Small intestine

Question 1

Duodenum

Answer 1

• 25 cm long
• Gastric acid neutralisation
• Digestion
• Iron absorption

Question 2

Jejunum

Answer 2

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

Question 3

Jejunum

Answer 3

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

Question 4

What does the Villus cell absorb:

Answer 4

• NaCl
• Monosaccharides
• Amino acids
• Peptides
• Fats
• Vitamins
• Minerals
• Water

Question 5

Crypt cell function

Answer 5

• Secretes Cl & Water

Question 6

Role of water secretion

Answer 6

1. Maintains lumenal contents in liquid state
2. Promotes mixing of nutrients with digestive enzymes
3. Aids nutrient presentation to absorbing surface
4. Dilutes and washes away potentially injurious substances

Question 7

Explain transport of Cl

Answer 7

1. Three ions: Cl,Na, K enter the enterocyte simultaneously
2. Sodium leaves, K enters
3. K then leaves via leaky ion channels
4. ATP à cAMP by adenylate cyclase
5. cAMP phosphorylates PKA, activating it
6. PKA activates CFTR protein channel, allowing Cl to enter cells
7. Water would leave via osmosis through tight junctions

Question 8

Segmentation

Answer 8

• Most common during meal
• Contraction and relaxation of short intestinal segments
• Contraction (few seconds) moves chyme (up & down) into adjacent areas of relaxation
• Relaxed areas then contract and push chyme back
• Provides thorough mixing of contents with digestive enzymes
• Brings chyme into contact with absorbing surface

Question 9

Generation of Segmentation contractions

Answer 9

• Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer
• The intestinal basic electrical rhythm produces oscillations in the smooth muscle membrane potential.
• If threshold is reached, action potentials are triggered that increase muscle contraction.
• The frequency of segmentation is set by the frequency of the intestinal basic electrical rhythm (BER) ; each successive region having a slightly lower frequency than the one above
• Action potential frequency determines strength of contraction
• Frequency of segmentation determined by BER
• BER decreases as you move down intestine ® rectum
• Segmentation produces slow migration of chyme towards large intestine (more chyme pushed down than up)

Question 10

Peristalsis in small intestine

Answer 10

After most of a meal has been absorbed, the segmenting contractions cease and are replaced by a pattern of peristaltic activity known as the migrating motility complex ( MMC ).

Question 11

What is the MMC and describe it

Answer 11

• A pattern of peristaltic activity
• Beginning in the lower portion of the stomach (antrum), repeated waves of peristaltic activity travel about 2 feet along the small intestine and then die out
• As one MMC ends (terminal ileum) another begins
• Arrival of food in stomach leads to the cessation of MMC and initiation of segmentation

Question 12

Role of MMC

Answer 12

1. Move undigested material into large intestine
2. Limit bacterial colonisation of small intestine

Question 13

Motilin

Answer 13

• Motilin (hormone) involved in initiation of MMC
• Feeding inhibits the release of motilin; motilin stimulates MMCs via both the enteric and autonomic nervous systems.

Question 14

Law of intestine

Answer 14

If intestinal smooth muscle is distended (eg 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

Question 15

Gastroileal Reflex

Answer 15

Gastric emptying leads to increased ­ segmentation activity in ileum

• Opening of ileocaecal valve (sphincter)
• Entry of chyme into large intestine
• Distension of colon
• Reflex contraction of ileocaecal valve (prevents backflux into small intestine)