Small Intestine Structure and Function Flashcards

1
Q

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

A

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.

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2
Q

State what the intestine absorbs and secretes.

A

Villus cell absorbs;

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

Crypt cell secretes;

  • Cl
  • Water
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3
Q

Describe intestinal fluid secretion.

A
  • 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.
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4
Q

Describe the Na-K-Cl co-transporter.

A
  • 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.
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5
Q

Describe segmentation.

A
  • 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.
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6
Q

Describe the generation of segmentation contractions.

A
  • 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.
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7
Q

Describe peristalsis.

A
  • 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.
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8
Q

Describe the law of the intestine.

A

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.
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9
Q

Describe the gastroileal reflex.

A

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).
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