GI #41
Mon. 03/03/03 8 a.m.
(Orig. Thur. 02/27/03 1 p.m.)
Dr. Gwirtz
Scribe: Malik Ali
Page 1 of 8
This scribe has not been reviewed and probably won’t be because Dr. Gwirtz will be
out of town on Wed. But I assure you I included EVERYTHING on the lecture
tape. I feel I did a thorough job. If you have any questions listen to the tape.
GI Physiology: Digestion and Absorption I
Digestion and absorption - Objectives
 Describe the process of digestion and absorption of carbohydrates (in the
small intestine)
 Describe the process of digestion and absorption of proteins
 Describe the process of digestion and absorption of lipids – how bile salts are
involved
 Describe the role of bile salts in the uptake of lipid digestion products
(monoacylglycerol and fatty acids) by the small intestine
 Describe the chylomicrons and very-low-density lipoproteins made by the
small intestine
 List the fat- and water-soluble vitamins and describe their mechanisms of
absorption by the gastrointestinal tract
 Describe the mechanisms of absorption of sodium, calcium, and iron by the
gastrointestinal tract
 Describe how water is absorbed in the GI tract
 Describe the malabsorption syndromes of the small intestine
 Digestion and absorption begin as soon as you start eating a meal. For example,
eating a hamburger will begin the process by digesting starches via salivary αamylase in the mouth, continues to the stomach, and mostly to the small intestine.
Absorption of a number of calories, proteins, carbos, other electrolytes and water
occurs.
Anatomical Basis of Absorption
 Most occurs in small intestine. Very little occurs in stomach b/c of tight
junctions among epithelial cells that inhibit absorption due to thick
mucosal barrier.
 Some very lipid-soluble substances absorbed in stomach due to poor
absorptive surface
- alcohol
- asprin
- absorbed unionized in stomach acid; ionized in epithelial cell 
causes damage
- some drugs, ex – digitalis
If you ate a 5 Lb bag of sugar, you would absorb it!
 Absorptive surface in small intestine increases 600 X:
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GI #41
Mon. 03/03/03 8 a.m.
(Orig. Thur. 02/27/03 1 p.m.)
Dr. Gwirtz
Scribe: Malik Ali
Page 2 of 8
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– simple tube
– folds - 3X
– villi - 10 X
– microvilli - 20 X
arrangement of blood vessels and central lacteal – important for
countercurrent exchange for nutrients in villi
pinocytic vesicles
unstirred layer exists among microvilli – cells are continuously secreting a
watery solution to allow nutrient absorption across epithelial cells to blood
capillary or lymphatic system.
Absorption in small intestine
 CHO, fat, amino acids, electrolytes, vitamins, minerals, water
 Absorptive capacity much greater than needed normally
- Normally absorb several hundred grams CHO, 100 g fats, 50-100 g amino
acids, 50-100 g ions, 8-9 L water
- Max capacity for water absorption is 20 L or more per day
- Cholera toxin can cause us to lose 20-30 L/day thus overpowering our
capacity to absorb fluid and we lose fluid instead
General Mechanisms of Digestion & Absorption
 The most basic mechanism of digestion is hydrolysis whether it is a protein,
CHO, or fat: R” - R’ + H2O → R”OH + R’H. Enzymes act on the R-R link.
 All starches, sugars converted to glucose, galactose, or fructose for absorption
 Proteins and amino acid peptides broken down into amino acids for absorption
 Lipids, cholesterol, cholesterol esters broken down to free fatty acids and glycerol
to diffuse into epithelial cells and then absorbed as triglycerides and cholesterol
Chemical digestion of carbohydrates (Fig 24.33a in Berne and Levy)
 Starches are begun to be broken down to oligosaccharides by salivary amylase in the mouth – 40% of starches digested by this enzyme until it is
inactivated by acid
 Pancreatic -amylase continues the digestion of oligosaccharides in the
small intestine – 60% of starches digested this way
 Lactose and Sucrose have no digestive activity in mouth or via pancreas
but are digested by intestinal brush border enzymes (located on the
microvilli) – lactose into galactose and glucose, maltose (a glucose
polymer) into glucose, and sucrose into glucose and fructose
 80% of sugar absorbed is glucose, 10% is galactose, 10% is fructose
Absorption of Carbohydrates
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GI #41
Mon. 03/03/03 8 a.m.
