Digestive System: The Small Intestine

Front 1

Basic Anatomy of the Small Intestine

Back 1

pyloric sphincter->duodenum->jejunum->ilium->ilocecal sphincter
1. duodenum: 1st 10-12in
-arcs around head of pancreas
-passes to the left & ends at sharp bend called duodenojejunal flexure
2. Jejunum: ~8ft long
3. Ileum: ~12ft & joins large intestine @ ileocecal valve

Front 2

Villi & Microvilli of Small Intestine

Back 2

3 components of brush border:
1. level 1 mucuosal lining
-referred to as "rugae" of SI
2. Villi: larger bumps, can see w/ naked eye or low powered microscope
3. Microvilli: bumps contained on larger bumps (increase SA)

Front 3

Simple Columnar Epithelium

Back 3

-enterocytes = simple columnar
-lots of mitochondria (high ATP) for absorption -> active transport
-highly energetic
-also have goblet cells: mucus producers, dumped into lumen
-protect from acid & help neutralize

Front 4

GI Secretions:
1. Saliva
2. Gastric Secretion
3. Pancreatic Secretion

Back 4

1. high HCO3, high K, hypotonic, a-amylase, lingual lipase
-stim by para/sympNS
-inhib by sleep, dehydration, atropine
2. HCl (Gastrin, paraNS, histamine), Pepsinogen (paraNS), Intrinsic Factor
-inhib by: low pH, chyme in duod (secretin/GIP), atropine, cimetidine, omeprazole
3. high HCO3 (secretin), isotonic (CCK, paraNS), pancreatic lipase/amylase/protease (CCK, paraNS)

Front 5

Type of digestion in...
1. mouth
2. stomach
3. duodenum
4. liver
5. pancreas
6. jejunum & ileum
7. colon

Back 5

1. mxl, biochem = no absorp
2. mxl, biochem = minimal absorp
3. coordination mxl, biochem = important absorp
4. biochem = no absorp
5. biochem = no absorp
6. mcl, biochem = absorp very imp
7. minimal digest = imp. absorp.

Front 6

Digestion of Carbohydrates

Back 6

-Polysaccharides: starch, glycogen, cellulose, fiber
-Disaccharide: sucrose, maltose, lactose
-Monosaccharide: glucose, fructose, galactose
-pancreatic amylase: degrades polysaccharides into disaccharides
-disaccharidases: disaccharides are degraded by group of brush border enzymes
-brush border enzymes: series/fam. of enzymes embedded in cell mem.
-imp that SI has good peristalsis so all chyme gets contact w/ brush border

Front 7

Absoprtion of Carbohydrates

Back 7

INTO ENTEROCYTES:
1. SGLT1: secondary active transport
-Na-dependent co-transport of Na w/ galactose or glucose
-ATP indirectly affects
2. GLUT5: fructose enters by facilitated diffusion

OUT OF ENTEROCYTES:
3. GLUT2: all monosaccharides to bloodstream
-core of villus = highly vascularized
-2 types of capillaries: 1. blood 2. lymph (lacteals)
3.

Front 8

Lactose Intolerance

Back 8

-lactose enzymes slow down = mxn unknown
-normal lactose digest = lactase enzyme
-intolerance= SI incapable of digest/absorp, creat osmotic gradient
-osm. gradient increases H2O retention in SI (SI stretch = increase motility)
-lactose not broken down->goes to LI, where bacteria ferment
-lots of particles w/ nowhere to go cause: gas, cramps, bloat, diarrhea

Front 9

Overview of Digestion & Absorption of Proteins

Back 9

-stomach denatures protein, but not as effective of cleave off aa (pepsin)
-cleave aa occurs mainly in SI
-pancreas release 4 inactive enzymes:
1. trypsinogen
2. chymotrypsinogen
3. procarboxypeptidase
4. proelastase
-when deposit in SI: trypsinogen converted into trypsin by brush border enzyme enterokinase (enteropeptidase)
-trypsin cleaves aa & convert/activate enzymes
-aa/small peptides after digest absorb via cotransport (same as glucose, Na-dependent) or facilitated diffusion

Front 10

What are the Brush Border Ezymes?

