Anatomy and Physiology of the
Gastrointestinal Tract
John P. Grant, MD, CNSP
Director Nutrition Support Service
Professor of Surgery
Duke University Medical Center
Durham, NC
Specialty Examination
 Rule of thirds:
 1/3 of questions are simple – no study needed.
 1/3 of questions you will answer from experience.
 1/3 of questions you will not know, studying will not
help (with combination of studying and experience
you can make an educated guess in 1/3).
What are the Functions of the Gut?
 Digestion…
Process by which large molecules in diet are
broken down into smaller ones, which are
acceptable to the enterocytes for absorption.
What are the Functions of the Gut?
 Absorption…
Process by which contents of the small bowel
enter the mucosal epithelial cells, and
eventually the portal vein or lymphatics.
What are the Functions of the Gut?
 Protection…
Barrier to entry of pathogens and toxins
Digestion and Absorption - ???
1. What digestive process does not occur in the
oropharynx?
A. Micelles are formed with fat
B. Salivary a-amylase degrades starch
C. Pharyngeal lipase hydrolyzes triglycerides to
diglycerides and fatty acids
D. Food particles are mechanically broken up
Digestion and Absorption - ???
1. What digestive process does not occur in the
oropharynx?
A. Micelles are formed with fat
B. Salivary a-amylase degrades starch
C. Pharyngeal lipase hydrolyzes triglycerides to
diglycerides and fatty acids
D. Food particles are mechanically broken up
Anatomy and Physiology of
Digestive System - Mouth
 Mouth - Digestive Action
 Food mechanically broken down
 Saliva - normally about 25 ml/hr is secreted,
increases up to 300 ml per hour with eating
 Salivary a-amylase degrades starch,
a-amylase is deactivated by gastric acid
Anatomy and Physiology of
Digestive System - Mouth
 Mouth - Protective Action
 Preventive bacteria in the mouth are important for
defense against invading microorganisms
 Contains specific antimicrobial proteins like lysozyme,
lactoferrin and lactoperoxidase, but also mucin, IgA,
and nitric oxide-donating substances such as nitrates
 Mucus covers food and follows it to colon. It can
attach to mucosal surfaces and forms protective
barrier
Anatomy and Physiology of
Digestive System - Mouth
 Inhibition of Saliva
 Anticholinergics, analgesics,
antispasmodics, antidiarrheals,
antidepressants, antihistamines,
antihypertensives, antipsychotics, and
diuretics
 Stimulation of Saliva
 Pilocarpine 5mg po tid
Anatomy and Physiology of
Digestive System - Esophagus
 Esophagus
 Transports food to stomach
 Pharyngeal lipase hydrolyzes triglycerides
to diglycerides and fatty acids
Anatomy and Physiology of
Digestive System - Stomach
Stomach:
Stores, mixes,
and grinds food
to form an
emulsion
Gastroesophageal
Junction
Incisura
Pylorus
Duodenal
Bulb
Cardia
Body
Antrum
F
u
n
d
u
s
Anatomy and Physiology of
Digestive System - Stomach
Hydrochloric acid
from parietal cells
denatures protein
Surface Epithelial Cell
Mucous Cell
Pepsinogen (Pepsin)
from zymogen
(chief) cells begins
proteolysis
Parietal Cell
Zymogen Cell
Lymph Nodule
Argentaffine Cell
Muscularis Mucosae
Pyloric Glands
Submucosa
Gastric or Fundic Glands
Anatomy and Physiology of
Digestive System - Stomach
Pepsin
Acid
pH 1-3
Mucous gel
layer
Bicarbonate / Mucus
pH 7
Gastric lumen
Gastric mucosa
Zymogen
Cell
Mucous
Cell
Parietal
Cell
Anatomy and Physiology of
Digestive System - Small Bowel
 Small Intestine - averages around 5
meters in length
 Duodenum: ~10” long, 2” in
diameter
 Jejunum: ~ 2/5 length of rest of
small bowel, 1 ½ to 1 ¼” in
diameter, thick, many blood
vessels.