(Orig. Thur. 02/27/03 1 p.m.)
Dr. Gwirtz
Scribe: Malik Ali
Page 3 of 8
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Sources: sucrose, lactose, starches
Absorbed as a monosaccharide: glucose, galactose, fructose
Glucose & galactose: via an active Na+- co-transport mechanism across
epithelial cell
Fructose: facilitated diffusion
CHO Digestion & Absorption in the Small Intestine
 Glucose-1 transport mechanism – glucose and Na+ attach to a carrier
molecule to bring glucose inside (Na-cotransport requires energy)
 Galactose via a similar mechanism w/ Na+ attached to a carrier
 Fructose comes in via a facilitated diffusion mechanism which requires no
energy
Absorption of Glucose From Various Carbohydrate-Containing Foods
 Glucose in its pure form has a glycemic index of 100 which means it
absorbs quickly
 Within other foods, absorption of glucose is a little less
Dietary Fiber
 CHO not digested by human GI tract: cellulose, hemicellulose, pectins
 Bind water
 Decrease transit time (meal spends less time in GI tract)
 ADsorb!!! minerals: Ca, Fe, Mg, Zn
 ADsorb organic material: bile salts (bile pool is decreased), lipids
 Bacterial flora digest  gas
 Failure to absorb  osmotic diarrhea (mechanism of laxatives to increase
motility)
 Too much fiber can be bad. If on a high fiber diet, you can end up with certain
malabsorption syndromes. Also can become dehydrated.
 Lactase deficiency can also cause osmotic diarrhea and lots of gas.
Chemical Digestion of Proteins (Fig. 24.33 – Berne and Levy)
 Protein digestion begins in stomach by pepsin which is activated by HCl
from pepsinogen
 In the small intestine, trypsinogen is activated by the brush border enzyme
enterokinase and then trypsin activates other proteolytic enzymes.
Enterokinase must be present!!!
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GI #41
Mon. 03/03/03 8 a.m.
(Orig. Thur. 02/27/03 1 p.m.)
Dr. Gwirtz
Scribe: Malik Ali
Page 4 of 8
Proteases and Peptidases in the Small Intestine
 Pancreatic proteases digest the proteins into oligopeptides which are futher
broken down by the brush border enzymes (amino peptidases,
carboxypeptidases, dipeptidases) into amino acids
 You still have some di- and tripeptides present
Absorption of Proteins
 Meats, plants
 Digestion by Epithelial Peptidases
 Absorbed as dipeptides, tripeptides and free amino acids
 Active Na+-co-transport mechanism - most amino acids absorbed by this
mechanism
 5 different amino acid and peptide transport proteins in luminal membrane
Absorption of Di- and Tripeptides Across Brush Border
 Even though you have di- and tripeptides entering across the brush border,
you have cytoplasmic peptidases which will further break down into
amino acids
 Only thing that enters portal circulation are amino acids which eventually
enter the blood
Amino Acid Transporters in Brush Border and Basolateral Membrane
- charts from book, don’t memorize
- several are Na+ dependent, some are Na+ independent
Conversion of Inactive Pancreatic Proenzymes to Active Enzymes
 Proenzymes like trypsinogen are secreted by the pancreas and are
converted to trypsin by enterokinase (in brush border) which then acts to
activate other proteolytic enzymes
Fat (Large, Lipid Droplets) Digestion / Absorption
 Emulsified by acid, pepsin in stomach; minimal digestion by gastric lipase
 Requirement for bile salts:
a) Continue emulsification in small intestine using bile salts in segmentation
contractions – breaking large droplets into small ones
b) Digestion by pancreatic lipases
c) Bile salt micelles transport fats to surface of epithelial cells. Lipid soluble
side attaches to lipid droplet, and enzymes attach to soluble side of bile
salt to begin digestion of lipids
d) Fats diffuse into epithelial cells (bile salt stays in lumen) – digested and
absorbed in small intestine
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GI #41
Mon. 03/03/03 8 a.m.
