Download Chap 65 - Digestion and Absorption in the Gastrointestinal Tract

Digestion and Absorption in the Gastrointestinal Tract
Physiology III, Tri 4
Guyton & Hall, Chapt. 65
I. Digestion of Various Foods
A. classification of foods (except vitamins and minerals)
1. carbohydrates
2. fats
3. proteins
B. carbohydrates
1. most are polysaccharides or disaccharides
2. monosaccharides bound to one another by condensation
(H+ removed from one of the monosaccharides and a OH- removed
from the next one)
3. hydrolysis - return of the hydrogen and hydroxyl ions to the
polysaccharides and separation of the monosaccharides
digestive enzyme
R'' - R' + H2O
R''OH + R"H
C. fats
1. triglycerides
a. neutral fats
b. 3 fatty acid + glycerol (-3 molecules of water)
2. digestion
a. addition of water to triglyceride molecule
b. splitting of fatty acid away from the glycerol
c. hydrolysis
D. proteins
1. amino acids with peptide linkages
2. bound by condensation
3. digestion is by hydrolysis
E. chemistry of digestion
1. three types of food
2. hydrolysis
3. enzymes
a. all are proteins
b. secreted by GI glands
II. Digestion of Carbohydrates (CHO)
A. CHO intake
1. 280 gm / day
2. varies widely
B. sources of CHO
1. sucrose
a. disaccharide
b. sugar cane
2. lactose
a. disaccharide
b. milk sugar
3. starches
a. large polysaccharides
b. found in nonanimal food (grains)
c. largest source of CHO; > ½ of all CHO
4. other sources:
a. amylose
b. glycogen
1. from meat
2. degrades quickly with ingestion in the
presence of -amylase
a. maltose (most common product)
b. maltotriose
c.  limit dextrins
3. highly branched
-amylase
maltose
glycogen   maltotriose
c. alcohol
-limit dextrins
d. lactic acid - buttermilk, yogurt
e. pyruvic acid
f. pectins
g. dextrins
1. oligosaccharide
2. has branched point
3. 1-4, 1-6
produces branch
h. derivatives in meats
i. cellulose
1. not a food source for humans
2. no enzymes for breakdown - no cellulase
C. digestion of CHO in the mouth and stomach
1. mouth - saliva
a. ptyalin = -amylase
b. secreted by parotid glands
c. hydrolyzes starch into the disaccharide maltose and glucose
polymers
d. about 5% of starches are hydrolyzed before swallowing
2. stomach
a. digestion continues in body and fundus for up to 1 hr.
b. food is mixed with stomach secretions
c. acidic gastric secretions block activity of salivary amylase
d. salivary amylase is inactive when pH is < 4.0
e. 30 - 40% of starch is hydrolyzed before mixing with
stomach acids
D. digestion by pancreatic amylase
1. pancreatic secretion contain large amounts of -amylase
2. identical function as salivary -amylase - but more powerful
3. 15 - 30 min. after chyme enters duodenum almost all starches are
digested
4. starches are converted to:
a. maltose
b. glucose polymers (small amount)
c. conversion takes place in the duodenum and upper jejunum
E. hydrolysis by the intestinal epithelial enzymes
1. enzymes of the enterocytes lining the villi:
a. lactase
b. sucrase
c. maltase
d. -dextrinase
e. all of the above can split lactose, sucrose, maltose, & other
glucose polymers into their constituent monosaccharides
f. enzymes are located in the membranes of the microvilli brush
border of the enterocytes
g. disaccharides are digested as they come in contact with the
membrane
h. lactose  1 galactose + 1 glucose
i. sucrose  1 fructose + 1 glucose
j. maltose & glucose polymers  glucose
k. monosaccharides = final product of CHO digestion
l. monosaccharides - absorbed into the portal blood
2. ordinary diet
a. CHO is mostly starches
b. >80% of the final CHO digestion product is glucose
F. absorption of carbohydrates
1. aided by increased surface area
a. folds of Kerckring = plica circularis
b. villi - increase surface area 30X
c. microvilli - increase surface area 600X
2. glucose and galactose taken up with Na
3. uses Na/K pump to create gradient
4. glucose & galactose compete for movement
5. movement is dependent on the presence of Na
6. absorption is assisted by:
a. diffusion
b. facilitated transport
7. fructose
a. probably receptor mediated
b. not dependent on Na
c. does not compete
8. glycemic index
a. not all foods release glucose at the same rate
b. high glycemic index
1. elevated glucose
2. elevated insulin
3. increased fat absorption
9. insulin blocks hormone sensitive lipase
10. glucagon stimulates hormone sensitive lipase
11. coffee and stimulants:
a. stimulate cyclic AMP
b. degradation of FFA
c. increase FFA release
G. RDA - CHO requirements
1. adult = 0.8 gm/Kg body wt.
