GI 3 Flashcards by sophie rosen

Fasting pattern (4)

Migrating Motility Complex
Motilin
Sweep intestines of undigested material
One every 90 minutes

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Feeding Pattern (4)

BER Slow waves in small intestine
Interstitial Cells of Cajal
3-12 waves/min
Controlled 1⁰ by ENS

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feeding pattern

Stimuli (4)

Distention of duodenum
Nutrient content of chyme
Gastroenteric Reflex – short feedback loop from stomach to small intestine
Hormones

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Hormones
Stimulated by: (4)
Inhibited by: (2)

CCK, Gastrin, Insulin, and Serotonin

secretin and glucagon

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Two patterns of Motility

Segmentation

2. Peristalsis

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Segmentation (4)

Mixing chyme with digestive enzymes
emulsifies fats
adjusts pH
exposes mucosa to chyme.

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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

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Duration of feeding pattern depends on: (2)

Caloric content of meal

Nutrient composition of meal

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Nutrient composition of meal (2)

Carb > Protein > Fats

Ex. 450 kcal nutrient mixed meal will disrupt MMC for ≈ 3 hours

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Net rate of movement of any substance across the intestinal epithelium is influenced by: (2)

Surface area

Motility

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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

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Factors influencing Digestion (5)

Motility
Large surface area
Appropriate pH
Hydrolytic enzymes
-carbohydrates, protein, fat
Emulsifying factors
-Fat

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Factors influencing Absorption (5)

Large surface area
Specialized cells
Specific transport mechanisms – carriers, pumps, pores
Energy
Blood or lymph flow

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Most substances completely digested and absorbed in

proximal (~25%) small intestine.

-Dietary fat is the (potential) exception

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This has consequences: (3)

↑intake produces ↑absorption, may produce ↑storage and obesity.
Diversion of chyme to distal gut is one treatment for obesity

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Digestive enzymes in small intestine require a — pH to function

neutral

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Pancreatic HCO3-

-Maximal stimulus of pancreas produces

pancreatic juice with [HCO3- ] of 145 mEq/L (5x plasma [])

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Two Sites for Digestion of Protein and Carbohydrate

Intraluminal (Stage I – pancreatic hydrolases):

2. Mucosal surface (Stage II – brush border hydrolases):

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Intraluminal (Stage I – pancreatic hydrolases): (2)

yields di- and tripeptides, amino acids, maltose, maltotriose, α-limit dextrins, glucose;
Fat digestion completed in lumen.

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Mucosal surface (Stage II – brush border hydrolases): (1)

End products: amino acids and di- and tripeptides, glucose, galactose, fructose.

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Typical American diet includes ≈ — carbs

250 – 300 g/day

≈ ½ Daily Caloric Intake

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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

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Carbohydrate Digestion

Begins in mouth when food exposed to

salivary amylase

Not significant
Enzyme inactivated by acidic gastric juice

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Carbohydrate Digestion
>95% digested in small intestine by (2)
Brush border hydrolases produce the

Starch and Glycogen digestion by pancreatic amylase

monosaccharides

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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

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

``` 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