Small Bowel Flashcards
What is the function of the small bowel?
To absorb nutrients, salt and water
Label the diagram.
What is the length and diameter of the small bowel?
6m long
3.5 cm in diameter
How long is the duodenum?
25cm
How long is the jejenum?
2.5m
How long is the ileum?
3.75m
What is the transition like between the DJI?
no sudden transition
they all have same basic histological organisation
What is the function 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.
Label the arteries in the mesentery.
What is the digestive epithelium like?
plicae circulares-> vilus
Describe the structure of the villus.
Endocrine cells
Absorptive cells (enterocytes)
Cells Paneth
Villus
Epithelium
crypt
brush border
Intestinal gland
Lacteal
blood capillaries
Where do villi occur?
Onl occur in the small intestine
How would you describe villi.
Motile
Have a rich blood supply & lymph drainage for absorption of digested nutrients
Have good innervation from the submucosal plexus.
Have a simple epithelium (1 cell thick)
What cells dominate the villi?
enterocytes (columnar absorptive cells)
Label this diagram.
What is the villi (mucosa) lined with?
simple columnar epithelium consisting of:
- primarily enterocytes (absorptive cells)
- scattered goblet cells
- enteroendocrine cells
What is the crypt called?
Crypts of Lieberkuhn
What does the crypt of lieberkuhn epithelium include?
Paneth cells
Stem cells
What is the most abundant cell in the small bowel?
Enterocytes
What are enterocytes?
Tall columnar cells with microvilli & a basal nucleus.
Specialised for absorption & transport of substances.
What is the lifespan of enterocytes?
1-6 days
Describe the surface area of small bowel.
Cylindrical internal surface area of small bowel is 0.4m2.
Folds, villi & microvilli ↑ surface area to ~200m2 (size of a tennis court)
- At least a 500 fold ↑
What are microvilli?
Microvilli (~0.5-1.5μm high) make up the “brush border”.
Several thousand microvilli per cell
Surface of microvilli covered with glycocalyx
What is glycocalyx?
Rich carbohydrate layer on apical membrane
Serves as protection from digestional lumen yet allows for absorption.
Traps a layer of water & mucous known as “unstirred layer”
Regulates rate of absorption from intestinal lumen
What is the 2nd most abundant epithelial cell type?
goblet cells
What is inside goblet cells?
Mucous containing granules accumulate at apical end of
cell, causing ‘goblet’ shape.
What is mucous?
Mucous → large glycoprotein that facilitates passage of material through bowel.
How does goblet cell abundance change through the small bowel?
abundance of goblet cells along entire length of bowel
↓ in duodenum
↑ in colon
What are enteroendocrine cells?
Columnar epithelial cells
scattered among enterocytes
most often found in lower part of crypts
Hormone secreting
- e.g. to influence gut motility (CCK)
- In older text books → referred to as chromaffin cells (affinity for chromium/silver salts).
What are paneth cells?
Found only in the bases of crypts
Contain large, acidophilic granules
Granules contain:
- antibacterial enzyme lysozyme (protects stem cells)
- Glycoproteins & zinc (essential trace metal for a no. of enzymes)
Also engulf some bacteria & protozoa
May have a role in regulating intestinal flora
What are stem cells in the small bowel?
Undifferentiated cells which remain capable of cell division to replace cells which die
Epithelial stem cells are essential in the GI tract to continually replenish the surface epithelium
Continually divide by mitosis
Migrate up to tip of villus, replacing older cells that die by apoptosis
- They are to digested and reabsorbed
Differentiate into various cell types (pluripotent)
Enterocytes & goblet cells of small bowel have a short life span (about 36 hrs)
Rapid turnover contrasts with lifespan of weeks/months for other epithelial cell types (e.g. lung, blood vessels)
Q. Why?
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.
Any lesions will be short-lived.
If escalator-like transit of enterocytes is interrupted through impaired production of new cells (e.g. radiation) severe intestinal dysfunction will occur
How is the duodenum distinguishable?
Distinguished by presence of Brunner’s glands
What are brunner’s glands?
Submucosal coiled tubular mucous glands secreting alkaline fluid
- Open into the base of the crypts
What do the alkaline secretion of brunner’s glands do?
Neutralizes acidic chyme from stomach, protecting proximal small bowel
Help optimise pH for action of pancreatic digestive enzymes
What is the difference between the jejunum and ileum?
Jejunum=
thick wall
plicae circulares
ileum=
not as prominent plicae circulares
thin wall
payer’ patches
smooth mucous membrane
What are the functions of 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 types of small bowel motility?
Segmentation (mixing)
Peristalsis (propelling)
Migration Motor Complex
What is segmentation?
