Week 3.9 - Small Intestine Structure and Function Flashcards
How long is the duodenum, jejunum and ileum?
25cm, 2.5M, 3.5M
What is the duodenum responsible for?
gastric acid neutralisation
digestion
iron absorption
What is the jejunum responsible for?
95% of nutrient absorption
What is the ileum responsible for?
NaCl and H2O absorption and chyme dehydration
What does the absorptive surface consist of?
folds (plica), villi, microvilli, crypts
What are crypts responsible for?
- indentings further increase SA
- secrete water and Cl to lumen
- stem cells to regenerate epithelium
What is the lifespan of small intestine epithelium?
3-5 days. regenerates by stem cells from crypts.
What are villus responsible for absorbing?
- NaCl,
- monosaccharides,
- amino acids and dipeptides
- fats,
- vitamins,
- minerals,
- water
How much water does the small intestine secrete per day?
1.5L normally
Why is it important for water secretion in the intestine?
- to form liquidy chyme.
- enzymes work better in a fluid.
- aids nutrient presentation
- dilute toxins
How does chloride enter intestinal lumen?
3 transporters.
- 1 allows 2 chloride in, 1 Na in and 1 K in.
- a K+ pump out.
- a NaK pump
all help move chloride in. then it exits via CFTR
How does cystic fibrosis affect water absorption in lumen?
significantly - CFTR transporter is vital for chloride movement which drives water movement.
What is segmentation?
occurs during meals to digest food. allows food to stay close to border for better absorption. contraction and relaxation at same time to cycle food.
What generates segmentation?
longitudinal pacemaker cells. create basic electrical rhythm. decreases down towards rectal direction to avoid chyme moving too fast into colon.
what autonomic control is on segmentation?
parasympathetic vagal nerve increases contraction, sympathetic decreases.
has no effect on BER as that is controlled by pacemaker cells in longitudinal muscles
What is peristalsis?
follows absorption of nutrients, segmentation stops and peristalsis starts. contraction on oral side and relaxation on anal side to push bolus down
What controls peristalsis?
MMC - migrating motility complex. hormone motilin initiates MMC.
Where does peristalsis MMC begin and end?
gastric antrum to bottom of small intestine.
What causes peristalsis to stop?
stops when food arrives in stomach and segmentation begins again.
What are aims of MMC?
- move undigested food revenue out small intestine
- eliminate bacterial colonisation of small intestine by removing unabsorbed food/nutrients.
- prevents bacteria entering small intestine from large by creating one way path
What nervous system mediates the movement in intestine?
myenteric plexus between longitudinal and circular layers in muscularis externa.
What is the gastro-ileal complex?
- food enters stomach
- stretch receptors trigger segmentation and the opening of ileocaecal valve and intensifies MMC allowing chyme to enter large intestine, and distending the colon.
- local reflex of ileocaecal sphincter closes the valve.
What is mostly absorbed in the terminal ileum?
bile salts and vitamin B12