 Ileum: ~3/5 length, 1 ¼ to 1”
diameter, thin wall, large Peyer’s
patches
Anatomy and Physiology of
Digestive System - Small Bowel
 Small Intestine
 Major organ for nutrient absorption
 Absorptive surface enhanced by plicae circulares,
foldings called villi with surface projections called
microvilli
 Final surface area about 1.7 m cm2
 About 800 cm2 to absorb 1 Kcal (100-200
cm2/Kcal minimum)
Anatomy and Physiology of
Digestive System - Small Bowel
Plica circularis or
Valve of Kerckring
Villus
Jejunum
Ileum
Digestion and Absorption - ???
Poor question…
2. The absorptive surface of the normal small
intestine is equal to:
A.
B.
C.
D.
3 tennis courts
5 tennis courts
1 tennis court
10 tennis courts
Digestion and Absorption - ???
Poor question…
2. The absorptive surface of the normal small
intestine is equal to:
A.
B.
C.
D.
3 tennis courts
5 tennis courts
1 tennis court
10 tennis courts
Digestion and Absorption - ???
Poor question…
3. How much small intestine can be removed
before a patient will develop short bowel
syndrome?
A. 1/3
C. 2/3
B. 1/2
D. 3/4
Digestion and Absorption - ???
Poor question…
3. How much small intestine can be removed
before a patient will develop short bowel
syndrome?
A. 1/3
C. 2/3
B. 1/2
D. 3/4
Digestion and Absorption - ???
It is always better to know the amount of bowel remaining
following surgical resection – rather than the amount
resected…Better question:
A patient is likely to, but not always, require HTPN if
only the following amount of small bowel can be
saved at the time of surgery:
A. 10 feet
C. 3 feet
B. 5 feet
D. 1 foot
Digestion and Absorption - ???
It is always better to know the amount of bowel remaining
following surgical resection – rather than the amount
resected…Better question:
A patient is likely to, but not always, require HTPN if
only the following amount of small bowel can be
saved at the time of surgery:
A. 10 feet
C. 3 feet
B. 5 feet
D. 1 foot
Digestion and Absorption
There is a high probability of transitioning patients
off HTPN if the ratio of remaining intestinal length
to body weight is: > 0.5 cm/kg
ie: > 40 cm for an 80 kg patient
Wilmore, et al., Ann. Surg., 226:288-293, 1997
Anatomy and Physiology of
Digestive System - Colon
 Absorbs water and
electrolytes
 Stores waste

“Organ within an Organ”
Colonic microflora ferment
malabsorbed nutrients and
soluble fiber to a form the
colonic mucosa can absorb
Nutrient Absorption
Sites of Nutrient
Absorption
Sites of Nutrient
Absorption
Good question…
5. What nutrient
deficiency might be
expected if 3 feet of
the terminal ileum is
resected?
A. Iron
C. Vit B-12
B. Magnesium
D. Calcium
Sites of Nutrient
Absorption
Good question…
5. What nutrient
deficiency might be
expected if 3 feet of
the terminal ileum is
resected?
A. Iron
C. Vit B-12
B. Magnesium
D. Calcium
Sites of Nutrient
Absorption
Much harder question…
6. What nutrient
deficiency might be
expected if 3 feet of
the terminal ileum is
resected?
A. Iron
C. Vit B-12
B. Magnesium
D. Fat
Absorption of Water
 8 L/day fluid reaches the small intestine of
which about 2 L is dietary in origin
 Small bowel absorbs about 7 L/d, mainly in
jejunum, colon absorbs 1 to 1.5 L/d (can
increase absorption up to 4 L/d)
Digestion and Absorption - ???
7. Water absorption by the small bowel may be
enhanced by adding the following to the enteral
formula:
A. Zinc Sulfate
C. Magnesium Chloride
B. Sodium Chloride
D.Medium-Chain Fat
Digestion and Absorption - ???
