Normal Physiology of Small Bowel and Colon

Question 1

What word is used to describe small intestine peristalsis in the fed state? What is that getting at?

Answer 1

“segmentation”

Contraction rates are different in different parts of the small intestine: faster in duodenum, slower in ileum.

Question 2

3 things that regulate peristalsis in the small bowel?

Answer 2

Distension - increases, particularly in duodenum.
Extrinsic nerves - parasymps increase, symps decrease
Gastrin - increases

Question 3

What is a “unit” of peristalsis in fasting? What initiates it?

Answer 3

A migrating motor complex (MMC).

Initiated by motilin (secreted by the ileum).

Question 4

Where’s the “vomiting center”?

Answer 4

Medulla - lateral reticular formation

Question 5

What things contract/relax during vomiting?

Answer 5

Pyloric sphincter closes.
Abdominal muscles and diaphragm contracts.
LES and esophagus dilate.
Glottis closes (so you don’t aspirate).

Question 6

What does small bowel secrete? In what disease can these secretions be messed up?

Answer 6

Water and mucus - for lubrication, protection from bugs, and digestion.
These secretions will be messed up in CF.

Question 7

Is absorption regulated?

Answer 7

No. Everything is absorbed that can be.

Question 8

Does the gut do any concentrating or diluting?

Answer 8

Nope.

Question 9

2 parts of the surface epithelium of the small intestine?

Answer 9

Villi and crypts

Question 10

Function of the villi in the small intestine?

Answer 10

Absorption

Question 11

Function of the crypt epithelium in the small intestine?

Answer 11

Secretion - Cl- channel (CFTR)

Stem cells

Question 12

What step of carbohydrate digestion to brush border enzymes do?
3 molecules that they break down?

Answer 12

Disaccharidases break down disaccharides to monosaccharides for absorption.
Maltose, sucrose, and lactose are disaccharides that get broken down like this.

Question 13

Monosaccharides are cotransported with…?

Answer 13

Na+

Question 14

How does the pancreas secrete digestive enzymes?

Answer 14

As pro-enzymes.

Question 15

How are pancreatic pro-enzymes activated?

Answer 15

Enterokinase (made by enterocytes) activates trypsinogen -> trypsin.
Trypsin activates other proteases.

Question 16

What happens to fat in the stomach?

Answer 16

Gastric lipase: TGs -> FAs and diglycerides.

DGs and phospholipids emulsify fats.

Question 17

What happens to fat in the duodenum?

Answer 17

Pancreatic lipase: TGs -> FAs + monoglycerides

Bile salts get added to stabilize emulsion.

Question 18

What does the final stable emulsion of fat-soluble stuff in the small intestine look like?

Answer 18

Hydrophobic on the inside: TGs, DGs, cholesterol, fat-soluble vitamins.
Hydrophilic on the outside: Phospholipids, MGs, FAs, bile salts.

Question 19

What does colipase do?

Answer 19

Secreted by pancreas, helps pancreatic lipase function.

Question 20

What does phospholipase A2 do? What activates it?

Answer 20

Digests phospholipids on the emulsion surface.

Activated by bile salts.

Question 21

Are brush border enzymes involved in fat digestion?

Answer 21

No.

Question 22

What must emulsions be transformed into before than can be absorbed? What allows this to happen?

Answer 22

They must be transformed into small micelles by bile salts.

Question 23

How are TGs, cholesterol, and phospholipids exported in lacteals?

Answer 23

They’re packed into chylomicrons… which have apolipoproteins.

Question 24

Which vitamins are fat soluble?

Answer 24

ADEK

Question 25

Which vitamins are water soluble?

Answer 25

B vitamins, vitamin C

Question 26

4 things you need for B12 absorption?

Answer 26

Acid, intrinsic factor, R protein, pancreatic enzymes to free from R protein.

Question 27

What pump really drives water absorption in the colon? Where is it?

Answer 27

The Na+/K+ ATPase (because pumps 3 Na+ out of the cell for every 2 K+ brought in).
It’s on the basolateral membrane.

Question 28

Take home point about Na+ transporters on the apical membrane?

Answer 28

They’re driven by concentration gradients.

Question 29

Functions of the colon? Is it strictly necessary?

Answer 29

Water and salt absorption.
Waste storage.
You can live without it.

Question 30

2 things that regulate cecal valve function?

Answer 30

Distention

Presence of fat in the ileum - closes it, mediated by Peptide YY.

Question 31

What, importantly, does the colon secrete?

Answer 31

K+ and HCO3

Question 32

Relationship between motility and water absorption?

Answer 32

More motility -> less absorption -> diarrhea.

Less motility -> more absorption -> constipation.

Question 33

What’s the gastrocolic reflex? What mediates it?

Answer 33

Having to poop after meals.

Mediated by gastrin and parasympathetics.

Question 34

What’s an important vitamin that gut bacteria make?

Answer 34

Vitamin K.

recall the MDTI cases about people who bled after diarrhea and antibiotics

Question 35

Where’s CNS control of defecation?

Answer 35

the hypothalamus

Question 36

Reflex control of defecation?

Answer 36

Rectal distension -> relaxation of internal anal sphincter.

Question 37

Last line of defense for fecal continence?

Answer 37

External anal sphincter.

Question 38

What does the puborectalis muscle do in defecation?

Answer 38

Relaxes to straighten out path of feces to anus.

Question 39

Are abs and diaphragm used in defecation?

Answer 39

Yeah.