(Orig. Thur. 02/27/03 1 p.m.)
Dr. Gwirtz
Scribe: Malik Ali
Page 5 of 8
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Greasy pan being washed by a detergent – similar to bile salts on lipids
Lipid Digestion
 Once emulsified, pancreatic lipases convert to monoglycerides and fatty
acids and also to glycerol and fatty acids
 Cholesterol esters must be digested by cholesterol esterases for cholesterol
to be absorbed
Action of Pancreatic Lipases
 Triglyceride is hydrolyzed into 2-Monoglyceride and free fatty acids by
Glycerol ester hydrolase
 Cholesterol ester is hydrolyzed into cholesterol and fatty acid by
cholesterol ester hydrolase
 Lecitin is hydrolyzed into lysolecitin and fatty acid by phospholipase A2
Absorption of Fats
 Once lipids in epithelial cell, need to absorb into circulation via
chylomicrons
 Formation of chylomicrons (big fat globules)
- Fats recombined to form new triglycerides in endoplasmic reticulum
- Aggregate into globules along with absorbed and newly formed
cholesterol, phospholipids in Golgi apparatus
- Coated by apoprotein
Absorption of Fats
 Exocytosis of chylomicrons through basolateral membrane and into
lymphatic capillary b/c too big to enter blood capillary
- Enter blood circulation
 Absorption can describe the whole process getting into the blood
circulation via the liver, not just to epithelial cell
 Small chain fats can diffuse through epithelial cell into blood (40% of
dietary lipids) – don’t require bile salts
 Absorption of bile salts
- Na-cotransport in terminal ileum
 Other 60% are medium/large chain and must have bile salts –
chylomicrons – for excretion through lymphatics
Unstirred Layer Figure
 diffusion of micelle with bile salts surrounding it. Micelle moves lipids
through unstirred layer so that they can move through epithelial layer to be
absorbed. Monoglycerides, cholesterol, and free fatty acids are moved
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GI #41
Mon. 03/03/03 8 a.m.
(Orig. Thur. 02/27/03 1 p.m.)
Dr. Gwirtz
Scribe: Malik Ali
Page 6 of 8
Figure –
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Inside epith cells, ER and Golgi aid in making chylomicrons. Make
trigylycerides, phospholipids, cholesterols are re-esterified
Form Chylomicrons
Exocytosis into lymphatics
Difference between small and large chain lipid absorption – know the
process
Absorption of Bile Acids in Terminal Ileum
 Transporter binds bile acid to one site and Na+ to another
 Move bile acid into epith cell and into portal blood (high in bile salt conc.)
 Each bile salt used about 3 times in a meal
Overall Fluid Balance in GI Tract
 Ingesting about 9 L/day total
1. Ingest 2 L/day of water
2. 1.5 L/day of saliva
3. 2 L/day of gastric secretions
4. .5 L/day of bile
5. 1.5 L/day of pancreatic juices
6. 1.5 L/day of intestinal secretions
 Small intestine absorbs 8.5 L/day
 Colon absorbs 400 mL/day
 100 mL/day water excreted
 Where is most of the water absorbed in the GI tract? – the small
intestine!!! – absorbs 8-10 L/day; Large intestine – approx. 1 L/day
Water Absorption in the Small Intestine
 Diffusion, mainly through pores between the epithelial cells
- 85-95% absorbed in upper small intestine (duodenum and jujenum)
 Follows osmotic gradient created by absorption of nutrients and
electrolytes
 Exception: entry of hyperosmotic chyme from stomach into duodenum
results in osmosis of water (secretion) into the intestinal lumen to make
iso-osmotic soln. relative to plasma. But that water is actually absorbed,
it doesn’t just sit there
 1-3 liters ingested, 7-10 liters secreted by gut, but only 150-200 ml
excreted in feces!
