Small Intestine Structure and Function

Question 1

1. what is the total length of small intestine?

2. What increases absorptive surface?

Answer 1

1. 6 metres (range 4.5 - 9 metres)

2. Folds, villi, microvilli

Question 2

How long is duodenum & what its function?

Answer 2

25cm long. Gastric acid neutralisation; digestion; iron absorption.

Question 3

How long is jejunum & what its function?

Answer 3

2.5m long (2/5 total length). Nutrient absorption (95%).

Question 4

How long is ileum & what its function?

Answer 4

3.5m long (3/5 total length). NaCl/H2O absorption leads to chyme dehydration.

Question 5

What does villus cell absorbs?

Answer 5

NaCl
monosaccharides 
amino acids
peptides
fats
vitamins
minerals
water

Question 6

Crypt cell secretes what?

Answer 6

Cl- & water

Question 7

1. How much fluid does intestine secrete per day?
2. Where does this fluid (H2O) secreted comes from & what happens to it after?
3. Does the above secretion require energy to take place?
4. Why is H2O secretion important for normal digestive process?

Answer 7

1. 1500ml H2O
2. epithelial cells lining crypts of Lieberkühn. It is reabsorbed by villi
3. Not really. H2O is secreted passively (osmotically) as a consequence of active secretion of chloride ions into intestinal lumen.
4. -Maintains luminal contents in liquid state
   - Promotes mixing of nutrients with digestive enzymes
   - Aids nutrient presentation to absorbing surface
   - dilutes & washes away potentially injurious substances

Question 8

Which receptor transports chloride ions from cell to intestinal lumen?

Answer 8

CFTR (cystic fibrosis transmembrane conductance regulator)

Question 9

Intestinal Motility:

What are the 2 distinct types of movement?

Answer 9

1. Segmentation

2. Peristalsis

Question 10

Explain segmentation.

Answer 10

• Contraction/relaxation of short intestinal segments.
• Contraction (few secs) moves chyme (up & down) into adjacent areas of relaxation.
• relaxed areas then contract & push chyme back
• provides thorough mixing of contents with digestive enzymes
• brings chyme into contact with absorbing surface.

Question 11

When does segmentation & peristalsis happen?

Answer 11

Segmentation occurs during the meal. Following absorption of nutrients: segmentation stops & peristalsis starts.

Question 12

Explain how segmentation contractions are generated & what determines their strength?

Answer 12

• Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer (cf gastric motility).
• Intestinal basic electrical rhythm (BER) produces oscillations in membrane potential which when reaches threshold generates AP which then produces contraction.
• AP frequency determines strength of contraction

Question 13

What determines frequency of segmentation?

Answer 13

Basic electrical rhythm (BER)

Question 14

What happens to BER as we move down the intestine to rectum & why?

Answer 14

Decreases. Therefore, segmentation produces slow migration of chyme towards large intestine (more chyme pushed down than up).

Question 15

What increases segmentation contraction?

Answer 15

Parasympathetic NS (vagus)

Question 16

What decreases segmentation contraction?

Answer 16

Sympathetic NS

Question 17

What effect does autonomic nervous system on BER?

Answer 17

No effect on BER.

Question 18

What is migrating motility complex (MMC)?

Answer 18

Pattern of peristaltic activity travelling down small intestine (starts in gastric antrum).
As one MMC ends (terminal ileum) another begins

Question 19

What happens to migrating motility complex (MMC) when food arrives in stomach?

Answer 19

MMC cessation & initiation of segmentation.

Question 20

What is the purpose of migrating motility complex (MMC)?

Answer 20

Move undigested material into large intestine.

Limit bacterial colonisation of small intestine.

Question 21

What is involved in migrating motility complex (MMC) initiation?

Answer 21

Motilin (hormone)

Question 22

Explain how bolus move down the intestine & what mediates it.

Answer 22

If intestinal smooth muscle is distended (e.g. by bolus of chyme):

• muscle on oral side of bolus contracts
• muscle on anal side of bolus relaxes
• bolus is moved into area of relaxation towards colon
• mediated by neurones in myenteric plexus (ascending neurone contracts circular muscle by releasing Ach & substance P; & relaxes longitudinal muscle by releasing NO this contracts area above the bolus & constricts it. Below the bolus, descending neurone relaxes circular muscle & contracts longitudinal muscle this makes the area bigger so bolus gets pushed down.

Question 23

What is gastroileal reflex?

Answer 23

Gastric emptying increases segmentation activity in ileum.

• Opening of ileocaecal valve (sphincter)
• entry of chyme into large intestine
• distension of colon
• reflex contraction of ileocaecal sphincter (prevents back flux into small intestine).