7. Water absorption by the small bowel may be
enhanced by adding the following to the enteral
formula:
A. Zinc Sulfate
C. Magnesium Chloride
B. Sodium Chloride
D.Medium-Chain Fat
Absorption of Water
 Water absorption is entirely passive - follows
absorption of solutes (can move in either
direction, depending on osmotic gradients)
 Particularly true of the solute Na+
 High sodium enteral diets enhance water
absorption
 Low sodium diets predispose to diarrhea
Absorption of Sodium and
Chloride - Small Bowel
 Sodium absorption is
directly coupled to
absorption of organic
solutes such as
glucose, amino acids,
water-soluble
vitamins, and bile
salts
Absorption of Sodium and
Chloride - Small Bowel
 Once inside cell,
sodium is extruded
against chemical and
electrical gradient via
a basolateral
membrane-associated
Na+-K+-ATPase
Absorption of Sodium and
Chloride - Small Bowel
 Chloride passively
follows absorption of
sodium
Absorption of Sodium and
Chloride – Distal Ileum and Colon
 Neutral NaCl co-transport, Na+ for H+ and Cl for
HC03
Absorption of Potassium
in Small Bowel
 Overall K+ movement is result of solvent
drag and is potential-dependent
 K+ actively secreted in colon
 Rectosigmoid colon has active K+
absorption - exchanges K+ for H+
Absorption of Calcium
 Passive - throughout the small intestine
 Predominates at concentrations  10
mmol/L
 Active - primarily in the duodenum
 Below 10 mmol/L, active transport
occurs with 1,25 dihydroxy vitamin D3
Absorption of Calcium
Absorption of Magnesium
 Absorbed from distal small intestine and all of
colon by passive diffusion
Digestion and Absorption - ???
Good question…
8. What mineral will be depleted if a patient has
a draining T-tube in the common bile duct ?
A. Zinc
C. Magnesium
B. Sodium
D. Copper
Digestion and Absorption - ???
Good question…
8. What mineral will be depleted if a patient has
a draining T-tube in the common bile duct ?
A. Zinc
C. Magnesium
B. Sodium
D. Copper
Absorption of Copper
 Dietary copper is absorbed in stomach and
duodenum
 Active process requires energy and involves
absorption of complexes of copper and amino
acids
 Copper is excreted by bile
urine losses = ~1-2% of intake
Digestion and Absorption - ???
Good question…
8. What mineral will be depleted if a patient has
marked diarrhea ?
A. Zinc
C. Magnesium
B. Sodium
D. Copper
Digestion and Absorption - ???
Good question…
8. What mineral will be depleted if a patient has
marked diarrhea ?
A. Zinc
C. Magnesium
B. Sodium
D. Copper
Absorption of Zinc
 Major loss of zinc is in the feces - during
intravenous nutrition it is suggested to give:
2 mg Zn
+ 17.1 mg Zn/kg stool lost
+ 12.2 mg Zn/kg of gastric/duodenal/or
jejunal fluid lost
Absorption of Zinc
 Zinc is absorbed primarily in jejunum
 Binds to a ligand in lumen – transports to mucosa
 Transferred to binding site on cell
 Active process requiring energy, oxygen, and Na+
 Absorption stimulated by glucose
Digestion and Absorption - ???
Good question…
9. Even in Short Bowel Syndrome, oral iron
supplementation can be effective.
A. True
B. False
Digestion and Absorption - ???
Good question…
9. Even in Short Bowel Syndrome, oral iron
supplementation can be effective.
A. True
B. False
Absorption of Iron
 Iron is absorbed in duodenum
ferrous salt > ferric salt
 At brush border, ferrous ion oxidized to ferric
and transported by various brush border
carrier proteins - regulated by body’s need for
iron
 At pharmacologic doses, passive diffusion
occurs
Digestion and Absorption - ???
Good question…
10. The major source of carbohydrate in the diet
is:
A. Starch
C. Lactose
B. Sucrose
D. Fructose
Digestion and Absorption - ???
