GI 3- Small Intestine and Colon- Motility, Digestion, Absorption Flashcards
What are the functions of the small intestine?
secretion
digestion
absorption
motility
What is secreted in the duodenum?
CCK
secretin
GIP
HCO3-
What are the motility patterns in the duodenum?
MMC
segmentation
peristalsis
What is absorbed in the ileum (the last segement of small intestine)?
bile acids
vitamin B12
What is the purpose of the migrating motility complex in the small intestine?
- sweeps intestines of undigested material
- 1 every 90 minutes
- motilin
When does the migrating motility complex happen in the small intestine?
fasting
What is the motility pattern of the small intestine during feeding?
➢ BER Slow waves in small intestine
➢ Interstitial Cells of Cajal
➢ 3-12 waves/min
➢ Controlled 1⁰ by ENS
- two types (segmentation and peristalsis)
What stimulates the feeding motility patterns in the small intestine?
- Distention of duodenum
- Nutrient content of chyme
- Gastroenteric Reflex – short feedback loop from stomach to small intestine
- Hormones
✓ Stimulated by: CCK, Gastrin, Insulin, and Serotonin
✓ Inhibited by: secretin and glucagon
What hormones stimulate motility in the small intestine?
CCK
gastrin
insulin
serotonin
What hormones inhibit motility in the small intestine?
secretin
glucagon
What are the two patterns of motility during feeding?
segmentation
peristalsis
What does segmentation motility do in the small intestine?
➢ Mixing
- chyme with digestive enzymes
- emulsifies fats
- adjusts pH
- exposes mucosa to chyme
What does peristalsis do in the small intestine?
➢ Propel chyme through small intestine at a velocity of 1 cm/min.
➢ Spread chyme across mucosal surface as it enters from stomach
➢ Can begin anywhere in the small intestine
➢ Normally weak and die out after traveling only 3 – 5 cm
➢ 3 - 5 hours from pyloric valve to ileocecal valve
What does the duration of feeding pattern depend on?
- caloric content
- nutrient composition (fats>proteins>carbs)
Net rate of movement of any substance across the intestinal epithelium is influenced by:
➢ Surface area (east-west)
➢ Motility (north-south)
What are the substances presented for digestion/absorption in the small intestine?
➢Macronutrients – carbohydrate, protein, fat – Require “digestion”
➢Electrolytes – Na+, K+, Ca++, Mg++, Fe++, Cl-, PO4—
➢Water
➢Bile salts
➢Vitamins – fat soluble, water soluble
➢Drugs
What affects digestion in the small intestine?
➢ Motility
➢ Large surface area
➢ Appropriate pH
➢ Hydrolytic enzymes – carbohydrates, protein, fat
➢ Emulsifying factors – Fat
What affects absorption in the small intestine?
➢ Large surface area
➢ Specialized cells
➢ Specific transport mechanisms – carriers, pumps, pores
➢ Energy
➢ Blood or lymph flow
Most substances completely digested and absorbed in __________ small intestine
proximal (~25%)
What are the consequences of most substances being completely digested in the proximal small intestine?
↑ intake produces ↑ absorption, may produce ↑storage and obesity
Digestive enzymes in small intestine require a ______ pH to function
neutral
What allows for a neutral pH in the small intestine when H+ is coming from stomach?
Pancreatic HCO3-
What are the two sites for digestion of proteins and carbs?
- intraluminal (stage 1; pancreatic hydrolases)
- mucosal surface (stage 2; brush border hydrolases)
What does intraluminal (stage 1) digestion result in?
- yields di- and tripeptides, amino acids, maltose, maltotriose, α-limit dextrins, glucose
- fat digestion (only in stage 1)
What does mucosal surface (stage 2) digestion result in?
End products: amino acids and di- and tripeptides, glucose, galactose, fructose.
Fats are digested intraluminal, mucosal surface, or both?
intraluminal (stage 1; pancreatic) only!
Starch and Glycogen digestion by __________________.
_____________________ produce
the monosaccharides
pancreatic amylase
Brush border hydrolases
What type of transport does glucose, galactose, and fructose use on the apical membrane?
glucose/galactose = active
fructose = passive
What type of transport does glucose, galactose, and fructose use on the basolateral membrane?
all passive
What begins protein breakdown?
pepsin in the stomach
- important for collagen digestion
When is pepsin inactivated?
when it reaches the small intestine (basic pH)
Small peptides and amino acids produced by __________ in the small intestine.
Trypsin, Chymotrypsin, Carboxypolypeptidases, Elastase (stage 1 from pancreas)
____________ produce amino acids in the small intestine
Brush border peptidases (stage 2)
What type fo transporter is the peptide transporter 1 (PEPT1)?
secondary active symporter
What is the pharmacological significance of PEPT1 (peptide transporter 1) in the small intestine?
Can be used for drug delivery
What is the most abundant dietary fat?
triglycerides (TG)
Digestion of fat requires emulsification via what…
bile salts
lecithin
What begins the digestion of fat?
lingual lipase (stomach)
- only 10% of fat digestion
What does most of the fat digestion?
pancreatic lipase (stage 1) in small intestine
The products of fat digestion are solubilized in what?
micelles
What do micelles allow for?
