Small Intestine Structure and Function Flashcards
Summarise the functions of the various sections of the small intestine.
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.
State what the intestine absorbs and secretes.
Villus cell absorbs;
- NaCl
- Monosaccharides
- Amino acids
- Peptides
- Fats
- Vitamins
- Minerals
- Water
Crypt cell secretes;
- Cl
- Water
Describe intestinal fluid secretion.
- 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.
Describe the Na-K-Cl co-transporter.
- 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.
Describe segmentation.
- 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.
Describe the generation of segmentation contractions.
- 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.
Describe peristalsis.
- 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.
Describe the law of the intestine.
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.
Describe the gastroileal reflex.
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).