Good question…
10. The major source of carbohydrate in the diet
is:
A. Starch
C. Lactose
B. Sucrose
D. Fructose
Average Carbohydrate Intake
Saccharides
Intake (gm) % Total
Polysaccharides
Starch
200
64
Glycogen
1
0.5
Disaccharides
Sucrose
80
26
Lactose
20
6.5
Monosaccharide
Fructose
10
3
Carbohydrate Digestion and
Absorption
 Primarily absorbed in duodenum and
proximal jejunum (75% in first 70 cm of
jejunum)
 Carbohydrate intolerance is nearly always
related to a defect in intestinal surface
digestion of a polysaccharide or disaccharide
Carbohydrate Digestion and
Absorption
 Luminal phase
 Hydrolysis of starch by salivary and
pancreatic a-amylases
Cleaves starches to yield a-limit
dextrans, maltotriose, and maltose
Carbohydrate Digestion and
Absorption
 Brush-border phase
 Hydrolysis by glycosidases to
monosaccharides:
glucose, galactose, and fructose
Carbohydrate Digestion and
Absorption
 Cellular phase - transport to venous system
 Glucose and galactose transported via
SGLT1 Na+-linked active transporter
 Fructose via GLUT5 facilitated diffusion
Carbohydrate Digestion and
Absorption
SGLT1
GLUT2
Galactose
Fructose
GLUT5
GLUT2
SGLT1
Glucose
Fiber Digestion and Absorption
 Non-starch carbohydrate of plant origin that
escapes enzymatic digestion in the small
intestine
 Two types
 Cellulosic: high molecular weight, non-soluble
(cellulose, wheat bran)
 Noncellulosic: soluble (hemicelluloses, pectin,
gums, mucilages)
Digestion and Absorption - ???
Good question…
11. Which type of fiber can contribute to the
energy needs of the colon?
A. Cellulosic
B. Non Cellulosic
Digestion and Absorption - ???
Good question…
11. Which type of fiber can contribute to the
energy needs of the colon?
A. Cellulosic
B. Non Cellulosic
Fiber Digestion and Absorption
 Non Cellulosic fiber is degraded rapidly by
anaerobic microflora of cecum and colon
(fermentation) to give short-chain fatty
acids:
Acetate, Propionate, N-butyrate
Fiber Digestion and Absorption
 Non Cellulosic fiber
 Enhances colonic blood flow
 Serves as fuel for colonocyte (70% )
 Increases colonocyte proliferation
 Enhances Na+ absorption
 Preserves colonic mucosal barrier
Fiber Digestion and Absorption
 Cellulosic fiber
 Contributes to fecal mass and water
content
 Reduces mean stool transit time
Fat Digestion and Absorption
  96% of ingested
fat is absorbed daily
 90% of ingested fat
is triglycerides, 10%
is cholesterol,
phospholipids, and
plant sterols
Triglycerides
 Majority of triglycerides contain long-chain
fatty acids (16 -18 C)
 A few dietary triglycerides contain mediumchain fatty acids (8 -12 C)
Lipid Absorption
 Lipids in diet are emulsified in the stomach
by mechanical grinding
 Pancreatic lipase in duodenum digests to free
fatty acids and 2-monoglycerides
 Bile acids form water soluble micelles which
diffuse easily across the unstirred water layer
Fat Digestion and Absorption
Digestion and Absorption - ???
Good question…
12. Which of the following fatty acid(s) is absorbed
directly into the enterocyte without micelle
formation?
A. Arachidonic acids
C. Chenodeoxycholic acids
B. Medium-chain fatty acids
D. Picric acids
Digestion and Absorption - ???
Good question…
12. Which of the following fatty acid(s) is absorbed
directly into the enterocyte without micelle
formation?
A. Arachidonic acids
C. Chenodeoxycholic acids
B. Medium-chain fatty acids
D. Picric acids
Fat Digestion and Absorption
 Medium-chain triglycerides, which are more
water soluble, may be absorbed intact with
direct transport to the portal system as free
fatty acids
Protein Digestion and Absorption
 Derived from animal and vegetable sources and
make up to 11 to 14% of average caloric intake
(70 to 100 gm/day)
 Primarily absorbed in the duodenum and
proximal jejunum, yet some does pass into and
is absorbed by the colon
Protein Digestion and Absorption
 Luminal gastric digestion
 Acid denaturation makes protein
susceptible to proteolysis by pepsin
resulting in large soluble oligopeptides,
peptones, and some amino acids
Protein Digestion and Absorption
 Luminal duodenal phase
 Three pancreatic endopeptidases and two
pancreatic exopeptidases reduce
oligopeptides to free amino acids and diand tripeptides
Protein Digestion and Absorption
 Luminal enterocyte phase
 Enterocyte brush border membrane
hydrolase produces amino acids,
dipeptides and tripeptides
Protein Digestion and Absorption
 There are 4 major Na+ dependent, group
specific, active transport systems
 Neutral amino acids
 Glycine, proline, hydroxyproline
 Dibasic amino acids and cystine
 Dicarboxylic amino acids
Digestion and Absorption - ???
