GI 3 Flashcards
Fasting pattern (4)
Migrating Motility Complex
Motilin
Sweep intestines of undigested material
One every 90 minutes
Feeding Pattern (4)
BER Slow waves in small intestine
Interstitial Cells of Cajal
3-12 waves/min
Controlled 1⁰ by ENS
feeding pattern
Stimuli (4)
Distention of duodenum
Nutrient content of chyme
Gastroenteric Reflex – short feedback loop from stomach to small intestine
Hormones
Hormones
Stimulated by: (4)
Inhibited by: (2)
CCK, Gastrin, Insulin, and Serotonin
secretin and glucagon
Two patterns of Motility
- Segmentation
2. Peristalsis
- Segmentation (4)
Mixing chyme with digestive enzymes
emulsifies fats
adjusts pH
exposes mucosa to chyme.
- Peristalsis (5)
Spread chyme across mucosal surface as it enters from —
Propel chyme through small intestine at a velocity of – cm/min.
Can begin anywhere in the
Normally weak and die out after traveling only —
— hours from pyloric valve to ileocecal valve
stomach 1 small intestine 3 – 5 cm 3 - 5
Duration of feeding pattern depends on: (2)
Caloric content of meal
Nutrient composition of meal
Nutrient composition of meal (2)
Carb > Protein > Fats
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: (2)
Surface area
Motility
Substances Presented for Digestion and/or Absorption (6)
Macronutrients – carbohydrate, protein, fat
Require “digestion”
Electrolytes – Na+, K+, Ca++, Mg++, Fe++, Cl-, PO4—
Water
Bile salts
Vitamins – fat soluble, water soluble
Drugs
Factors influencing Digestion (5)
Motility Large surface area Appropriate pH Hydrolytic enzymes -carbohydrates, protein, fat Emulsifying factors -Fat
Factors influencing Absorption (5)
Large surface area Specialized cells Specific transport mechanisms – carriers, pumps, pores Energy Blood or lymph flow
Most substances completely digested and absorbed in
proximal (~25%) small intestine.
-Dietary fat is the (potential) exception
This has consequences: (3)
↑intake produces ↑absorption, may produce ↑storage and obesity.
Diversion of chyme to distal gut is one treatment for obesity
Digestive enzymes in small intestine require a — pH to function
neutral
Pancreatic HCO3-
-Maximal stimulus of pancreas produces
pancreatic juice with [HCO3- ] of 145 mEq/L (5x plasma [])
Two Sites for Digestion of Protein and Carbohydrate
- Intraluminal (Stage I – pancreatic hydrolases):
2. Mucosal surface (Stage II – brush border hydrolases):
- Intraluminal (Stage I – pancreatic hydrolases): (2)
yields di- and tripeptides, amino acids, maltose, maltotriose, α-limit dextrins, glucose;
Fat digestion completed in lumen.
- Mucosal surface (Stage II – brush border hydrolases): (1)
End products: amino acids and di- and tripeptides, glucose, galactose, fructose.
Typical American diet includes ≈ — carbs
250 – 300 g/day
≈ ½ Daily Caloric Intake
Polysaccharides/Disaccharides (2)
Need to be broken down into building blocks (Glucose, Fructose, and Galactose) before it can be absorbed.
Fiber (Cellulose) cannot be digested and is lost in feces
Carbohydrate Digestion
Begins in mouth when food exposed to
salivary amylase
Not significant
Enzyme inactivated by acidic gastric juice
Carbohydrate Digestion
>95% digested in small intestine by (2)
Brush border hydrolases produce the
Starch and Glycogen digestion by pancreatic amylase
monosaccharides
Typical American diet includes ≈ — protein
60 – 90 g/day
1/6 average daily caloric intake/day
≈50 g endogenous protein added
≈50 g endogenous protein added (3)
protein added to lumen in form of mucus and enzymes
Disintegration of epithelial cells
Polypeptides must be digested into small peptides or Amino Acids
Digestion
Begins in stomach by pepsin (3)
10-20% total protein digestion
Important for collagen digestion
Enzyme inactivated by basic pH in Small Intestine
80-90% digested in small intestine by:
Small peptides and amino acids produced by (4)
Brush border peptidases produce —
Trypsin, Chymotrypsin, Carboxypolypeptidases, Elastase
amino acids
Peptide Transporter 1 (PEPT1)
Pharmacological Significance –
Extremely broad substrate availability
Can be used for drug delivery
Typical American diet includes ≈ — fats
70 – 100 g/day
≈ 1/3 Daily Caloric Intake
Most abundant dietary fats =
triglycerides (TG)
Most abundant dietary fats = triglycerides (TG)
Smaller amounts of (3)
cholesterol, cholesterol esters, and phospholipids
Digestion of fat requires emulsification (2)
Bile Salts
Lecithin (phospholipid)
fat digestion reduces the
interfacial surface tension of fat
Agitation of the fat (Segmentation), breaks the fat globule down into smaller fragments, increasing
surface area for digestion.
