Small Intestine Structure and Function Flashcards
What is the length of the small intestine?
About 6m
What does the small intestine consist of?
Duodenum
Jejunum
Ileum
How long is the duodenum?
25cm long
What are some functions of the duodenum?
Gastric acid neutralisation
Digestion
Iron absorption
How long is the jejunum?
2.5m long (2/5 of total length)
What are some functions of the jejunum?
Nutrient absorption
What percentage of nutrient absorption occurs in the jejunum?
95%
How long is the ileum?
3.5m (3/5 total length)
What is the function of the ileum?
NaCl/H2O absorption
What is the absorptive surface of the small intestine enhanced by?
Folds, villi and microvilli
What are the circular folds of the small intestine called?
Plicae
What is the relative surface area increase of the small intestine due to plicae?
3
What is the relative surface area increase of the small intestine due to villi?
30
What is the relative surface area increase of the small intestine due to microvilli?
600
What is the classification of the epithelium in the small intestine?
Simple columnar
What does villus cells absorb?
NaCl
Monosaccharides
Amino acids
Peptides
Fats
Vitamins
Minerals
Water
What are the functions of crypt cells?
Secrete Cl and water
How much water does the small intestine secrete everyday?
1500ml
Where does the water that the small intestine secretes come from?
Epithelial cells lining the crypts of Lieberkuhn
How does the small intestine secrete water passively?
Osmotically as a consequence of secreting Cl into the lumen of the gut
Why is H2O secretion important for normal digestive processes?
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 happens to H2O secreted by crypts?
Normally reabsorbed by villi
What are the 2 distinct kinds of movements that makes up intestinal motility?
Segmentation
Perstalsis
What is segmentation?
Contraction of the circular muscles in the digestive tract
What is perstalsis?
Contraction of the longitudinal muscles in the digestive tract
When is segmentation most common?
During a meal
What is the function of segmentation?
Alterations between contraction and relaxation of short intestinal segments moves chyme up and down into adjacent areas of relaxation
Provides thorough mixing of contents with digestive enzymes
Brings chyme into contact with absorbing surface
How long does each cycle of segmentation last?
A few seconds
Explain the mechanism of segmentation contractions?
1) Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer
2) Intestinal basic rhythm (BER) produces oscillations in membrane potential
3) Reaches threshold which generates an action potential
4) Causes contraction
What is BER?
Basic electrical rhythm
What determins the strength of contraction for segmentation?
Action potential frequency
What is frequency of segmentation determined by?
BER
How does BER change as chyme moves down the intestine to the rectum?
Decreases
What supplies the innervation for segmentation?
Parasympathetic (vagus) increases contraction strength
Sympathetic decreases contraction strength
Does the autonomic nervous system have an effect on BER?
No
What effect does the parasympathetic nervous system have on segmentation?
Increases contraction strength
What effect does the sympathetic nervous system have on BER?
Decreases contraction strength
What does segmentation cause?
Slow migration of chyme towards large intestine (more chyme is pushed towards it than away from it)
What occurs following absorption of nutrients in terms of segmentation and peristalsis?
Segmentation stops and peristalsis begins
What are the pattern of contractions that creates peristalsis known as?
Migrating motility complex (MMC)
Explain the mechanism of peristalsis?
1) Starts in gastric antrum
2) As one MMC ends at the terminal ileum another begins
Where does peristalsis begin and end?
Begins at the antrum of the stomach and ends at the terminal ileum
How does the arrival of food into the stomach impact segmentation and peristalsis?
Causes cessation of MMC and initiation of segmentation
What is the function of peristalsis?
Move undigested material into large intestine
Limit bacterial colonisation of small intestine
What is a hormone involved in the initiation of MMC?
Motilin
What happens when intestinal smooth muscle is distended such as due to chyme?
Muscle on oral side contracts
Muscle on anal side relaxes
Bolus is moved into area of relaxation towards colon
What is the bolus moving towards the colon mediated by?
Enteric nervous system
Explain the mechanism of the gastroileal reflex?
1) Gastric emptying causes increase in segmentation activity in ileum
2) Opening of ileocaecal valve (sphincter)
3) Distention of colon
4) Reflex contraction of ileocaecal sphincter (prevents backflux into small intestine)
What reflex controls chyme entering the large intestine?
Gastroileal reflex
