Small Bowel Flashcards
What is the function of the small bowel?
The small bowel absorbs nutrients, salt & water
Small bowel structure:
How long is the small bowel? And what is its diameter?
How long are each of the following components:
- duodenum
- jejunum
- illeum
Describe the transition between the diff components of the small bowel
~ 6m long & 3.5cm in diameter
Duodenum: 25 cm
Jejunum: 2.5 m
Illeum: 3.75 m
No sudden transition between them
All have same basic histological organisation
Mesentery:
What are the functions of the mesentery?
Suspends what?
Provides a conduit for?
Mesentery:
Suspends small & large bowel from posterior abdominal wall
-anchors them in place but still allows some movement
Provides conduit for blood vessels, nerves & lymphatic vessels
Small bowel - villi:
Where do they occur?
Movement?
Blood supply?
Innervation?
What type of epithelium?
How many cells? What cell type?
Villi only occur in the small intestine
Villi are motile
Rich blood supply & lymph drainage for absorption of digested nutrients
Good innervation from the submucosal plexus
Simple epithelium
- 1 cell thick
- dominated by enterocytes (columnar absorptive cells)
Small bowel - cell types
What cells is the villi lined with?
The crypts of lieberkühn epithelium includes what cell types?
Villi (mucosa) lined with:
Simple columnar epithelium consisting of:
Primarily enterocytes (absorptive cells)
Scattered goblet cells
Enteroendocrine cells
Crypts of lieberkühn - epithelium includes:
- paneth cells
- stem cells
Enterocytes:
Describe the structure of enterocytes
What are enterocytes specialised for?
What is the lifespan of enterocytes?
What structures increase the surface area of the small bowel and by how much?
Most abundant cells in small bowel
Tall, columnar cells w microvilli & basal nucleus
Specialised for absorption & transport of substances
Short lifespan: 1-6 days
SA:
Folds, villi & microvilli increase SA to 200m^2 (from 0.4^2) - 500x increase
Microvilli:
What do microvilli make up?
How many microvilli per cell?
What is the surface of microvilli covered with?
What is the glycocalyx?
What does the glycocalyx serve as protection from?
What is the unstirred layer?
What is the purpose of the unstirred layer?
Microvilli (~0.5-1.5 micrometers high) make up brush border
Several thousand microvilli per cell
Surface of microvilli covered w glycocalyx
Glycocalyx:
Rich carbohydrate layer on apical membrane
Protection from digestional lumen yet allows for absorption
Traps a layer of water and mucous - unstirred layer
->Regulates rate of absorption from intestinal lumen
Goblet cells:
Abundance?
What causes goblet shape?
What is mucuous and what is its function?
An increase in abundance of goblet cells along entire length of bowel results in?
2nd most abundant epithelial cell type
Mucous containing granules accumulate at apical end of cell -> causing goblet shape
Mucous - large glycoprotein that facilitates passage of material through bowel
Increase in abundance of goblet cells along entire length of bowel
- decrease in duodenum
- increase in colon
Enteroendocrine cells/chromaffin cells:
Describe the structure of enteroendocrine cells
Describe the organisation of enteroendocrine cells: where are they found?
What is the function of enteroendocrine cells?
Columnar epithelial cells
Scattered among enterocytes
Most often found in lower part of crypts
Hormone secreting
-eg to influence gut motility
Paneth cells:
Where are paneth cells found?
What type of granules do paneth cells contain?
What do these granules contain?
What are the defence against infection roles of paneth cells?
Only found in the bases of crypts
Contains large, acidophilic granules
Granules contain:
- lysozyme - antibacterial enzyme (protects stem cells)
- glycoproteins & zinc (essential trace metal for lots of enzymes)
Also engulf some bacteria and Protozoa
Potential role in regulating intestinal flora
Stem cells:
What are stem cells?
Why are epithelial stem cells essential in the GI tract?
What type of stem cells are found in the GI tract?
Stem cells - 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
-> digested and reabsorbed
Pluripotent - Differentiate into various cell types
Rapid turnover of enterocytes:
What is the lifespan of enterocytes & goblet cells of the small bowel? This contrasts the lifespan of weeks/months for other epithelial cell types (eg lung, blood vessels) why?
Enterocytes & goblet cells of small bowel have short life span (~36 hrs)
This is because:
Enterocytes are the first line of defence against GI pathogens & may be directly affected by toxic substances in diet
Effects of agents which interfere w cell function, metabolic rate etc will be diminished
Any lesions will be short lived
If transit of enterocytes is interrupted through impaired production of new cells (eg radiation) severe intestinal dysfunction will occur
Small bowel - duodenum:
What distinguishes the duodenum from the jejunum and ileum?
