4/29: Small Intestine and Colon: Motility, Digestion, Absorption Flashcards
What does the duodenum secrete?
CCK
Secretin
Gip, HCO3
What does the ileum secrete?
PYY
HCO3-
Rank the duodenum, jejunum, and ileum from high to low in function of intraluminal and surface digestion?
Duodenum > jejunum > Ileum
What does the duodenum absorb?
Fe
What does the ileum absorb?
Bile acids
Vitamin B12
What is the motility of the duodenum?
MMC
Segmentation
Peristalsis
What complex controls the fasting pattern of the small intestine?
Migrating motility complex
What hormone stimulates the fasting pattern of the small intestine?
Motilin
What does the fasting pattern do?
Sweep intestines of undigested material
How often does fasting pattern occur?
One every 90 min
Describe the waves of the feeding pattern?
BER Slow waves in small intestine
What cells control the feeding pattern of the small intestine?
Interstitial Cells of Cajal
What stimulates the feeding pattern?
- Distention of duodenum
- Nutrient content of chyme
- Gastroenteric Reflex – short feedback loop from stomach to small intestine
- Hormones
How often does feeding pattern occur?
3-12 waves/min
What is the primary control of the feeding pattern?
ENS
What is the small intestine stimulated by?
CCK
Gastrin
Insulin
Serotonin
What is the small intestine inhibited by?
Secretin
Glucagon
What are the two feeding patterns of the small intestine?
- Segmentation
- Peristalsis
What is the mixing pattern of motility?
- chyme with digestive enzymes
- emulsifies fats
- adjusts pH
- exposes mucosa to chyme
What is the function of peristalsis?
- Propel chyme through small intestine at a velocity of 1 cm/min.
- Spread chyme across mucosal surface as it enters from
stomach
Where can peristalsis begin?
Anywhere in the small intestine
Normally weak and die out after traveling only 3-5cm
How long does peristalsis take?
3-5 hours from pyloric valve to ileocecal valve
What does duration of feeding pattern depend on?
- Caloric content of meal
- Nutrient composition of
meal
Rank the duration of proteins, fats, and carbs
Fats > proteins > carbs
Ex. 450 kcal nutrient mixed
meal will disrupt MMC for ≈
3 hours
Net rate of movement of any
substance across the intestinal
epithelium is influenced by:
- surface area
- motility
What is the north-south vector influenced by?
Motility -> transit time
What is the east-west vector influenced by?
Surface area
What are substances presented for digestion and/or absorption?
- Macronutrients – carbohydrate, protein, fat (Require “digestion”_
- Electrolytes – Na+, K+, Ca++, Mg++, Fe++, Cl-, PO4—
- Water
- Bile salts
- Vitamins – fat soluble, water soluble
- Drugs
What factors influence digestion?
- Motility
- Large surface area
- Appropriate pH
- Hydrolytic enzymes
(carbohydrates, protein, fat) - Emulsifying factors (Fat)
What factors influence absorption?
- Large surface area
- Specialized cells
- Specific transport
mechanisms – carriers,
pumps, pores - Energy
- Blood or lymph flow
Where are most substances completely digested and abosrbed?
In proximal small intestine
- dietary fat is the (potential) exception
What does an increase intake produce?
Increase absorption, may produce increase storage and obesity
- diversion of chyme to distal gut is one treatment for obesity
What pH do digestive enzymes in the small intestine require?
Neutral pH to function
Where does the pH come from?
H+ from stomach
Bile HCO3-
Pancreatic HCO3-
What are the two sites for digestion of protein and carbohydrate?
- Intraluminal (stage I - pancreatic hydrolases)
- Mucosal surface (stage II - brush border hydrolases)
What does intraluminal site yield?
di- and tripeptides, amino acids, maltose, maltotriose, α-limit dextrins, glucose;
- Fat digestion completed in lumen
What are the end products of the mucosal surface?
amino acids and di- and tripeptides, glucose, galactose, fructose
What are examples of carbohydrates in the diet?
Polysaccharides/disaccharides
What do carbohydrates need to be broken down into before being absorbed?
building blocks (Glucose,
Fructose, and Galactose)
_____________ cannot be digested and is lost in feces
Fiber (cellulose)
Where does carbohydrate digestion begin?
In mouth when exposed to salivary amylase
- not significant
- enzyme activated by acidic gastric juice
What are carbohydrates digesting in the small intestine by?
