lymphatics and immune cells
Role of muscularis mucosa in the vili of the small intestine
Move vili to prevent any areas where vili can’t absorb nutrients from fluid
Features of surface of duodenum 1 2 3 4
1) Short (25cm)
2) Has only low plica circulares
3) Fewer goblet cells
4) Receeves secretions from gall bladder and pancreas (bile for digestion of fat
Duodenal functions
1
2
3
1) Receives acidic chyme with active pepsin from stomach
2) Has glands in submucosa (Brunner’s glands) that release alkaline mucus
3) The pH is raised to ~7.3, which inactivates pepsin
Features of surface of jejunum and ileum
1
2
3
1) More plica, shorter vili
2) Lots of goblet cells (increase in number in ileum)
3) Ileum has a lot of Peyer’s patches
Amount of fluid absorbed by small intestine each day
~8.5L
Role of large intestine
Compacts faeces, recovers water and electrolytes
Structure of the colon
Smooth surface (no vili) Many tubular glands (Crypts of Liberkuhn, mainly producing mucus)
Composition of faeces
Water, roughage, bacteria, fat, inorganic material, bile pigment.
Stuck together with mucus
Problem with bacteria living in small and large intestine
Produce acid. HCO3 released from crypts to neutralise this acid
Amount of water lost each day in faeces
~50-100mL