Back 10

aminopeptidase
carboxypeptidase

Front 11

Main players in lipid digestion:

Back 11

1. emulsification
-critical step: w/o can't go further
-lecithin & bile acid = detergents (break down into smaller bits for step 2)
2. pancreatic lipase & colipase
-fat hydrolysis
-emuls. makes more vulnerable for lipase to attack
3. micelles
-non stable, org. of lecithin to allow absorp across brush border
4. chylomicrons
-core of fast in center surround by protein coat (formed by enterocytes)
-hold FFA, chol, phospholipid, TGs
-lecithins recycled back to SI
5. lacteals
-CM too bulky for bloodstream, capillaries so goes thru lymph

Front 12

Transport of Chylomicrons

Lipemia

Back 12

-don't last in circulation very long w/in hr is cleared
-once CM enter lymph-> transported up thru thoracic duct
- then emptied into venous circulation at juncture of jugular & subclavian veins

-occur w/in hr after eat fatty meal

Front 13

Fate of Chylomicrons

Back 13

-removed from blood by liver
-CM removed from blood as pass thru capillaries of adipose tissue & liver
-both contain lipoprotein lipase (tears apart CM)
-enzyme esp. active in capillary endothelium: hydrolyze the TGs of CMs into FA & glycerol which diffuse into adipocytes (storage) & hepatocytes (repacked into lipoprotein & export to blood)
1) some stored in liver
2) most repack in liver-> lipoprotein new protein coat-> store in adipocytes or used as energy

Front 14

Types of Lipoproteins

Back 14

5 classes:
1. CM
2. VLDL
-high TGs, moderate chol & phospholipids
3. IDL: interm. dens lipoprotein
-moderate TGs, chol, & phospholipids
4. LDL
-low TGs, high chol & phospholipids
5. HDL
-low TGs, chol & phospholipids

Front 15

1. LDL

2. HDL

Back 15

1. bad cholesterol:
-cells take up via receptor mediated endocytosis
-LDL receptor on surface of most cells
-cell digest coat & extract fat (store/use)
-steroid hormone product. of cells (lydig-make male testosterone)
-increase risk factor for heart attack

2. good cholesterol
-low risk of heart attack
-inflated beach ball
-liver makes HDL shell->circulate
-scavengers: looks for excess chol. that body doesn't need/want
-inflated->returns to liver
-liver extracts fat/chol put into bile (waste)

Front 16

Na, K, & Water Absorption

Back 16

-all H2O absorp in SI is passive & secondary to solute mvmt
-solutes can be electrolytes (Na) or nonelectrolytes (glucose)
-Na/glucose & Na/aa mxn stim water absorp
-most K absorbed passively when luminal [K] rise bc absorp of water (solvent drag)
-don't absorp all, some goes thru LI & incorp into fecal matter

Front 17

Ca absorption

Back 17

-hormone regulated
-occur in duodenum & jejunum
-prim. reg. by VitD (1,25 dihydroxycholecalciferol) stim synth of Ca binding protein called calbindins in enterocytes
-Ca removed from lumen across brush border via passive diffucion (regulate by VitD)
-inside enterocyte Ca binds to Calbinding->allow intracell level of free Ca to reamin low & maintain Ca gradient for Ca to move across brush border
-at basal mem = Ca ATPase pump
-in blood is bound to something (~40% albumin, 10% phosphourus)

Front 18

Iron absorption

Back 18

-complex & critical for hemoglobin production
-enterocytes full of carriers sp. for iron
1) heme iron: ingeset attach to Hg (steak)
-heme transporter into cell
-heme oxidase: extracts & frees it to Fe2+
2) non-heme iron: free iron
-reduced and absorp by iron reductase
*Ferroxidase: takes Fe2+->Fe3+ to either bind to protein or storage via Ferritin
-pumped out of cell via IREG1
-transferrin: carrier molecule thru blood: either stored in liver or bone marroe to make RBC

Front 19

Vitamin Absorption

Back 19

-fat sol: ADEK, absorp sim. to fat
-water sol: absorp by simple diff.
-VitB12: water sol absorp w/ intrinsic factor (ileal cells across brush border)

Front 20

Peristalsis & Segmentation in SI

Back 20

-peristaltic: propel & mix food along GI (esoph, stomach, & SI)
-takes chyme ~3-5hr move thru entire SI via peristalsis
-seg: series of contract & relax period mxl digest
-both controlled by ANS
-symp inhibit
-para stim

Front 21

Regulation of SI
1) Fat & AA

Back 21

CCK:
+ gallbladder contraction -> increase bile secretion -> increase fat digest
+ pancreatic enzymes -> 6 enzymes
- slow motility/empty of stomach

Front 22

Regulation of SI
2) Acid & AA

Back 22

SECRETIN:
- inhib gastric motility
+bile (allow cells to secrete electrolytes & water into bile)
+ pancreatic alkaline (HCO3 rich fluid)

GIP:
- inhib. gastric motility & empty

Front 23

Regulation of SI
3) Vagal Nerve

Back 23

+ intestine segmentation & peristalsis
+ secretion of pancreatic enzymes

Front 24

Pancreatic Enzymes

Back 24

1. lipase
2. amylase
3. trypsinogen
4. chymotrypsinogen
5. procarboxypeptidase
6. proelastase