Small Intestine Structure and Function Flashcards
How long is the total length of the duodenum?
~6m
What is the role of the duodenum?
- Neutralises gastric acid as digestion can only occur at a neutral pH
- Site of the majority of digestion (95%)
- Site of iron absorption as epithelial cells express DMT1 transporter
What is the role of the Jejunum?
- Flat shaped as nearly always empty and appears pink as highly vascular but empty
- Absorbs nutrient that aren’t absorbed in duodenum
What is the role of the ileum?
Specialised to absorb NaCl and H2O which dehydrates the chyme
Absorb left over nutrients i.e. Vitamin B12 and bile acids for reuse
How is the absorptive surface area in the small intestine increased?
Folds (plicae circulares)
Villi
Microvilli
How does plicae circulares help with mixing?
They’re folds which causing the mixture to spin, so all nutrients comes into contact with epithelium
What are interspersed between the villi and what is their role?
Crypts - blind end sacs which contain:
• Paneth cells - defensive function
• Endocrine cells - produce secretin, somatostatin, enteroglucagon and serotonin
• Stem cells - divide continuously to replace enterocytes
What type of epithelium does the small intestine contain?
Tall columnar epithelial cells (with each villi containing a vein and artery which anatomies through a capillary network)
Name 8 molecules that the villi can absorb?
NaCl Monosaccharides Amino acids Peptide Fats Vitamins Mineral Water
What 2 molecules do the crypts secrete?
Water and Cl
Describe how nutrients are absorbed across the villi?
- Enter through Na-coupled transporter
- Na immediately transported out via Na-K-ATPase
- Nutrient transported into the blood via a basolateral membrane protein
- Created osmotic gradient as Na is being pumped into blood, so water diffuses out of lumen into blood
Why is H2O secretion important?
- Maintains luminal contents in liquid state
- Promotes mixing of nutrients with digestive enzymes
- Aids nutrient presentation to absorbing surface
- Dilutes and washes away potentially injurious substances
Describe how NaCl is secreted out of crypt cells
- Na+, K+, 2 Cl- enter by cotransport (NKCC)
- Na pumped back into ECF via NaK-ATPase and K pumped out via leaky K channels
- Adenylyl cyclase activated, generating cyclicAMP
- cAMP activate CFTR which is an apical Cl- channel, allowing Cl to enter the lumen
- Movement of negative Cl-, draws Na+ down electrical gradient through leaky cell junctions
- Water follow through osmotic gradient, creating isotonic saline solution
How is the CFTR channel in cyst membrane regulated to release Cl-?
- Enzyme, adenylate cyclase, in the basolateral membrane converts ATP -> cyclic AMP
- CAMP phosphorylates its own protein kinase A
- PKA corresponding with specialised sites on the CFTR membrane to open the channel
What are the two distinct types of movement during intestinal mobility?
Segmentation
Peristalsis
What is segmentation?
Contraction relaxation of short intestinal segments
What is the benefit of segmentation?
- Contraction moves chyme up and down into adjacent areas of relaxation
- Relaxed areas then contact and push chyme back
- Provides mixing of content with digestive enzymes and brings chyme into contact with epithelium
- Most common during meal
What is Migrating Motility Complex?
- Pattern of peristaltic activity travelling down small intestine
- As one MMC end (terminal ileum) another begins
- Arrival of food in stomach –> cessation of MMC and initiation of segmentation
What is the benefit of peristalsis/MMC?
- Occurs after segmentation to move undigested material into large intestine
- Limit bacterial colonisation of small intestine
What hormone is involved in the initiation of MMC?
Motilin through migration in the blood
What are the laws of the intestine when smooth muscle is distended due to bolus of chyme?
- Muscle on oral side contacts
- Muscle on anal side relaxes
- Bolus is moved into area of relaxation toward colon
(All mediated by neurone in the myenteric plexus)
Where is the myenteric plexus located?
Between the inner circular and outer longitudinal smooth muscle layers
Describe the outline of the Gastroileal reflex (movement of chyme from stomach to colon)
- Gastric emptying –> increase segmentation
- Opening of iliocaecal valve (sphincter)
- Entry of chyme into large intestine
- Distension of colon
- Reflex contraction of iliocaecal sphincter (prevents reflux into small intestine)
How is segmentation initiated?
- Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer (cf gastric motility)
- Intestinal basic electrical rhythm (BER) produces oscillations in membrane potential threshold (with nerve input) AP contraction
- AP frequency determines strength of contraction
- BER decreases as you move down intestine
Why does BER decrease as you move down the intestine?
Frequency of segmentation is determined by BER, so the slower it goes, more chyme is push down than up as there is more contraction in the upper parts
What is the effect of parasympathetic NS on gut motility?
Increase in contraction via vagus nerve
What is the effect of sympathetic NS on gut motility?
Decrease contraction
