Small intestine structure and function Flashcards
What is the average total length of the small intestine?
6m
Why is it advantageous to have a long small intestine?
to prevent material passing through too quickly
What is the shortest and longest parts of the small intestine?
duodenum (25cm)
jejunum (2.5m)
ileum (3.5m)
What processes occur in the duodenum?
neutralisation, digestion and iron absorption
What processes occur in the jejunum?
95% nutrient absorption
What processes occur in the ileum?
NaCl/H2O absorption to dehydrate the chyme
Name 3 ways in which the surface area of the small intestine is increased
folds (plicae)
villi and microvilli
What are plicae?
corkscrew folds of the mucous membranes
What are the crypts of Lieberkuhn?
Short gland invaginations between the villi
Explain the structure of a villi and the components making it up
simple columnar epithelium
capillaries for exchange and lacteals for fat absorption
Goblet cells present for mucus secretion
What is the main difference between the villi and the crypts?
villi absorb and crypts secrete
What things does the villi absorb?
amino acids, fat, NaCl, minerals, vitamins, monosaccharides, water
What does the crypts secrete?
water and chloride
What are some molecules which are sodium coupled for transport?
glucose and galactose
amino acid, nucleotides, water soluble vitamins
How much water is secreted a day in the small intestine and from where EXACTLY?
1.5L
epithelial cells of the crypts of Lieberkuhn
What is the water secretion in the small intestine due to? (what drives it)
active chloride secretion into the lumen
List the 4 things water is important for in the small intestine
wash away toxins
mixing
maintains liquid state of lumen
Aids nutrient presentation to the absorbing surface
What is chloride coupled to for movement across the basolateral membrane?
sodium and potassium ie 2 chlorides transported at a time
How is chloride expelled from the cell to the apical membrane and through what receptor?
adenylate cyclase catalyses ATP –> cAMP
turns on protein kinase A which turns on CFTR
What are the 2 types of intestinal motility?
segmentation
peritalsis
When is segmentation most likely to be occurring?
during a meal
Briefly explain what segmentation is
the contracting of one part of the intestine and the relaxing of the part before and after the contraction which interchanges to give a mixing of the contents
How is segmentation generated?
pacemaker cells depolarise cells in longitudinal muscle layer and the BER produces AP until contraction
What determines the strength of contraction?
AP frequency
BER decreases or increases from intestine to rectum?
decreases
How does the nervous system stimulate the contraction of segmentation?
Para - vagus to increase
sympathetic - decrease
autonomic - no control
When does peristalsis kick in?
After the nutrients have been absorbed
What is the MMC?
Pattern of peristaltic activity from the gastric antrum
When does another MMC start after the other?
When the one before terminates at the ileum
If food enters the stomach what happens to peristalsis?
stops and segmentation begins
What are the 2 roles of MMC?
move undigested eg cellulose material to the large intestine
prevent bacterial colonisation of the small intestine
What is motilin and how is it increased?
A hormone which initiates MMC
nutrient content drops
If the smooth muscle in the intestine is distended what are the 3 rules which happen?
muscle on anal side of bolus relax
muscle on oral side contract
bolus moved into relaxation toward colon
What mediates the law of the intestine?
neurons in the myenteric plexus
What is the general movement of a bolus in the intestine?
to the anal end
What happens to a bolus once it has reached the ileum (gastroileal reflex)
peristalsis starts as the stomach is emptying
The ileocaecal sphincter opens and the chime enters the large intestine. This distends the colon and closes the valve behind it to prevent backflux.
