Small Bowel Flashcards
What is the function of the small bowel?
To absorb nutrients, salt and water
What are the dimensions of the small bowel?
Approx. 6m long & 3.5cm in diameter.
Which artery supplies the jejunum and the ileum?
Superior mesenteric artery
What are the two functions of the mesentery?
Suspends small & large bowel from posterior abdominal wall anchoring them in place whilst still allowing some movement
Provides a conduit for blood vessels, nerves & lymphatic vessels.
Which part of the GI tract is the only place where villi are found?
In the small intestine
Which nervous plexus innervate the villi?
Submucosal plexus - meissners plexus
Which cell type are the villi of the small bowel dominated by?
dominated by enterocytes (columnar absorptive cells)
Which cells do the crypts of Lieberkühn - epithelium include?
Stem cells
Paneth Cells
Where are the Crypts of Lieberkühn found?
In the base of the epithelial villi
What are villi mucosa lined with?
Simple columnar epithelium consisting of:
primarily enterocytes (absorptive cells)
scattered goblet cells
enteroendocrine cells
What is the life span of an enterocyte?
1-6 days
Which are the most abundant cell in the small bowel?
Enterocytes
What is the purpose of the villi on the enterocytes?
To increase the surface area for absorption by at least 500 fold
What are enterocytes specialised for?
Specialised for absorption & transport of substances.
What structures make up the brush border?
Microvilli
What is the surface of the microvilli covered in?
Glycocalyx
What is the glycocalyx?
A rich carbohydrate layer on the apical membrane which serves as protection from digestional lumen yet allows for absorption
What does the glycocalyx trap?
An unstirred layer
What is the unstirred layer?
a layer of water and mucous which helps to regulate the rate of absorption from intestinal lumen
Which side of the goblet cells do mucus containing granules accumulate on?
The apical end - this causes the goblet shape to form
What is mucous?
A glycoprotein which helps to facilitate the passage of material through the bowel
What happens to the abundance of goblets cells along the length of the bowel?
increased - abundance of goblet cells along entire length of bowel
Low in duodenum, higher in colon
Where are most enteroendocrine cells found?
found in lower parts of the crypts
What do enteroendocrine cells secrete?
Hormones to influence gut motility eg histamine
Where are paneth cells found?
At the bases of crypts
What do paneth cells contain?
Large, acidophilic granules
What do the granules in paneth cells contain?
antibacterial enzyme lysozyme (protects stem cells)
Glycoproteins & zinc (essential trace metal for a no. of enzymes)
Which cells in the small bowel engulf bacteria and protozoa?
Paneth cells
Why are epithelial stem cells essential in the GI tract?
Epithelial stem cells are essential in the GI tract to continually replenish the surface epithelium
how do stem cells from the crypts replace cells that have died?
Migrate from the crypts to the top of the villus, replacing older cells that die by apoptosis
Why do enterocytes have such a high turnover rate?
Enterocytes are first line of defense against GI pathogens & may be directly affected by toxic substances in diet.
Effects of agents which interfere with cell function ,metabolic rate etc will be diminished. Therefore, any lesions will be short lived
How is the duodenum differentiated from the jejunum and the ileum?
Duodenum has Brunners glands which secrete bicarbonate
What is the role of the alkaline secretions from the Brunners Glands?
Neutralizes acidic chyme from stomach, protecting proximal small bowel
Help optimise pH for action of pancreatic digestive enzymes.
What us the difference in the blood supply between the jejunum and the ileum?
Jejunum = fewer arcades and longer branches to the jejunum
Ileum = more arcades and short branches to the ileum
What is the difference in the wall of the jejunum and the ileum?
jejunum = thicker, wider and redder than the iluem
Ileum = thinner walls and contains peyers patches
What are the functions of the small intestine motility?
To mix ingested food with digestive secretions & enzymes
To facilitate contact between contents of intestine & the intestinal mucosa
To propel intestinal contents along alimentary tract
What are the three types of motility in the small bowel?