Mixes contents of lumen
Occurs by stationary contraction of circular muscles at intervals.
More frequent contractions in duodenum cf. ileum
Allow pancreatic enzymes & bile to mix with chyme
Although chyme moves in both directions, net effect is movement
→ colon
What is peristalsis?
Involves sequential contraction of adjacent rings of smooth muscle
Propels chyme towards colon
Most waves of peristalsis only travel about 10cm
Segmentation & peristalsis result in chyme being segmented, mixed & propelled → colon
What is migrating motor complex?
Cycles of smooth muscle contractions sweeping through gut
Begin in stomach → small intestine → colon → next wave starts in
duodenum
Prevents migration of colonic bacteria into ileum
Label this diagram.
What is digestion like in the duodenum?
Digestion in small bowel occurs in an alkaline environment
Pancreatic digestive enzymes & bile enter duodenum from MPD & CBD (common bile duct)
Duodenal epithelium also produces its own digestive enzymes
Digestion occurs in lumen & in contact with the membrane
How much fo the western diet is carbohydrates?
roughly 50%
Where does digestion start?
In mouth by salivary alpha amylase
ow is salivary amylase destroyed?
in stomach by acid pH
Where does most digestion of carbs happen?
In the small intestine
What are examples of simple carbohydrates monosaccharides?
glucose and fructose
What are examples of simple carbohydrates disaccharides?
sucrose and maltose
What are examples of complex carbohydrates and what are they?
starch, cellulose, pectins
-> sugars bonded together to form a chain
What is pancreatic alpha-amylase? (5 things)
Secreted into duodenum in response to a meal
Continues digestion of starch & glycogen in small bowel (started by salivary amylase)
Needs Cl- for optimum activity & neutral/slightly alkaline pH
Acts mainly in lumen (some also adsorbs to brush border)
Digestion of amylase products & simple carbohydrates occurs at the brush border
Are you happy with this diagram?
Circled nos= approx. % of substrate hydrolysed by each brush border enzyme
What are the types of alpha-amylase?
alpha-dextrins
Maltotriose
Maltose
How are each carbohydrates absorbed?
Absorption of glucose & galactose is by 2ary active transport
- Carrier protein = SGLT-1 on apical membrane
Absorption of fructose is by facilitated diffusion.
- Carrier protein = GLUT-5 on apical membrane
GLUT-2 facilitates exit at basolateral membrane
Human small intestine can absorb 10kg of simple sugars/day
What are the steps of digestion of carbs?
- Digestion of starch in lumen
- digestion of oligosaccharides at brush border
- absorption of monosaccharides
Where does protein digestion start?
in lumen of stomach by pepsin
When is pepsin from the stomach inactivated?
in alkaline duodenum
How are proteases secreted for digestion of protein in small bowel?
5x pancreatic proteases secreted as precursors in the lumen of small bowel e.g., trypsinogen
How is trpysin activated?
Trypsin is activated by enterokinase
Where is enterokinase located?
on duodenal brush border
What does trypsin do?
activates other proteases
hydrolyse proteins → single amino acids (AA) & oligopeptides (AA)
What progressively hydrolysis amino acid chains to amino acids?
peptidases at brush borders of enterocytes
Enterocytes directly absorb some of small (AA)n via action of H+/oligopeptide cotransporter PepT1
These small peptides are digested to AAs by peptidases in cytoplasm of enterocytes
This is the action of luminal, brush-border & cytosolic peptidases
Are lipids soluble?
No,they are poorly soluble in water
What does the lipid’s solubility affect?
Makes it more complicated to digest
What are the stages of digestion of lipids?
- Secretion of bile salts & pancreatic lipases
- Emulsification (↑s surface area for digestion)
- 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
How are lipids different to AA and simple sugars?
lipids transformed as absorbed via enterocytes
Describe the absorption of lipids.
Fatty acids (FAs) & monoglycerides (MG) leave micelles and enter enterocytes
FAs & MG resynthesized into triglycerides (TGS) by 2x pathways:
- Monoglyceride acylation (major)
- Phosphatidic acid pathway (minor)
Chylomicrons - lipoprotein particles synthesised as an emulsion (80-90% TGs, 8-9% phospholipids, 2% cholesterol, 2% protein, trace carbohydrate) in Golgi apparatus
ow are chylomicrons secreted?
Chylomicrons secreted across basement membrane by exocytosis
Chylomicrons enter a lacteal (lymph capillary) → lymph transports them away from bowel
How do you know which side is closer to the blood,. apical or basolateral?
think basolateral= blood
How is the ileum separated from the colon?
Ileocaecal valve
What control passage of material into colon?
Relaxation and contraction
What does relaxation and contraction also help?
prevent back flow of bacteria into ileum
