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Overview of Gastrointestinal Function
George N. DeMartino, Ph.D.
Department of PhysiologyUniversity of Texas Southwestern Medical Center
Dallas, TX 75390
Functions of the gastrointestinal system
• Digestion
• Absorption
• Secretion
• Motility
• Immune surveillance and tolerance
GI-OP-13
X
X
X
X
X
Lumen Blood Tight junctions
Apical membrane Basolateral
membrane
GI-OP-19
Movement of substances across the epithelial layer
transcellular
paracellular
Motility in the gastrointestinal system
Propulsionnet movement by peristalsis
Mixingfor digestion and absorption
Separationsphincters
Storagedecreased pressure
GI-OP-42
Neural control of the GI system
• Extrinsic nervous system
autonomic central nervous system
• Intrinsic (enteric) nervous system
entirely with the GI system
GI-OP-14
The intrinsic nervous system formscomplete functional circuits
Sensory neurons
Interneurons
Motor neurons
(excitatory andinhibitory)
Composition of Saliva
Proteinsα−amylase lactoferrinlipase RNaselysozyme et almucus
Electrolyte solutionwaterNa+ , K +
HCO3-
Functions of saliva
• Oral protectionbuffering of hot, cold, acid, base
• Oral hygienebacteriostatic
• Lubrication swallowing
• Digestioncarbohydrates and lipids
GI-ME-11
Structure of the esophagus
Skeletal muscle
Skeletal and smooth muscle
Smooth muscle
Pharynx
UpperEsophagealSphincter(UES)
Diaphragm
Lower EsophagealSphincter(LES)
Stomach GI-010
Reflex relaxation(neural)
Persistalsis
Reflex contraction(neural)
Regulation of the UES during a swallow
Reflex contraction(myogenic)
Reflex relaxation(neural)
VIP(NO ?)
Regulation of the UES during a swallow
Anatomical and functionaldivisions of the stomach
G cell
Gastrin
pylorus
Pyloric gland area
Antrum
Body(Corpus)
Fundus
Oxynticgland area
Orad
duodenum
Caudad
Gastric juice Oxyntic
gland
LES
GI-S-15
H20 H+ + OH -
Mechanism of acid secretion by the parietal cell
Cl-Cl-
CO2
H+K+
K+
Na+ HCO3-
K+
Na+
Carbonicanhydrase
Cl-
H2O
H2O Lumen -
ATP ADP
ATP
ADP
+Blood GI-S-07
Stimulation of acid secretion in parietal cells
Histamine (H2)
AC
GACh
PL-C
GGastrin
PL-C
G
H+
Receptors
Lumen
H+
GI-S-13
Stimulation of acid secretion in parietal cells
SequesteredH+ / K+ pumps
Histamine (H2)
AC
GACh
PL-C
GGastrin
PL-C
G
Substrates
H+SequesteredH+ / K+ pumps
Membrane-exposedH+ / K+ pumps
Receptors
Lumen
cAMP
Protein kinase A
Substrates
IP3 + DAG
Protein kinaseC
Ca2+
GI-S-13
G cell
ECL cell
Parietal cell
GRP
Gastrin
ACh
Histamine
H+
Cephalic phase of acid secretion
thought, sight, smell, taste, or chewing of food
vagal nucleusvagus nerve
circulation
Efferent pathways
GI-S-05
G cellECL cell
Parietal cell
GRP
Gastrin
ACh
Histamine
H+
Gastric phase of acid secretion
distension in the stomach,presence of amino acids and peptides
vagal nucleusvagus nerve
circulation
distension sensed by mechanoreceptors
ACh
Efferent pathways
Afferent pathways
Amino acidsand peptides
GI-S-06
Protection of the epithelial lining of the stomach
pH 7
pH 2
HCO3- HCO3
-
Gastric lumen
Mucus layer
Mucus
GI-S-19
Motility in the stomach
• Reservoir
storage without increased pressure
• Grinding and mixing
mechanical disruption for digestion and absorption
• Regulated pumping
optimal delivery to duodenumGI-S-32
Pancreatic Secretion
Aqueous Component (ductule cells)
water Function: acid neutralizationbicarbonate
Enzymatic Component (acinar cells)
proteases Function: digestionlipasessaccharidases
vagus nerve
acinar cells
Ach
Enzymes
Intestinal phase of pancreatic secretion
acid and digestion productsin the duodenum
pancreas
GI-SILPGB-02
I cells
S cells
protein
fatH+
Secretin
amino acidspeptides
CCK HCO3-
H2O
ductule cells
afferentpathways for vagalreflex
efferentpathways forvagal reflexcirculation
vagal nucleus
Secretion from the liver and gallbladder
Na+ Cl - K+Na+ Cl - K+
waterwater
Ca2+Ca2+Bile acidsBile acids
Gallbladder
PortalcirculationPortalcirculation
HCO3-
Na+
H20 HCO3-
Cl-Na+
H20
HCO3-
Cl-Na+
H20
BileBile
CCK Ach
Bile acidsBile acids
Figure15
Figure14
Figure11
Sphincter of Oddi
Liver
Secretin
IleumDuodenum
Functions of Bile
• Emulsification of dietary lipids
required for lipid digestion
• Solubilization of lipid digestion products
required for lipid absorption
• Excretion of waste products
bilirubin and cholesterolGI-SILPGB-30
Organic components of bile
• Bile acids / bile salts 70%
• Phospholipids 20%
• Cholesterol 5%
• Bilirubin 1%
• Everything else 4%
GI-SILPGB-40
Bile acids package products of lipid digestion into mixed micelles for absorption
Products of lipid digestion Mixed micelle
+-- +
--
+
+--
-
-
Free fatty acid Lysolecithin
Cholesterol Monoglyceride
GI-SILPGB-36
Digestion and absorption in the small intestine
Proteinsdegradation to amino acids and peptidescombined action of lumenal and membrane-
bounds proteases absorption by multiple transporters
Carbohydratesdegradation to monosaccharidescombined action of lumenal and membrane-
bounds proteases absorption by several transporters
Lipids
Absorption of monosaccharidesin the small intestine
ATP ADP
Glucose /Galactose
Fructose
Na+
K+
Blood
Na+/ K+ ATPase
Glut 2
SGLT1
Na+
Glucose /Galactose
FructoseGlut 5
Lumen
(Apical) (Basolateral)GI-SILPGB-16
Fluid balance in the GI tract
~ 20% totalbody water !
Volumeabsorbed by intestines
Volumeabsorbed by intestines
Volumeentering intestines
Volumeentering intestines
9000 ml 8900 ml
100 mlVolumeexcreted
Important Principle
The absorption of water is dependent on and proportional to the absorption of solutes
GI-LI-25