Where does the brunners glands open into?
What are the functions of the alkaline secretions from the brunners glands?
Duodenum:
Distinguished by presence of brunners glands
Submucosal coiled tubular mucous glands secreting alkaline fluid
Open into the base of the crypts
Alkaline secretions of brunners glands:
- neutralises acidic chyme from stomach, protecting proximal small bowel
- helps optimise pH for action of pancreatic digestive enzymes
What are the functions of small intestine motility?
- to mix ingested food with digestive secretions & enzymes
- to facilitate contact between contents of intestine & intestinal mucosa
- to propel intestinal contents along alimentary tract
Small bowel - motility
What is segmentation?
What does it do?
Where does it occur?
Allows pancreatic enzymes and bile to mix with?
What is the direction of net movement of _____?
- Segmentation (mixing)
- 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 w chyme
Chyme moves in both directions but net effect is movement through colon
Small bowel - motility:
Peristalsis: What is it What type of contraction and contraction of what? What does it propel? How far do waves of peristalsis travel?
- Peristalsis (propelling)
Sequential contraction of adjacent rings of smooth muscle
Propels chyme towards colon
Most waves of peristalsis only travel about 10 cm
Segmentation & peristalsis result in chyme being segmented, mixed & propelled -> colon
Small bowel - motility
Migrating motor complex
Contractions of what? Through where?
Detail the journey of the contractions: begins in _____ -> ________ -> _______ -> next wave starts in ________
What does the migrating motor complex prevent the migration of?
- 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
Digestion & absorption mechanisms:
What type of environment does digestion in the small bowel occur in?
From where do digestive enzymes and bile enter the duodenum from?
What does the duodenal epithelium produce?
Where does digestion occur also?
Digestion in duodenum:
-digestion in small bowel occurs in an alkaline environment
- pancreatic digestive enzymes & bile enter duodenum from MPD & CBD
- duodenal epithelium produces its own digestive enzymes
- digestion occurs in lumen & in contact w the membrane
Digestion of carbohydrates:
Where does carbohydrate digestion begin and by what enzyme?
Where does most of the digestion of carbohydrates occur?
What are examples of simple and complex carbohydrates?
Carbohydrate digestion begins in mouth by salivary alpha-amylase (destroyed in stomach bc of acidic pH)
Most of the digestion of carbs occurs in small intestine
Simple carbohydrates:
Monosaccharides - glucose & fructose
Disaccharides - sucrose & maltose
Complex carbohydrates:
Starch, cellulose, pectins
Digestion of carbohydrates:
Action of pancreatic alpha amylase:
-where is it secreted into in response to a meal?m
- what does it continue the digestion of and where?
- what does pancreatic alpha amylase require for optimum activity? And what pH?
- Where does pancreatic alpha amylase mainly act?
- where does the digestion of amylase products & simple carbohydrates occur?
Pancreatic alpha amylase
-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
- digestion of amylase products & simple carbohydrates occurs at the brush border
Digestion of carbs:
How are glucose, fructose & galactose absorbed and via which transporters on what type of membrane?
What is the function of GLUT-2
How much simple sugar can the human small intestine absorb a day?
Absorption of glucose & galactose is by secondary 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
Small intestine can absorb 10kg of simple sugars/day
Digestion of proteins:
Where does protein digestion begin and by what enzyme?
Where is this enzyme inactivated?
Pancreatic proteases are secreted as precursors into where?
What activates trypsin? Where is this enzyme located?
What is the role of trypsin?
Protein digestion:
Begins - lumen of stomach by pepsin
-> Pepsin then inactivated in alkaline duodenum
5x pancreatic proteases secreted as precursors into lumen of small bowel (eg trypsinogen)
Trypsin activated by enterokinase - enzyme located on duodenal brush border
Trypsin -> activates other proteases
Hydrolyse proteins -> single amino acids (AA) & oligopeptides (AA)n
Digestion of proteins - action of luminal, brush-border & cytosolic peptidases:
Via the action of what do enterocytes directly absorb some of the small (AA)n?
Small peptides are digested to AAs by what enzyme and where of what type of cell?
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
Digestion of lipids:
Why are lipids more complicated to digest?
What is the 4 stage process of digestion of lipids in the small bowel?
What is the purpose of colipase complexes with lipase?
Lipids are poorly soluble in water
-more complicated to digest
4 stage process in small bowel:
- secretion of bile salts and pancreatic lipases
- emulsification (increases 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
