Small Intestine Structure and Function

Question 1

How long is the total length of the duodenum?

Answer 1

~6m

Question 2

What is the role of the duodenum?

Answer 2

• 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

Question 3

What is the role of the Jejunum?

Answer 3

• Flat shaped as nearly always empty and appears pink as highly vascular but empty
• Absorbs nutrient that aren’t absorbed in duodenum

Question 4

What is the role of the ileum?

Answer 4

Specialised to absorb NaCl and H2O which dehydrates the chyme
Absorb left over nutrients i.e. Vitamin B12 and bile acids for reuse

Question 5

How is the absorptive surface area in the small intestine increased?

Answer 5

Folds (plicae circulares)
Villi
Microvilli

Question 6

How does plicae circulares help with mixing?

Answer 6

They’re folds which causing the mixture to spin, so all nutrients comes into contact with epithelium

Question 7

What are interspersed between the villi and what is their role?

Answer 7

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

Question 8

What type of epithelium does the small intestine contain?

Answer 8

Tall columnar epithelial cells (with each villi containing a vein and artery which anatomies through a capillary network)

Question 9

Name 8 molecules that the villi can absorb?

Answer 9

NaCl
Monosaccharides 
Amino acids
Peptide 
Fats
Vitamins
Mineral 
Water

Question 10

What 2 molecules do the crypts secrete?

Answer 10

Water and Cl

Question 11

Describe how nutrients are absorbed across the villi?

Answer 11

1. Enter through Na-coupled transporter
2. Na immediately transported out via Na-K-ATPase
3. Nutrient transported into the blood via a basolateral membrane protein
4. Created osmotic gradient as Na is being pumped into blood, so water diffuses out of lumen into blood

Question 12

Why is H2O secretion important?

Answer 12

• 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

Question 13

Describe how NaCl is secreted out of crypt cells

Answer 13

1. Na+, K+, 2 Cl- enter by cotransport (NKCC)
2. Na pumped back into ECF via NaK-ATPase and K pumped out via leaky K channels
3. Adenylyl cyclase activated, generating cyclicAMP
4. cAMP activate CFTR which is an apical Cl- channel, allowing Cl to enter the lumen
5. Movement of negative Cl-, draws Na+ down electrical gradient through leaky cell junctions
6. Water follow through osmotic gradient, creating isotonic saline solution

Question 14

How is the CFTR channel in cyst membrane regulated to release Cl-?

Answer 14

1. Enzyme, adenylate cyclase, in the basolateral membrane converts ATP -> cyclic AMP
2. CAMP phosphorylates its own protein kinase A
3. PKA corresponding with specialised sites on the CFTR membrane to open the channel

Question 15

What are the two distinct types of movement during intestinal mobility?

Answer 15

Segmentation

Peristalsis

Question 16

What is segmentation?

Answer 16

Contraction relaxation of short intestinal segments

Question 17

What is the benefit of segmentation?

Answer 17

• 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

Question 18

What is Migrating Motility Complex?

Answer 18

• 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

Question 19

What is the benefit of peristalsis/MMC?

Answer 19

• Occurs after segmentation to move undigested material into large intestine
• Limit bacterial colonisation of small intestine

Question 20

What hormone is involved in the initiation of MMC?

Answer 20

Motilin through migration in the blood

Question 21

What are the laws of the intestine when smooth muscle is distended due to bolus of chyme?

Answer 21

• 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)

Question 22

Where is the myenteric plexus located?

Answer 22

Between the inner circular and outer longitudinal smooth muscle layers

Question 23

Describe the outline of the Gastroileal reflex (movement of chyme from stomach to colon)

Answer 23

1. Gastric emptying –> increase segmentation
2. Opening of iliocaecal valve (sphincter)
3. Entry of chyme into large intestine
4. Distension of colon
5. Reflex contraction of iliocaecal sphincter (prevents reflux into small intestine)

Question 24

How is segmentation initiated?

Answer 24

• 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

Question 25

Why does BER decrease as you move down the intestine?

Answer 25

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

Question 26

What is the effect of parasympathetic NS on gut motility?

Answer 26

Increase in contraction via vagus nerve

Question 27

What is the effect of sympathetic NS on gut motility?

Answer 27

Decrease contraction