- Removes TG digestion products from fat globules so fat digestion can continue
- Transports TG digestion products to brush border membrane
Digestion of cholesterol esters by pancreatic _______________
cholesterol ester hydrolase
Digestion of phospholipids by Pancreatic _____________
Phospholipase A2
Micelle Formation and Breakdown
are in ____________
equilibrium
Fatty acids and MonoGlycerides cross ___________ membrane via simple diffusion
apical
In what organelle are TryGlycerides resynthesized?
smooth ER
TG and other hydrophobic substances packaged into ____________
chylomicrons
Chylomicrons are secreted across what membrane via exocytosis?
basolateral
- enter lymphatic vessels (NOT blood)
What is the job of surface cells in the small intestine?
- Mature intestinal epithelial cells
- Absorb Na+, Cl-, and H2O
What is the job of crypt cells in the small intestine?
- Found deep in the spaces between the circular folds
- Immature intestinal epithelial cells
- Produced by stem cells
- Mature and migrate to surface
- Secrete Na+, Cl-, and H2O
Secretion crypt cells and absorption of surface cells create what?
➢ Provides flowing H2O for absorbing intestinal digestates
➢ Normally balance between Absorption and Secretion
If the secretion from the crypt cells is not enough what occurs?
cystic fibrosis
If the secretion from the crypt cells is too much what occurs?
infectious diarrhea
What is the prandial state of surface cells in the small intestine?
➢ Electrogenic
➢ Anions and water follow
What is the post-prandial state of surface cells in the small intestine?
➢ Electroneutral
➢ H2O follows
What is happens during secretion in the crypt cells?
➢ Electrogenic
➢ Na+ and H2O follows
What do cholera and E. coli infections produce that lead to diarrhea?
produce enterotoxins that increase cAMP
What doe enterotoxins do?
↑ [cAMP]
➢ cAMP activates Cl- secretion via CFTR into gut lumen: massive diarrhea.
What is the oral rehydration theory?
drink water, sugar, and salt to cause the sodium-glucose cotransport to pull water back through basolateral side into the blood
Enterotoxins do not affect ___________ cotransport.
sodium-glucose
What happens to vitamin B12 absorption in the stomach?
➢ Binds to the R-binding protein
➢ Stabilizes B12 in acidic environment
➢ Intrinsic factor secreted by gastric parietal cells cannot interact with B12 at low pH
What happens to vitamin B12 absorption in the duodenum?
➢ Proteases digest R-binding protein
➢ B12 binds to intrinsic factor (pH
neutral)
What happens to vitamin B12 absorption in the ileum?
➢ Intrinsic factor/Vit B12 binds to IFCR and taken into cells via receptor-mediated endocytosis
What happens to vitamin B12 absorption inside the cell?
➢ Intrinsic factor degraded
➢ B12 binds to TCII
➢ Complex crosses BL via exocytosis
What can vitamin B12 bind to intrinsic factor?
when the pH is neutral
- small intestine
What can cause maldigestion?
➢ Pancreatic “insufficiency”
➢ Liver disease
➢ Biliary obstruction
What can cause malabsorption?
➢Achlorhydria (no acid in stomach)
➢Short bowel
➢Mucosal disease (i.e. Celiac Disease – autoimmune destruction of intestinal cells)
➢Lactose intolerance
What does the first half of the colon do?
Absorbing Colon
➢ Absorption of water and electrolytes from chyme to form solid feces
What does the second half of the colon do?
Storage Colon
➢ Storage of Fecal matter until it can be expelled
Takes _____ hours to move contents from ileocecal valve to anus
8-15 hours
Poor motility in the colon causes…
greater absorption and hard feces (constipation)
Excess motility in the colon causes…
less absorption and diarrhea (loose feces)
What do haustrations (mixing movements) in the colon do?
➢ Fecal material slowly dug into and rolled over
➢ All fecal material gradually exposed to mucosal surface
➢ Facilitates absorption of water and dissolved substances
➢ Movement of material from cecum through ascending colon
What do mass movements (propulsive movements) do?
➢ 1-3x each day
➢ Transverse to Sigmoid
➢ Occurs throughout colon for 10 min.
➢ Force feces into rectum
➢ Gastrocolic and Duodenocolic Reflexes
➢ Irritation can stimulate
What is the role of colon bacteria?
➢Slow motility promotes growth
➢ Ileocecal valve keeps bacteria in colon
➢ Ferment undigested carbs to short-chain fatty acids
➢ Produce Vitamin K
➢ Increase resistance of the intestinal mucosa to colonization with pathogenic microorganism
➢ Produce Flatus
What nerve controls the external anal sphincter?
pudendal nerve
- skeletal motor nerve from conscious cortex
What controls the internal anal sphincter?
parasympathetic nerves (pelvic nerves)
What are the sensory receptors on the rectum for?
afferent nerve fibers
- sense stretch in rectum
