The small intestine Flashcards
Describe the gross structure of the small intestine, including the route
6m long
Stomach leads on to: - Duodenum 25cm - Jejunum 2.5m - Ileum 3.75m this goes to large intestine
All have SAME basic histological organisation
What is the mesentery?
Allows folding of small intestine and supports blood supply
Describe digestive eptihelium and mucosa organisation
Lot of mucosal layers
- provides layer of tissue for nerve plexuses and blood vessels
- Internal mucosa are arranged in circular . folds
- Mucosa are covered in villi
Describe the villi
Situated in small intestine
Motile with rich blood supply and lymph drainage
Good innervation
Presence of enterocytes for absorption
What are the types of cell in the small intestine?
Mucosa
- Enterocytes
- Goblet cells
- Enteroendocrine cells
In the crypts
- Paneth cells
- Stem cells
What are enterocytes?
Tall COLUMNAR cells
Face lumen, containing microvilli
Polarised with specialisation to absorb and transport items
SHORT LIFESPAN
What are microvilli?
Attached to cytoskeleton, on apical side of villi
Surface covered with glycocalyx
What is the function of the glycocylax?
Rich carbohydrate layer, which allows for protection from digestional lumen, not stopping absorption
Traps layer of water and mucous - unstirred layer - regulates rate of absorption
What are goblet cells?
2nd most abundant
Granules containing mucus
Mucus is large glycoprotein
As you go down, more goblet cells
- becomes drier and needs lubrication
Enteroendocrine cells?
Hormone releasing cells, in the epithelial wall of the gut
- found on the tip and crypt
CHROMAFFIN CELLS
What are Paneth cells?
Only in the base of crypts
- Has granules containing
- antibacterial lysozyme
- glycoproteins and zinc
Protective function - stem cells
Regulate intestinal flora
What are stem cells?
Undiff cells, can undergo differentiation
Can from any type of surface epithelium
Escalator
Have memory
- cells in the colon will form colon cells regardless of location
Why do epithelial cells have such a high turnover?
First line of defence against GI pathogens
- direct affect by toxins
Effects of agents that damage cell function diminish
Damage is short lived
If the escalator is interrupted - sever dysfunction occurs
How does cholera cause disease?
Results in prolonged chloride ion channel opening
- uncontrolled water secretion as water jus moves out
What are the distinguishing features of the duodenum?
Presence of Brunner’s glands
- tubular mucous
- open into crypts for alkaline secretion
- neutralise damage of stomach acid on chyme
- optimises pH for pancreas to act on it
What are the distinguishing features of the jejunum?
Presence of large submucosal folds
- much more pronounced - tall and thin
PLICAE CIRCULARES
What are the distinguishing features of the ileum?
Lot of Peyer’s patches
- similar to large intestine - immune system link
Used to prime immune system against bacteria
Well positioned to prevent bacteria migrating back into small intestine from large
What are the functions of small intestine motility?
Mix ingested food with digestive enzymes
Facilitate contact with contents of lumen and mucosa
Propel intestinal contents along tract
How is food mixed?
Segmentation
- circular muscle contraction
- more in the duodenum than ileum
- net movement is towards colon despite movement in both direction
Peristalsis
- sequential muscle contraction
- 10cm
- more well done
Migrating motor complex - in fasting
- smooth muscle contraction that start from stomach
- to ensure any undugested items are sweeped all the way through
- prevent bacteria from returning from large intestine
What is digested in the small intestine?
Carbs
Proteins
Lipids
How is food digested in the SI?
ALKALINE
Digestive enzymes and BILE enter duodenum from pancreatic and bile duct
MIxed in the Sphincter of Oddi
Duodenal epithelium also makes its own enzymes
How are carbohydrates digested?
Begins in mouth by alpha amylase
Most in small intestine
- all types of carb
Major enzyme - pancreatic alpha amylase
- secreted into duodenum
- breaks starch and glycogen
- NEEDS CL- AND ALKALINE
- acts in lumen, by breaking into medium sized carbs
- digestion of smaller and simple carbs happens at the brush border membrane
How are carbs absorbed?
SECONDARY ACTIVE TRANSPORT - not direct ATP hydrolysis
Energy released from Na re-entering cell after being pumped out is used to pump GLUCOSE in - SGLT-2 prot
Fructose diffuses in via GLUT_2
How are proteins digested?
Pepsin acts but only in acid condition
- Inactivated by alkaline env
- TRYPSIN from trypsinogen from pancreas
- Converted by enterokinase ONLY in gut
- Activates other proteases
How are proteins absorbed?
Brush border peptidases break larger peptides
AAs absorbed by fac diff and secondary act trans
Di, tripeptides absorb using distinct carrier proteins
Cytoplasmic peptidases break down these dipeptides as they cross basolateral membrane - in blood its just AAs
How are lipids digested?
Poor water solubility
Bile and lipase secretion
Emulsification - increase SA and allow lipase to split TGs into 2 FAs and monoglyceride
Enzymatic hydrolysis of ester linkages - LIPASE, needs colipase which is activated in gut - stop bile salt from displacing fat droplet
Solubilization of lipolytic products using bile salts
What are the two other important lipid enzymes?
Phospholipase A2
- FA hydrolysis at 2 position of phospholipids - forms free FA and lyso-phospholipids
Pancreatic cholesterol esterase
- hydrolyses cholesterol esters to free cholesterol and FA
Describe bile salt molecule
Amphipathic
Hydrophilic - hydroxyl and carboxyl
Hydrophobic - nucleus and methyl
Form mixed micelles - product of lipid digestion in centre, with bile salts on outside
- needed for transport to gut wall
How are lipids absorbed?
Micelles present products to brush border - drops off
- Micelle is split
- bile salts absorbed at ileum
- lipid absorption is complete at jejunum
- bile salts are transported to liver to be recirculated
following absorption, what occurs to lipids?
Monoglycerides and FFAs are absorbed and resynthesised
2 paths
- Monogylceride acylation
- add FAs in SER
- Phosphatidic acid pathway
- acyl coA added to phospholipid - new FA
Then, transported around body in CMs
- 80-90pc TG
- 8-9pc PL
- Secreted out of basement membrane
- Too big to go through capillaries so goes into lymph then reenters
What separates the ileum and colon?
Ileocaecal sphincter
- relaxation and contraction controls movement