Fat Digestion
Begins in
stomach
lingual lipase
< 10% (Insignificant)
> 90 % in Small Intestine of Fat Digestion (4)
Pancreatic lipase (and Colipase)
Enormous quantities secreted
Digests all TG it can reach in < 1 min.
Produces fatty acid (FA) (X2) and mono-glyceride (MG).
Digestion products solubilized in micelles
Removes
Transports
TG digestion products from fat globules so fat digestion can continue
TG digestion products to brush border membrane
Fat Digestion of: (3)
Cholesterol esters by
Phospholipids by
pancreatic cholesterol ester hydrolase
Pancreatic Phospholipase A2
Packaged into micelles with TG digestion products
Micelle Formation and Breakdown are in —
equilibrium
FA and MG cross apical membrane via
simple diffusion
In smooth ER, — resynthesized
TG
In smooth ER, TG resynthesized
Keeps (2) low inside cell
Maintains gradient for (2)
[FA] and [MG]
simple diffusion and absorption
(3) absorbed by same mechanism
Fat-soluble vitamins (A, D, E, K),
phospholipids, and
cholesterol esters
TG and other hydrophobic substances packaged into chylomicrons and secreted across
BL membrane via exocytosis
Chylomicrons enter lymphatic vessels via
the lacteals and are transported to systemic veins and enters circulation
Surface Cells (2)
Mature intestinal epithelial cells
Absorb Na+, Cl-, and H2O
Crypt Cells (5)
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
Fluid secreted by Crypt cells flow to surface cells where —
reabsorbed
Provides flowing H2O for
absorbing intestinal digestates
Normally balance between (2)
Absorption and Secretion
Significance of secretion:
Not enough:
Too much:
cystic fibrosis
infectious diarrhea
Prandial State (2)
Electrogenic
Anions and water follow
Post –Prandial (2)
Electroneutral
H2O follows
Sodium in GI Lumen (3)
30g/day secreted into lumen
5-8g/day ingested
Normally >95.5% absorbed
(2) infections are major cause of death in undeveloped countries.
Cholera and E. coli
Cholera and E. coli infections are major cause of death in undeveloped countries.
Both produce enterotoxins that
↑ [cAMP]i.
cAMP activates Cl- secretion via CFTR into gut lumen:
massive diarrhea.
Up to –L/day of diarrhea can be produced
20
Vitamin B12 (Cobalamin) Absorption In stomach (3)
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
Vitamin B12 (Cobalamin) Absorption In Duodenum (2)
Proteases digest R-binding protein
B12 binds to intrinsic factor (pH neutral)
Vitamin B12 (Cobalamin) Absorption In Ileum (1)
Intrinsic factor binds to IFCR and taken into cells via receptor-mediated endocytosis
Vitamin B12 (Cobalamin) Absorption Inside cells (3)
Intrinsic factor degraded
B12 binds to TCII
Complex crosses BL via exocytosis
Absorption of Water Soluble Vitamins
Thiamin, riboflavin, niacin, pyridoxine, pantothenate, biotin, and ascorbic acid: (2)
Cotransport with Na+
Completed in upper small intestine.
Other Actively Absorbed Substances (4)
Ca++, PO4—
Mg++
Fe++
Bile salts
Function Absorbing Colon (3)
Proximal ½
Absorption of water and electrolytes from chyme to form solid feces
≈ 1.4 L/day
Function Storage Colon (2)
Distal ½
Storage of Fecal matter until it can be expelled
Takes – hours to move contents from ileocecal valve to anus
8-15
Mixing Movements “Haustrations” (4)
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
Propulsive Movements “Mass Movements” (6)
1-3x each day Transverse to Sigmoid Occurs throughout colon for 10 min. Force feces into rectum Gastrocolic and Duodenocolic Reflexes Irritation can stimulate
– different bacterial species (# bacteria in colon > # cells in human body)
1500
Slow motility promotes —
growth
– valve keeps bacteria in colon
Ileocecal
Ferment undigested carbohydrates to short-chain fatty acids: (3)
Soluble and easily absorbed.
Nutrient salvage
Evidence obesity associated with biasing microbes to species efficient at nutrient salvage
Colon Bacteria
Produce
Vitamin K (vital since amount in diet not sufficient)
Colon Bacteria
Increase resistance of the intestinal mucosa to colonization with ]
pathogenic microorganism
Colon Bacteria
Produce —
Flatus