- Starch and Glycogen
digestion by pancreatic amylase - Brush border
hydrolases produce
the monosaccharides
What does amylase do?
Take polysaccharides and break them down to disaccharides
How is protein added to the lumen?
In form of mucus and enzymes
- disintegration of epithelial cells
What must polypeptides be digested into?
Small peptides or amino acids
What does digestion begin in the stomach by?
Pepsin
What is pepsin important for?
collagen digestion
What is pepsin inactivated by?
Basic pH in small intestine
80-90% digested in small intestine by?
Small peptides and amino acids
What are small peptides and amino acids produced by?
Trypin, chymotrypsin, carboxypolypeptidases, elastase
What do brush border peptidases produce?
Amino acids
Describe peptide transporter 1 (PEPT1)
- Extremely broad substrate availability
- Pharmacological Significance – Can be used for drug delivery
What is the most abundant dietary fat?
Triglycerides
What do fats have smaller amounts of?
Cholesterol, cholesterol esters, and phospholipids
Digestion of fat requires emulsification of what?
Bile salts
Lecithin (phospholipid)
What does agitation of the fat (segmentation) break the fat globule down into?
Smaller fragments, increasing area for digestion
Where does fat digestion begin?
In stomach
- lingual lipase
Where does 90% of fat digestion occur?
In small intestine by pancreatic lipase (and colipase)
Where do digestion products solubilize in?
Micelles
What do micelles remove?
TG digestion products from fat
globules so fat digestion can continue
What do micelles transport?
TG digestion products to brush border membrane
What is digestion of cholesterol esters by?
Pancreatic colesterol ester hydrolase
What is digestion of phospholipids by?
Pancreatic phospholipase A2
What are fats packaged into?
Micelles with TG digestion products
Micelle formation and breakdown are in _________
Equilibrium
How do FA and MG cross apical membrane via?
Simple diffusion
What happens to TG in smooth ER?
Resynthesized
- keeps [FA] and [MG] low inside cell
- maintains gradient for simple diffusion and absorption
- Fat-soluble vitamins (A, D, E, K),
phospholipids, and cholesterol
esters absorbed by same
mechanism
TG and other hydrophobic
substances packaged into ________ and secreted across BL membrane via __________
chylomicrons; exocytosis
How do chylomicrons enter lymphatic vessels via?
Lacteals and are transported to systemic veins and enters circulation
What are surface cells?
Mature intestinal epithelial cells
What do surface cells absorb?
Na, Cl, and H2O
Where are crypt cells found?
Deep in the spaces between the circular folds
What are crypt cells?
Immature intestinal epithelial cells
What are crypt cells produced by?
Stem cells
Where do crypt cells migrate to?
Surface
What do crypt cells secrete?
Na, Cl, and H2O
Where does fluid secreted by crypt cells flow?
To surface cells where reabsorbed
What do crypt cells provide flowing H2O for?
Absorbing intestinal digestates
What do crypt cells balance?
Absorption and secretion
Describe the prandial state of surface cells?
Electrogenic
Anions and water follow
Describe the post-prandial state of surface cells?
Electroneutral
H2O follows
Describe the Na, Cl, and H2O secretion of crypt cells
Electrogenic
Na and H2O follow
What do cholera and E.coli produce?
Enterotoxins that increase cAMP
What is the function of cAMP?
Activates Cl- secretion via CVTR into gut lumen: massive diarrhea
_________ do not affect sodiun-glucose cotransport
Enterotoxins
What does vitamin B12 do 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 does vitamin B12 do in the duodenum?
- Proteases digest R-binding protein
- B12 binds to intrinsic factor (pH
neutral)
What does vitamin B12 do in the ileum?
Intrinsic factor binds to IFCR and
taken into cells via receptor-
mediated endocytosis
What does vitamin B12 do inside cells?
- Intrinsic factor degraded
- B12 binds to TCII
- Complex crosses BL via exocytosis
How are Thiamin, riboflavin, niacin, pyridoxine, pantothenate, biotin, and ascorbic acid absorbed?
- cotransport with Na
- completed in upper small intestine
What are other actively absorbed substances?
- Ca++, PO4—
- Mg++
- Fe++
- Bile salt
What is maldigestion?
- Pancreatic “insufficiency”
- Liver disease
- Biliary obstruction
Steatorrhea occurs early
What is malabsorption?
- Achlorhydria
- Short bowel
- Mucosal disease (i.e. Celiac
Disease – autoimmune destruction of intestinal
cells) - Lactose intolerance
