Small Intestine Structure and Function

Question 1

Summarise the functions of the various sections of the small intestine.

Answer 1

Total length = 6 metres

Duodenum;

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

Jejunum;

• 2.5m long
• Nutrient absorption (95%)

Ileum;

• 3.5m long
• NaCL/H2O absorption –> chyme dehydration (absorbs salt so water follows)

Absorptive surface area enhanced by folds, villi and microvilli.

Question 2

State what the intestine absorbs and secretes.

Answer 2

Villus cell absorbs;

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

Crypt cell secretes;

• Cl
• Water

Question 3

Describe intestinal fluid secretion.

Answer 3

• Small intestine secretes 1500ml H2O per day.
• H2O secretion comes from epithelial cells lining crypts of Lieberkuhn.
• H2O passively (osmotically) secreted as a consequences of active secretion of Cl into intestinal lumen.
• Normally H2O secreted by crypts absorbed by villi.

H20 secretion important for normal digestive process;

• Maintains lumenal 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 4

Describe the Na-K-Cl co-transporter.

Answer 4

• Takes place in crypts of Lieberkuhn.
• Uses Na electrochemical gradient produced from Na-K pump to bring 2Cl into the cell.
• Inside of cell now has negative potential so Cl leaves through channel in apical membrane.

Question 5

Describe segmentation.

Answer 5

• Most common during meal.
• Contraction/relaxation of short intestinal segments.
• Contraction (few seconds) move chyme 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 6

Describe the generation of segmentation contractions.

Answer 6

• Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer.
• BER produces oscillations in membrane potential –> threshold –> action potential –> contraction.
• Action potential frequency determines strength of contraction.
• Frequency of segmentation determined by BER.
• BER decreases as you move down intestine (slow migration of chyme towards large intestine).
• Parasympathetic NS (vagus) –> increase in contraction.
• Sympathetic NS –> decrease.
• No effect of autonomic NS on BER.

Question 7

Describe peristalsis.

Answer 7

• Peristalsis follows segmentation (absorption of nutrients).
• Motilin (hormone) involved in initiation of MMC.

Migrating motility complex (MMC);

• Pattern of peristaltic activity travelling down small intestine (starts in gastric antrum).
• As one MMC ends (at terminal ileum), another begins.
• Arrival of food in stomach –> MMC stops and segmentation begins.

MMC acts to;

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

Question 8

Describe the law of the intestine.

Answer 8

If intestinal smooth muscle is distended e.g. by bolus of chyme;

• Muscle of oral side of bolus contracts.
• Muscle on anal side relaxes.
• Bolus is moved into area of relaxation towards colon.
• Mediated by neurons in myenteric plexus.

Question 9

Describe the gastroileal reflex.

Answer 9

Gastric emptying –> increase of segmentation activity in ileum.

• Opening of ileocaecal sphincter.
• Entry of chyme into colon.
• Distension of colon.
• Reflex contraction of ileocaecal sphincter (prevents back flux into small intestine).