Overview of Gastrointestinal Function

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Overview of Gastrointestinal Function
George N. DeMartino, Ph.D. Department of
Physiology University of Texas Southwestern
Medical Center Dallas, TX 75390
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The gastrointestinal system
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• Digestion
• Absorption
• Secretion
• Motility
• Immune surveillance and tolerance

GI-OP-13
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Histology of the GI tract
Blood or Serosal Side
Lumenal or Mucosal Side
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Structure of a villus
Villus
Lamina propria
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Movement of substances across the
epithelial layer
X
Tight junctions
Lumen
Blood
Apical membrane
Basolateral membrane
X
X
transcellular
X
X
GI-OP-19
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Histology of the GI tract
Blood or Serosal Side
Lumenal or Mucosal Side
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Motility in the gastrointestinal system
GI-OP-42
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Intercellular signaling in the gastrointestinal
system

• Neural
• Hormonal
• Paracrine

GI-OP-10
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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
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The extrinsic nervous system
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The intrinsic nervous system forms complete
functional circuits
Sensory neurons Interneurons Motor neurons
(excitatory and inhibitory)
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Parasympathetic nerves regulate functions of the
intrinsic nervous system
Y
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Reflex control of gastrointestinal functions
Afferent
Vago-vagal reflex
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Salivary Glands
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Composition of Saliva

• Proteins
• a-amylase lactoferrin
• lipase RNase
• lysozyme et al
• mucus
• Electrolyte solution
• water
• Na , K
• HCO3-

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Functions of saliva

• Oral protection
• buffering of hot, cold, acid, base
• Oral hygiene
• bacteriostatic
• Lubrication
• swallowing
• Digestion
• carbohydrates and lipids

GI-ME-11
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Structure of the esophagus
Pharynx
Upper Esophageal Sphincter (UES)
Skeletal muscle Skeletal and smooth
muscle Smooth muscle
Diaphragm
Lower Esophageal Sphincter (LES)
Stomach
GI-010
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Reflex relaxation (neural)
Reflex contraction (neural)
Persistalsis
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Reflex contraction (myogenic)
Reflex relaxation (neural)
VIP (NO ?)
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Anatomical and functional divisions of the stomach
LES
Fundus
Pyloric gland area
Oxyntic gland area
Orad
Gastric juice
Oxyntic gland
pylorus
G cell
Caudad
Body (Corpus)
Gastrin
duodenum
Antrum
GI-S-15
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Mechanism of acid secretion by the parietal cell
H2O
Lumen
-
Cl-
H
K
H2O
Cl-
Cl-
K
ATP ADP
K
ATP ADP
H20 H OH -
Na
HCO3-
Na
Carbonic anhydrase
CO2
Blood

GI-S-07
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Stimulation of acid secretion in parietal cells
Receptors
Histamine (H2)
ACh
Gastrin
H
H
Lumen
GI-S-13
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Stimulation of acid secretion in parietal cells
Receptors
Histamine (H2)
ACh
Gastrin
IP3 DAG
Protein kinase C
Ca2
Sequestered H / K pumps
Substrates
Sequestered H / K pumps
H
Membrane-exposed H / K pumps
Lumen
GI-S-13
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(No Transcript)
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Cephalic phase of acid secretion
vagal nucleus
vagus nerve
thought, sight, smell, taste, or chewing of food
H
ACh
Parietal cell
GRP
ECL cell
G cell
Histamine
Gastrin
Efferent pathways
circulation
GI-S-05
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Gastric phase of acid secretion
vagal nucleus
vagus nerve
distension sensed by mechanoreceptors
distension in the stomach, presence of amino
acids and peptides
H
Amino acids and peptides
ACh
Parietal cell
G cell
GRP
ECL cell
ACh
Histamine
Gastrin
Efferent pathways Afferent pathways
circulation
GI-S-06
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Protection of the epithelial lining of the stomach
pH 2
Gastric lumen
Mucus layer
HCO3-
HCO3-
pH 7
Mucus
GI-S-19
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Motility in the stomach

• Reservoir
• storage without increased pressure
• Grinding and mixing
• mechanical disruption for digestion and
  absorption
• Regulated pumping
• optimal delivery to duodenum

GI-S-32
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(No Transcript)
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The gastrointestinal system
Small Intestine Digestion Absorption
Secretion Motility
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Pancreatic Secretion
Aqueous Component (ductule cells)
water
Function acid neutralization bicarbonate E
nzymatic Component (acinar cells) proteases
Function digestion lipases sacchari
dases
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Intestinal phase of pancreatic secretion acid
and digestion products in the duodenum
vagal nucleus
vagus nerve
afferent pathways for vagal reflex
protein fat H
amino acids peptides
I cells
efferent pathways for vagal reflex
S cells
circulation
HCO3- H2O
CCK
Ach
pancreas
Secretin
Enzymes
acinar cells
ductule cells
GI-SILPGB-02
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Secretion from the liver and gallbladder
Figure15
Na Cl - K
Ca2
Bile acids
Liver
water
Portal circulation
Figure11
HCO3- Na H20
Gallbladder
Secretin
HCO3- Cl- Na H20
Figure14
Ach
CCK
Sphincter of Oddi
Bile acids
Bile
Ileum
Duodenum
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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 cholesterol

GI-SILPGB-30
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Organic components of bile

• Bile acids / bile salts 70
• Phospholipids 20
• Cholesterol 5
• Bilirubin 1
• Everything else 4

GI-SILPGB-40
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Components of bile
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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
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(No Transcript)
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Proteins degradation to amino acids and
peptides combined action of lumenal and
membrane- bounds proteases absorption by
multiple transporters Carbohydrates degradation
to monosaccharides combined action of lumenal
and membrane- bounds proteases absorption by
several transporters Lipids
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Absorption of monosaccharides in the
small intestine

K
ATP?ADP
Na
Na
Glucose / Galactose
Glucose / Galactose
Fructose
Fructose
Lumen
Blood
(Apical)
(Basolateral)
GI-SILPGB-16
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(No Transcript)
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Fluid balance in the GI tract
9000 ml
8900 ml
Volume entering intestines
Volume absorbed by intestines
100 ml
20 total body water !
Volume excreted
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Important Principle
The absorption of water is dependent on and
proportional to the absorption of solutes
GI-LI-25
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Bad things happen when solutes are not absorbed
Colon
Jejunum Ileum
Na
Lactose glucose galactose
H2O H2O
Normal
Diarrhea
SCFA CO2
Lactose Lactose H2O
Lactose
H2O
Lactase deficiency
GI-LI-24
H2O