Normal Physiology of Small Bowel and Colon Flashcards
What word is used to describe small intestine peristalsis in the fed state? What is that getting at?
“segmentation”
Contraction rates are different in different parts of the small intestine: faster in duodenum, slower in ileum.
3 things that regulate peristalsis in the small bowel?
Distension - increases, particularly in duodenum.
Extrinsic nerves - parasymps increase, symps decrease
Gastrin - increases
What is a “unit” of peristalsis in fasting? What initiates it?
A migrating motor complex (MMC).
Initiated by motilin (secreted by the ileum).
Where’s the “vomiting center”?
Medulla - lateral reticular formation
What things contract/relax during vomiting?
Pyloric sphincter closes.
Abdominal muscles and diaphragm contracts.
LES and esophagus dilate.
Glottis closes (so you don’t aspirate).
What does small bowel secrete? In what disease can these secretions be messed up?
Water and mucus - for lubrication, protection from bugs, and digestion.
These secretions will be messed up in CF.
Is absorption regulated?
No. Everything is absorbed that can be.
Does the gut do any concentrating or diluting?
Nope.
2 parts of the surface epithelium of the small intestine?
Villi and crypts
Function of the villi in the small intestine?
Absorption
Function of the crypt epithelium in the small intestine?
Secretion - Cl- channel (CFTR)
Stem cells
What step of carbohydrate digestion to brush border enzymes do?
3 molecules that they break down?
Disaccharidases break down disaccharides to monosaccharides for absorption.
Maltose, sucrose, and lactose are disaccharides that get broken down like this.
Monosaccharides are cotransported with…?
Na+
How does the pancreas secrete digestive enzymes?
As pro-enzymes.
How are pancreatic pro-enzymes activated?
Enterokinase (made by enterocytes) activates trypsinogen -> trypsin.
Trypsin activates other proteases.
What happens to fat in the stomach?
Gastric lipase: TGs -> FAs and diglycerides.
DGs and phospholipids emulsify fats.
What happens to fat in the duodenum?
Pancreatic lipase: TGs -> FAs + monoglycerides
Bile salts get added to stabilize emulsion.
What does the final stable emulsion of fat-soluble stuff in the small intestine look like?
Hydrophobic on the inside: TGs, DGs, cholesterol, fat-soluble vitamins.
Hydrophilic on the outside: Phospholipids, MGs, FAs, bile salts.
What does colipase do?
Secreted by pancreas, helps pancreatic lipase function.
What does phospholipase A2 do? What activates it?
Digests phospholipids on the emulsion surface.
Activated by bile salts.
Are brush border enzymes involved in fat digestion?
No.
What must emulsions be transformed into before than can be absorbed? What allows this to happen?
They must be transformed into small micelles by bile salts.
How are TGs, cholesterol, and phospholipids exported in lacteals?
They’re packed into chylomicrons… which have apolipoproteins.
Which vitamins are fat soluble?
ADEK
Which vitamins are water soluble?
B vitamins, vitamin C
4 things you need for B12 absorption?
Acid, intrinsic factor, R protein, pancreatic enzymes to free from R protein.
What pump really drives water absorption in the colon? Where is it?
The Na+/K+ ATPase (because pumps 3 Na+ out of the cell for every 2 K+ brought in).
It’s on the basolateral membrane.
Take home point about Na+ transporters on the apical membrane?
They’re driven by concentration gradients.
Functions of the colon? Is it strictly necessary?
Water and salt absorption.
Waste storage.
You can live without it.
2 things that regulate cecal valve function?
Distention
Presence of fat in the ileum - closes it, mediated by Peptide YY.
What, importantly, does the colon secrete?
K+ and HCO3
Relationship between motility and water absorption?
More motility -> less absorption -> diarrhea.
Less motility -> more absorption -> constipation.
What’s the gastrocolic reflex? What mediates it?
Having to poop after meals.
Mediated by gastrin and parasympathetics.
What’s an important vitamin that gut bacteria make?
Vitamin K.
recall the MDTI cases about people who bled after diarrhea and antibiotics
Where’s CNS control of defecation?
the hypothalamus
Reflex control of defecation?
Rectal distension -> relaxation of internal anal sphincter.
Last line of defense for fecal continence?
External anal sphincter.
What does the puborectalis muscle do in defecation?
Relaxes to straighten out path of feces to anus.
Are abs and diaphragm used in defecation?
Yeah.