2. child = 2 gm - 4 gm/ Kg body wt.
3. may need to increase with exercise
Digestion of Carbohydrates
Starches
Ptyalin (saliva) = 20 to 40% CHO digestion (3-5% of digestion is in the mouth, remaining is in
stomach; up to pH of < 4, approx. 30-40 min)
Pancreatic amylase = 50 -80% of CHO digestion (begins in duodenum, ~10 min., most CHO
digestion is in jejunum)
Lactose
Sucrose
Maltose and 3 to 9 glucose polymers
Maltase & -dextrinase
(intestine)
isomaltase, maltoligosaccharides
Glucose
Lactase
(intestine)
Galactose
III. Digestion of Protein
A. dietary protein
1. long chains of aminos acids (A.A.)
2. bound by peptide linkages
B. characteristics determined by:
1. types of A.A. in molecule
2. arrangement of A.A.
C. sources of protein:
1. endogenous protein:
a. salivary secretions
b. bile
c. epithelium
d. digestive secretions
e. ~ 180 gm/day
2. exogenous protein:
a. diet
b. 80 - 100 gm/day - American diet
Fructose
Sucrase
(intestine)
3. egested protein:
a. GI cells (mostly from the colon)
b. GI flora
D. digestion of proteins in the stomach
1. pepsin:
a. important peptic enzyme of stomach
b. most active at pH of 2.0 - 3.0
c. inactive at pH > 5.0
d. activity is dependent on acid environment created by HCl
e. can digest collagen
1. an albuminoid
2. major constituent of intercellular connective tissue
f. only begins the process of protein digestion
g. provides 10 - 20% of protein digestion
Digestion of Proteins
pepsinogen
HCL

Proteoses
Peptones
Polypeptides
pepsin
1. Proteins
Proteoses
2. Peptones
Polypeptides
3.
Polypeptides
+
Amino Acids
(in the stomach)
Polypeptides
+
trypsin, chymotrypsin, carboxypolypeptidase, proelastase
Amino Acids
Amino Acids
peptidases
E. digestion of proteins by pancreatic secretions
1. endogenous protein:
a. salivary secretions
b. bile
c. epithelium
d. digestive secretions
e. 180 gm / day
2. exogenous protein
a. American diet
b. 80 100 gm / day
3. egested protein
a. GI cells of colon
b. GI flora
4. protein digestion occurs mainly in the duodenum and jejunum in
alkaline environment
5. digestion occurs via proteolytic enzymes of the pancreas
6. forms of protein leaving the stomach:
a. proteoses
b. peptones
c. large polypeptides
7. major proteolytic pancreatic enzymes in the small intestine:
a. trypsin  splits protein molecules into small polypeptides
b. chymotrypsin 
c. carboxypolypeptidasecleaves A.A. from carboxyl ends of
polypeptides
d. proelastase  gives rise to elastase, which digests elastin fibers
8. after digestion by pancreatic juices:
a. some A.A. constituents
b. dipeptides
c. tripeptides
d. larger peptides
F. digestion of peptides by peptidases in the enterocytes
1. enterocytes
a. perform last digestion of proteins in intestinal lumen
b. line the villi of small intestine
c. mainly in duodenum and jejunum
d. possess brush border with hundreds of microvilli on surface
e. peptidases are contained in cell membrane of microvilli
2. 2 types of brush border peptidase enzymes of importance:
a. aminopoly-peptidase
1. amino - oligopeptidase
2. amino - pepidase
3. dipeptidly aminopeptidase
b. several dipeptidases
3. peptidase enzymes split large polypeptides into tripeptide and
dipeptides, and some into A.A.
4. A.A., dipeptides, and tripeptides are transported through the microvillar
membrane to the interior of the enterocyte
G. digestion of protein in cytosol of enterocyte
1. multiple peptidases
a. prolidase
b. dipeptidase
c. tripeptidase
2. peptidases specific for linkages between amino acids
3. digestion to final stage of single A.A. occurs within minutes
4. A.A. pass through opposite site of enterocyte into the blood
5. about 99% of absorbed digested protein is single A.A.
