Small Intestine Structure and Function Flashcards
how long is the duodenum?
25 cm long
What is the length of the small intestine?
About 6m
What does the small intestine consist of?
Duodenum
Jejunum
Ileum
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?
as a consequence of active secretion of chloride into intestinal lumen
Why is H2O secretion important for normal digestive processes?
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
What happens to H2O secreted by crypts?
Normally H2O secreted by crypts reabsorbed by villi
What are the 2 distinct kinds of movements that makes up intestinal motility?
segmentation
peristalsis
What is segmentation?
occurs when the intestine divides into many sections due to simultaneous transient contractions, and when these contractions subside, contractions appear more or less at the sites in between the original contractions giving a checkered appearance.
Contraction ⇔ relaxation of short intestinal segments
Contraction (few seconds) moves chyme (up & down) into adjacent areas of relaxation
Relaxed areas then contract and push chyme back
When is segmentation most common?
most common during a meal
What is the function of segmentation?
Provides thorough mixing of contents with digestive enzymes
Brings chyme into contact with absorbing surface
How long does each cycle of segmentation last?
few seconds
Explain the mechanism of segmentation contractions?
Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer (cf gastric motility)
Intestinal basic electrical rhythm (BER) produces oscillations in membrane potential ⇒ threshold ⇒ action potential ⇒ contraction
Action potential frequency determines strength of contraction
Frequency of segmentation determined by BER
What is BER?
The basal or basic electrical rhythm (BER) or electrical control activity (ECA) is the spontaneous depolarization and repolarization of pacemaker cells known as interstitial cells of Cajal (ICCs) in the smooth muscle of the stomach, small intestine, and large intestine.
What determins the strength of contraction for segmentation?
Action potential frequency determines strength of contraction
What is frequency of segmentation determined by?
Frequency of segmentation determined by BER
How does BER change as chyme moves down the intestine to the rectum?
BER decreases as move down intestine → rectum
What supplies the innervation for segmentation?
Parasympathetic NS (vagus) ⇒ ↑ contraction
Sympathetic NS ⇒ ↓ contraction
No effect of autonomic nervous system on BER
Does the autonomic nervous system have an effect on BER?
no effect
What effect does the parasympathetic nervous system have on segmentation?
↑ contraction
What effect does the sympathetic nervous system have on segmentation?
↓ contraction
What does segmentation cause?
Following absorption of nutrients: segmentation stops, and peristalsis starts
What occurs following absorption of nutrients in terms of segmentation and peristalsis?
Migrating Motility Complex (MMC)
What are the pattern of contractions that creates peristalsis known as?
Migrating Motility Complex (MMC)
Explain the mechanism of peristalsis?
Pattern of peristaltic activity travelling down small intestine (starts in gastric antrum)
As one MMC ends (terminal ileum) another begins
Arrival of food in stomach ⇒ cessation of MMC and initiation of segmentation
Where does peristalsis begin and end?
gastric antrum
terminal ileum
How does the arrival of food into the stomach impact segmentation and peristalsis?
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 (hormone) involved in initiation of MMC
What happens when intestinal smooth muscle is distended such as due to chyme?
If intestinal smooth muscle is distended (eg by bolus of chyme):
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
What is the bolus moving towards the colon mediated by?
neurones in myenteric plexus
Explain the mechanism of the gastroileal reflex?
Gastric emptying ⇒ ↑ segmentation activity in ileum
Opening of ileocaecal valve (sphincter)
Entry of chyme into large intestine
Distension of colon
Reflex contraction of ileocaecal sphincter (prevents backflux into small intestine)
What reflex controls chyme entering the large intestine?
Gastroileal Reflex
