Small Intestine Structure and Function Flashcards
3 divisions of the small intestine? Functions?
Duodenum - acid neutralization, digestion, Fe absorption
jejunum - Nutrient absorption
ileum - NaCl/H2O absorp. (dehydrate chyme), B12 absorption
How long is the small intestine? Approx. size of each division?
Ranges from 4.5 - 9m
Duodenum - 25cm
Jejunum - 2.5m
Ileum - 3.5m
How does the small intestine increase SA for nutrient absorption?
Through folds and fingerlike projections (villi)
Intestine is a cylinder - inner layer arranged in circular folds - on circular folds are villi - on the villi are microvilli
What are the crypts of Lieberkuhn?
They are indents into the lamina propria between villi - contain gut stem cells
Stem cells begin differentiating at the bottom of the crypts, move up the villi and mature until they become villus enterocytes
How much do microvilli increase the surface area for absorption compared to a flat cylindrical intestinal lumen?
Increased about 600 fold
About 200m squared available for absorption in the SI
What secretes the mucosal lining in the SI?
Goblet cells
Responsible for protection and acid microclimate
What are the functions of villi and crypts in the intestinal wall?
Villi - absorb nutrients, vitamins, salt, minerals & water
Crypts - secrete Cl and water (water follows Cl - passive secretion)
Why is a CF patients mucous thick in the SI?
Because the CFTR transporter in the crypts is not functional, can’t transport Cl into the lumen and therefore can’t get water in to the lumen - thicker mucous
Why does the small intestine need to secrete water? Secretes about 1500ml/day
- Keep lumen contents in liquid state (not thick like CF)
- Allows nutrients to mix with digestive enzymes
- Helps nutrients reach absorptive surface
- Dilutes and washes away toxins
Describe the mechanism crypt cells use to transport water into the intestinal lumen
Cl in via a Na/K/Cl symporter, K back out through leaky channels - Na out through Na/K exchanger
Eventually large Cl gradient builds up in cell, crosses apical membrane into lumen through CFTR (if open)
Water follows the Cl into the lumen
What regulates the CFTR channel in the crypt cells?
CFTR is only activated once phosphorylated by PKA (protein kinase A)
PKA is only activated after phosphorylated by cAMP, which is activated by phosphorylation via ATP hydrolysis done by adenylate cyclase
What are the mechanisms of motility in the small intestine? When is each performed?
Segmentation - during meal (mix)
Peristalsis - after meal (clear up)
Describe segmentation
Short segments of the SI contract, move chyme into other segments that are relaxed - then these segments contract while the others relax, moving it back (slow process)
Provides thorough mixing of contents for digestion, and brings contents into contact with absorptive surface
How are segmentation contractions regulated?
In much the same way as peristalsis in the stomach, pacemaker cells in the muscle set the BER - that how often segmentation contractions occur
Strength of contraction can change with stimuli (neural/hormonal) - but in segmentation the rate of contraction always follows the BER
How does BER change along the intestine? What does this allow?
BER decreases down the intestine - rectum
allows the lumen contents to slowly move down the gut tube during segmentation
Effect of nervous systems on segmentation?
Parasympathetic - increases force of contraction
Sympathetic - decreases force of contraction
Neither can affect the BER (frequency of contractions)
What is the migrating motility complex? Describe its mechanism
Mechanisms of peristaltic clearing of the small intestine - contract at the proximal end, relaxed at the distal - moves chyme along
As one MMC ends (ileum), another begins
When there is new food in the stomach MMC stops and segmentation starts up again
What effect does the MMC have on bacterial colonization?
Reduces it, bacteria get washed away with the chyme
What hormone activates the MMC?
Motilin - secreted by proximal small intestine
What is the “Law of the Intestine?”
If pressure created by a food bolus distends the intestine, the oral side (proximal) will contract and the anal (distal) side will relax - moving the chyme down the intestine and reducing pressure
What is the gastroileal reflex?
Gastric emptying occurs - increases ileal segmentation
ileocaecal sphincter opens and chyme moves into the large intestine
Distension of colon causes reflex closing of ileocaecal sphincter (preventing reflux)