6. very rare to have peptide and protein absorption
a. more common in fetal gut
b. used in rehab of injury via receptor mediated transport
c. creates allergic and immunological disturbances
H. no one method for AA uptake
1. L more easily absorbed than D type
2. AA carriers
a. dibasic AA
b. dicarboxycylic AA
c. neutral AA
d. imino acid-glycine
3. sodium transport
4. diffusion - major transport mechanism
IV. Digestion of fat
A. fat
1. mostly triglycerides - glycerol nucleus and 3 fatty acids
2. triglycerides = neutral fat
a. major constituent in food
b. mainly animal origin
3. also seen in the diet:
a. triacyelglicerides = triglycerides
b. phospholipids
1. a fat
2. contains fatty acid
c. cholesterol
1. sterol compound
2. contains no fatty acid, but is considered a dietary fat
3. derived from fat
4. has physical and chemical characteristics of fats
5. metabolized similarly as fats
d. cholesterol esters
1. a fat
2. contains fatty acid
B. digestion of fats in the small intestine
1. lingual lipase:
a. secreted by lingual glands
b. travels to stomach in saliva
c. digests small amount of triglycerides in stomach (<10%)
2. most fat digestion occurs in small intestine
C. emulsification of fat by bile acids and lecithin
1. emulsification
a. first step in fat digestion
b. occurs by:
1. agitation
2. bile:
a. prevents hydrolysis of fats
b. secreted by the liver
c. contains no digestive enzymes
d. contains large quantities of :
1. bile salts
2. phospholipid lecithin
2. bile salts and phospholipid lecithin
a. both are soluble in fat & water
b. act as detergents
c. increase surface area of fats
3. lipases are water-soluble compounds
a. can attack the fat globules only on their surfaces
b. co-lipase knocks bile off of fat so that lipase can
hydrolyze the fat
D. digestion of triglycerides by pancreatic lipase
1. pancreatic lipase = glycerol hydrolase
a. most important enzyme for triglyceride digestion
b. found in pancreatic juice
c. produced in enormous quantities
d. digests all triglycerides within a few minutes
2. enterocytes
a. found in small intestine
b. contain small quantity of enteric lipase
E. end products of fat digestion
1. most triglycerides are split by pancreatic lipase into free fatty acids
and 2-monoglycerides
2. small amount remains as diglycerides
F. role of bile salts in fat digestion
1. accelerates fat digestion
2. forms micelles
3. hydrolysis of triglycerides is reversible
4. bile salts remove monoglycerides and free fatty acids to prevent the
reversing of triglyceride hydrolysis
5. during triglyceride digestion:
a. monoglycerides and FFA are formed
b. fatty portion becomes dissolved in the central fatty portion
of micelles
c. concentrations of end products of digestion are reduced
d. digestive process continues
G. bile salt micelles:
a. transport medium to carry monoglycerides and FFA
b. transport to brush border of epithelial cells for absorption
c. bile salts are released at the brush border and reenter the chyme
Fat
Globule
_
Bile &
Agitation
bile salt
fat
_
bile salt
Micelles
bile salt
bile salt
B
r
u
s
h
B
o
r
d
e
r
fat
H. digestion of cholesterol ester and phospholipids
1. cholesterol esters
a. most common form in diet
b. free cholesterol + fatty acid molecule
2. phospholipids - contain fatty acid chains
3. lipases that free the fatty acids
a. cholesterol ester hydrolase - hydrolyzes cholesterol ester
b. phospholipase A2 - hydrolyses phospholipid
4. micelles are used for "ferry" free cholesterol and digested
phospholipids
5. essentially no cholesterol can be absorbed without micelles
I. chylomicrons - take ingested fat to CV side
J. plasma lipoproteins:
1. chylomicrons
2. VLDL
3. LDL
4. HDL
K. lipoprotein lipase:
1. stimulated by apo C-II
2. promotes fat storage
3. can be taken to monocyte
V. Basic Principles of GI Absorption
A. total fluid absorbed each day
1. 1.5 L ingested
2. 7 L secreted in GI secretions
3. 8 - 9 L total
4. all but 1.5 L is absorbed in the small intestine
B. absorption:
1. surface area
a. stomach does not absorb well
1. lacks villus membrane
2. tight junctions
b. duodenum and jejunum
1. folds of Kerckring = valvulae conniventes
2. villi
3. microvilli
2. mechanisms
a. active transport
b. diffusion
c. solvent drag
3. water
a. moved by diffusion
b. moves with absorbed substances
c. maintains osmotic environment
4. ions
a. sodium
b. bicarbonate
5. nutrients
a. carbohydrates
b. proteins
c. fats
6. large intestine
a. formation of feces
b. electrolytes and water