- Segmentation
- Peristalsis
- Migrating motor complex
Describe what happens during the process of segmentation?
Mixing contents of the lumen - occurs due to stationary contraction of circular muscles at intervals to allow pancreatic enzymes and bile to mix with chyme
Where in the small bowel is segmentation more frequent?
in the duodenum
Describe what is meant by peristalsis?
Involves sequential contraction of adjacent rings of circular smooth muscle - in order to propel chyme towards the colon
What is meant by migrating motor complex
Cycles of smooth muscle contractions sweeping through gut
What prevents the migration of colonic bacteria into the iluem?
Migrating motor complex
How do pancreatic enzymes enter into the duodenum?
From the main pancreatic duct and the central bile duct
At what pH does digestion in the small bowel occur in?
Digestion in small bowel occurs in an alkaline environment
What kickstarts the digestion of carbohydrates?
salivary amylase
What happens to salivary alpha-amylase in the stomach?
Destroyed due to acidic pH
Where does the majority of carbohydrate digestion occur?
In the small intestine
What is maltose
A disaccharide formed from two glucose molecules
What are three complex carbohydrates?
Starch, cellulose and pectins - sugars bonded to form a chain
Where are pancreatic amylases secreted into?
The duodenum in response to a meal
What is the role of the pancreatic amylase?
Continues digestion of starch & glycogen in small bowel (started by salivary amylase)
What does pancreatic amylase need for optimal activity?
Needs Cl- for optimum activity & neutral/slightly alkaline pH
Where does digestion of amylase products mainly occur?
at the brush border
What are the substrates of amylase enzyme?
dextrins, maltose, lactose, sucrose, trehalose, maltotriose
How is glucose and galactose absorbed?
Secondary active transport
Which carrier protein on the apical membrane is involved in the absorption of glucose and galactose?
SGLT-1
How is fructose absorbed?
facilitated diffusion
Which carrier protein is involved in the transport of fructose?
GLUT-5
which transporter channel facilitates the exit of carbs from the basolateral membrane?
GLUT-2
Where does protein digestion begin and by what?
In the lumen of the stomach by pepsin
What happens to pepsin in the duodenum?
Inactivated due to the alkaline conditions
How many pancreatic proteases are secreted as precurosors?
5
What is the role of enterokinase?
To activate trypsinogen into trypsin
What does trypsin do?
Activate other proteases which then hydrolyse proteins into single amino acids
What is the role of peptidases at the brush border of enterocytes?
progressively hydrolyse (AA)n → AAs
how do enterocytes directly absorb some small (AA)n
Enterocytes directly absorb some of small (AA)n via action of H+/oligopeptide cotransporter PepT1
What is the four stage process of the digestion of lipids?
Secretion of bile salts & pancreatic lipases
Emulsification
Enzymatic hydrolysis of ester linkages
Colipase complexes with lipase – prevents bile salts displacing lipase from fat droplet
Solubilisation of lipolytic products in bile salt micelles
What is the purpose of emulsification in the digestion of lipids?
Emulsification (↑s surface area for digestion)
What is the purpose of colipase?
Colipase complexes with lipase – prevents bile salts displacing lipase from fat droplet
What separates the ileum from the colon?
ileocaecal value
what does the relaxation and contraction of the ileocaecal valve allow?
the controlled passage of material into the colon, and prevention of backflow of bacteria into the ileum
Describe how fatty acids are absorbed into the enterocytes?
- FA and MG leave micelle and enter enterocyte
- Made back into TGs
- TGs packed into chylomicrons
- Chylomicrons secreted across basement membrane by exocytosis
- Chylomicrons enter a lacteal (lymph capillary) → lymph transports them away from bowel
What are the two pathways by which FA and MGs are resynthesized into TGs?
Monoglyceride acylation (major)
Phosphatidic acid pathway (minor)
In which organelle are chylomicrons made?
Golgi