Good question…
13. What is the di- and tri-peptide H+ dependent
transport system in the small intestine?
A. GLUT5
C. PepT1
B. Endopeptidase
D. SGLT1
Digestion and Absorption - ???
Good question…
13. What is the di- and tri-peptide H+ dependent
transport system in the small intestine?
A. GLUT5
C. PepT1
B. Endopeptidase
D. SGLT1
Protein Digestion and Absorption
 There is a H+ dependent di- and tripeptide
transport system (PepT1 Transporter)
 Cephalosporin antibiotics, containing a
peptide bond, share the same transport
system
Protein Digestion and Absorption
 Amino acids absorbed by either route efflux
from the basolateral membrane via
transporters whose kinetic characteristics are
sensitive to circulating amino acid
concentrations
Protein Digestion and Absorption
 Mucosal uptake of peptides has an important
role in protein absorption - absorption of
a-amino nitrogen is greater during perfusion
of di- and tripeptides
 Yet have less effect on Na+ and water uptake
than free amino acids or complex proteins
(diarrhea)
Protein Digestion and Absorption
 Although theoretical advantages may exist
for enteral products containing peptides vs
intact protein or free amino acids, little
experimental data exists to support their
advantage. Any advantage would be more
apparent with increased “protein load” as
during cyclical feedings
Malabsorption
 Clinical symptoms include unexplained weight
loss, steatorrhea, diarrhea, anemia, tetany,
bone pain, pathologic fractures, bleeding,
dermatitis, neuropathy, glossitis, and edema
Malabsorption
Normal Stool Composition
 Water
100 ml
 Sodium
4 mEq
 Potassium
9 mEq
 Chloride
2 mEq
 Bicarbonate
0 mEq
From 3/week to 3/day considered normal
Diarrhea = stool weight > 200 to 500 g/24 h
Digestion and Absorption - ???
Good question…
13. Each of the following are screening tests for
intestinal absorption except:
A. Gross inspection of stool
B. Fat content of stool in random collection
C. Microscopic examination of stool
D. Lactose tolerance test
Digestion and Absorption - ???
Good question…
13. Each of the following are screening tests for
intestinal absorption except:
A. Gross inspection of stool
B. Fat content of stool in random collection
C. Microscopic examination of stool
D. Lactose tolerance test
Malabsorption - Screening Tests
 Gross inspection of stool
 Microscopic examination of stool
 Fat content of stool in random collection
 Protein content of stool in random collection
Malabsorption - Screening Tests
 Serum carotene
 D-xylose absorption
 Radiologic evaluation of transit time,
motility, mucosal diseases, fistulas,
strictures/obstructions
Malabsorption - Specific Tests
 Lactose tolerance test (Lactase deficiency,
short gut, mucosal disease)
 Schilling test (B12 absorption, tests terminal
ileum and gastric production of intrinsic
factor)
Malabsorption - Specific Tests
 Small bowel biopsy (celiac disease, tropical
sprue, Whipple’s disease, etc.)
 Radioactive compounds (iron, calcium, amino
acids, folic acid, pyridoxine, vit D, bile salts,
and others)
Malabsorption - Balance Tests
 3-5 day timed stool collection for quantitative
fat absorption (standard fat intake = 100
g/d)
Usually less than 5% lost in stool
Malabsorption - Balance Tests
 Radioactive tracer studies
 14C-triolein and 13C-trioctanion breath tests
for neutral fat absorption
 131I-albumin, 51Cr-albumin given IV with
stool measurement of radioactivity for
protein-losing enteropathies
Specialty Examination
Good Luck !!