Small Intestine Structure and Function Flashcards
How long is the small intestine?
Roughly 6m
What are the 3 components of the small intestine and their lengths?
- Duodenum 25cm
- Jejunum 2.5m
- Ileum 3.5m
What takes place in the duodenum?
- Gastric acid neutralisation
- Digestion
- Iron absorption
What takes place in the jejunum?
95% of nutrient absorption
What takes place in the ileum?
NaCL and H2O absorption leading to chime dehydration
How is the absorptive area of the small intestine enhanced?
- Folds
- Villi
- Microvilli
What does a villus cell absorb?
- NaCl
- Monosaccharides
- Amino acids
- Peptides
- Fats
- Vitamins
- Minerals
- Water
What do crypt cells secrete?
- Cl
- H2O
Describe the Na coupled secondary active transport that takes place at the villi.
- Cooperative transport means that Na enters alongside glucose/galactose/amino acids/nucleosides
- Na is then pumped into the blood while K is pumped in.
- Another transporter pumps K into the blood
- Meanwhile a transporter pumps glucose/glactose/amino acids/nucleoside into the blood
How much water does the small intestine secrete each day?
1500ml
Where does H2O secretion come from?
Epithelial cells lining crypts of Lieberkuhm
How is H2O secreted?
Passively as a consequence of active secretion of chloride into intestinal lumen
Why is H2O important for normal digestion?
- Maintains luminal contents in liquid state
- Promotes mixing of nutrients with digestive enzymes
- Aids nutrient presentation to absorbing surface
- Dilutes and washes away potentially injurious substances
What normally happens to H2O secreted by crypts?
Reabsorbed by villi
Slide 9
Slide 9
What are the 2 distinct types of movement within the intestine?
- Segmentation
- Peristalsis
When is segmentation most common?
During a meal
What is segmentation?
Contraction and relaxation of short intestinal segments
Describe the process of segmentation.
- Contraction moves 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
How is segmentation generated?
-Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer
What does the intestinal basic electrical rhythm produce?
- Oscillations in the membrane potential
- Meets threshold
- Initiates action potential
- Leads to contraction
What determines the strength of contraction?
Action potential frequency
What is the frequency of segmentation determined by?
BER
What happens to BER as it moves down the intestine to the rectum?
Decreases
What does segmentation produce?
Slow migration of chyme towards large intestine (more chyme pushed down that up)
How does the parasympathetic system affect segmentation?
Via the vagus increases contraction
How does the sympathetic system affect segmentation?
Decreases contraction
How does the autonomic nervous system affect BER?
No effect
When does peristalsis occur?
Following absorption of nutrients, segmentation stops and peristalsis starts
What is the migrating motility complex?
Pattern of peristaltic activity travelling down small intestine ( starts in gastric antum)
-As one MMC enda another begins
What causes cessation of MMC?
Food arriving in the stomach which causes initiation of segmentation
What does MMC act to do?
- Move undigested material into large intestine
- Limit bacterial colonisation of small intestine
What is involved in the initiation of MMC?
Hormone motilin
Law of Intestine
If intestinal smooth muscle is distended:
- 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