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GI #41
Mon. 03/03/03 8 a.m.
(Orig. Thur. 02/27/03 1 p.m.)
Dr. Gwirtz
Scribe: Malik Ali
Page 7 of 8
Table 34.1 – Transport of Na+, K+, Cl-, and HCO3- in the large and small intestines
 Na+ is absorbed in all regions of the gut – jujenum, ileum, colon. Active
transport is a co-transporter which requires Na+ along with other nutrients
– glucose, galactose, bile salt co-transport in ileum – in order to be
absorbed. Co-transporters require energy in active transporter
mechanisms
 Cl- is normally absorbed in small intestine following electro-chemical
absorption gradient by Na+, HCO3- usually follows as well
 Na+ is passively absorbed in the small intestine following water through
the pores
 In the ileum and colon, Cl- is absorbed in exchange for the bicarb ion
secretion
 The bicarb is secreted because it is needed in the colon to neutralize the
acid secreted by the bacteria there
Major Ion Transport in Jejunum
 Most nutrients absorbed here
 Na+ co-transport with different sugars and amino acids (active transport)
 Cl- follows Na+
 Na+/H+ exchange for absorption of Na+
 K+ is passively absorbed between the cells
 Active Na+/K+ exchange in order to keep Na+ low in the cell
Major Ion Transport in Ileum
 Na+ co-transport with sugars, amino acids, and particularly bile salts
 Cl- follows by the electrical-chemical gradient
 Na+/H+ exchange and also Na+ pump at basolateral membrane
 Active Cl- absorption in exchange for HCO3- secretion (neutralizes acid
by bacteria)
Major Ion Transport in Colon
 Na+/H+ exchange
 Only electrolytes, water, and things like vitamin K that are made by
bacteria
 No absorption of CHO, amino acids, or lipids
Absorption of Sodium Ions
 25-35 g daily ingested
 Mechanisms at luminal membrane
1. diffusion through Na+ channel
2. coupled Na+- Cl– co-transport
3. Na+-H+ co-transport
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GI #41
Mon. 03/03/03 8 a.m.
(Orig. Thur. 02/27/03 1 p.m.)
Dr. Gwirtz
Scribe: Malik Ali
Page 8 of 8
Na+-solute co-transport (glucose, galactose, amino acids, bile
salts)
Active transport through basolateral membrane into intercellular spaces to
blood circulation
4.
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Absorption of Chloride Ions
 Follows electrochemical gradient created by absorption of Na+
 Active absorption in distal ileum and large intestine in exchange for
HCO3- ions (for buffering acidic products formed by bacteria)
Absorption of Bicarbonate
 Absorbed in jejunum with Na+
 Active secretion in distal ileum and large intestine in exchange for Cl(wrong in ppts) ions (for buffering acidic products formed by bacteria)
Absorption of Ions
 Potassium
- passive, through tight junctions and lateral spaces
 Calcium
- active
- absorbed relative to body’s need via parathyroid hormone and
vitamin D
 Iron
- active
- absorbed relative to body’s need
 Calcium and Iron absorbed according to body’s needs
Absorption of Ca++
 Ca++ binding protein inside cell
 Amount of Ca++ conc. inside cell dependent upon vit D converting
25-hydroxycholcalciferol in the liver to Ca++ binding protein
synthesis
 If you need more Ca++ in cell, you will make more Ca++ binding
protein to bring in more Ca++
 If you don’t need Ca++, the Ca++ stays in the cell and when cell is
sloughed off it is excreted
Effects of Vitamin D3 on Ca++ Absorption
 With Vit D, you make more binding protein and absorb lots of Ca++
in duodenum
 Without Vit D (control), you absorb some Ca++ but to a lesser extent
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