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 and 3.5cm in diameter
What are the dimensions of the 3 main parts of the small bowel?
- Duodenum 25 cm
- Jejunum 2.5 m
- Ileum 3.75 m
Do the 3 main parts of the small bowel have a sudden transition? How about their histological organisation?
- No sudden transition
- Same basic histological organisation
Which artery supplies the jejunum and the ileum?
Superior mesenteric artery
What are the two functions of the mesentery?
- Suspends small and large bowel from posterior abdominal wall to anchor them in place whilst still allowing some movement
- Provides a conduitfor blood vessels, nerves and lymphatic vessels
Which part of the bowels do the jejunal and ileal arteries supply? Where do they originate?
- Supply all of the ileum
- Originate from superior mesenteric artery which comes from the inferior border of the pancreas
Which parts of the bowels does the ileocolic artery supply?
- Terminal ileum
- Caecum
- Ascending colon
Which part of the bowels does the right colic artery supply?
Ascending colon
Which parts of the bowels does the middle colic artery supply?
- Hepatic flexure
- Transverse colon
- Splenic flexure
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?
Enterocytes (columnar absorptive cells) dominate the 1 cell thick epithelium
What features do villi have?
- Motile
- Rich blood supply and lymph drainage for nutrient absorption
- Good innervation from submucosal plexus
- 1 cell thick epithelium of enterocytes
- Only occur in small intestine
Which cells do the crypts of Lieberkühn 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 and 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 and what is its purpose?
- A rich carbohydrate layer on the apical membrane
- Serves as protection from digestional lumen yet allows for absorption
What is the unstirred layer?
A layer of water and mucous which helps to regulate the rate of absoprtion from intestinal lumen
Which side of the scattered goblet cells do mucous containing granules accumulate on?
The apical end, which causes the goblet shape to form
What is mucous and what does it do?
A glycoprotein which facilitates the passage of material through the bowel
What happens to the abundance of goblets cells along the length of the bowel?
- Abundance of goblet cells increases along entire length of bowel
- Low in duodenum, higher in colon as solid faeces need lubricating
Where are most enteroendocrine (chromaffin) cells found?
Lower parts of the crypts, scattered among enterocytes
What type of epithelial cells are enteroendocrine (chromaffin) cells?
Columnar
What do enteroendocrine (chromaffin) cells secrete?
Hormones to influence gut motility (e.g. histamine)
What do enteroendocrine cells have a high affinity for in terms of staining?
Chromium and silver salts
Where are paneth cells found?
At the bases of crypts
What do paneth cells contain?
Large, acidophilic granules with anti-bacterial lysozyme, glycoproteins and zinc
What are the functions of the contents of the granules in paneth cells?
- Lysozyme protects stem cells
- Glycoproteins
- Zinc is an essential trace metal for some enzymes
Which cells in the small bowel engulf bacteria and protozoa?
Paneth cells
Why are epithelial stem cells essential in the GI tract?
To continually replenish the surface epithelium
How do epithelial stem cells from the crypts replace cells that have died?
- They continually undergo mitosis and migrate from the crypts to the top of the villus, replacing older cells that die by apoptosis
- They differentiate into various cell types that are pluripotent
Why do enterocytes and goblet cells have such a high turnover rate?
- They are the first line of defense against GI pathogens and may be directly affected by toxic substances in the diet
- High turnover diminishes the effects of agents that affect cell function and metabolic rate
- Lesions are short-lived
What can affect enterocytes and cause severe intestinal dysfunction?
Impaired production of new enterocytes due to factors like radiation, interrupting the escalator-like transit of enterocytes
How is the duodenum differentiated from the jejunum and the ileum?
Duodenum has Brunner’s glands which secrete alkaline fluid (bicarbonate)
What is the role of the alkaline secretions from the Brunner’s Glands?
- Neutralizes acidic chyme from stomach, protecting proximal small bowel
- Optimises pH for action of pancreatic digestive enzymes
What is the difference in the blood supply between the jejunum and the ileum?
- Jejunum – 1/2 archades with long, infrequent branches to the jejunum
- Ileum – 3/4 archades with short, more frequent 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 as its plicae circulares muscles are larger, more numerous and closely set
- 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
- This allows pancreatic enzymes and bile to mix with chyme
- Net movement of chyme is towards colon
Where in the small bowel is segmentation more frequent?
Duodenum
Describe what is meant by peristalsis?
- Sequential contraction of adjacent rings of circular smooth muscle
- Propels 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-alpha-amylase in the mouth
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 is pancreatic-alpha-amylase secreted into and when?
The duodenum in response to a meal
What is the role of the pancreatic-alpha-amylase?
Continues digestion of starch and glycogen in the small bowel (started by salivary-alpha-amylase)
What does pancreatic amylase need for optimal activity?
- Cl-
- Neutral or slightly alkaline pH
Where does digestion of amylase products mainly occur?
Brush border
What are the substrates of amylase enzyme?
dextrins, maltose, lactose, sucrose, trehalose, maltotriose
How are 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 carrier protein
How is fructose absorbed?
Facilitated diffusion
Which carrier protein on the apical membrane is involved in the transport of fructose?
GLUT-5
Which transporter channel facilitates the exit of carbohydrates 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
List the 5 pancreatic proteases secreted as precursors into the small bowel lumen and their activated forms
- Trysinogen → trypsin via enterokinase
- Chymotrypsinogen → chymotrypsin
- Proelastase → elastase
- Procarboxypeptidase A → carboxypeptidase A
- Procarboxypeptidase B → carboxypeptidase B
What is the role of enterokinase?
To activate trypsinogen into trypsin
Where is enterokinase located?
Duodenal brush border
What are oligopeptides?
Peptides whose molecules have a small number of amino acid residues but more than 2
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 multi-amino acids to mono-amino acids
How do enterocytes directly absorb some small oligopeptides
Action of the H+/oligopeptide cotransporter PepT1
What is the four stage process of the digestion of lipids?
- Secretion of bile salts and pancreatic lipases
- Emulsification, increasing surface area for digestion
- Enzymatic hydrolysis of ester linkages forms colipase complexes with lipase (to prevent bile salts from displacing lipase from the 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 valve
What does the relaxation and contraction of the ileocaecal valve allow?
- Controlled passage of material into the colon
- Prevention of backflow of bacteria into the ileum
Describe how fatty acids are absorbed into the enterocytes?
- Fatty acids and monoglycerides leave micelles and enter enterocytes
- They are made back into triglycerides by the monoglyceride acylation pathway (major) and phosphatidic acid pathway (minor)
- TGs packed into chylomicrons made in the golgi apparatus
- Chylomicrons secreted across basement membrane by exocytosis
- Chylomicrons enter a lacteal (lymph capillary) and 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
