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Overview of Gastrointestinal Function

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Title: Overview of Gastrointestinal Function


1
Overview of Gastrointestinal Function
George N. DeMartino, Ph.D. Department of
Physiology University of Texas Southwestern
Medical Center Dallas, TX 75390
2
The gastrointestinal system
3
  • Digestion
  • Absorption
  • Secretion
  • Motility
  • Immune surveillance and tolerance

GI-OP-13
4
Histology of the GI tract
Blood or Serosal Side
Lumenal or Mucosal Side
5
Structure of a villus
Villus
Lamina propria
6
Movement of substances across the
epithelial layer
X
Tight junctions
Lumen
Blood
Apical membrane
Basolateral membrane
X
X
transcellular
X
X
GI-OP-19
7
Histology of the GI tract
Blood or Serosal Side
Lumenal or Mucosal Side
8
Motility in the gastrointestinal system
GI-OP-42
9
Intercellular signaling in the gastrointestinal
system
  • Neural
  • Hormonal
  • Paracrine

GI-OP-10
10
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
11
The extrinsic nervous system
12
The intrinsic nervous system forms complete
functional circuits
Sensory neurons Interneurons Motor neurons
(excitatory and inhibitory)
13
Parasympathetic nerves regulate functions of the
intrinsic nervous system
Y
14
Reflex control of gastrointestinal functions
Afferent
Vago-vagal reflex
15
Salivary Glands
16
Composition of Saliva
  • Proteins
  • a-amylase lactoferrin
  • lipase RNase
  • lysozyme et al
  • mucus
  • Electrolyte solution
  • water
  • Na , K
  • HCO3-

17
Functions of saliva
  • Oral protection
  • buffering of hot, cold, acid, base
  • Oral hygiene
  • bacteriostatic
  • Lubrication
  • swallowing
  • Digestion
  • carbohydrates and lipids

GI-ME-11
18
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
19
Reflex relaxation (neural)
Reflex contraction (neural)
Persistalsis
20
Reflex contraction (myogenic)
Reflex relaxation (neural)
VIP (NO ?)
21
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
22
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
23
Stimulation of acid secretion in parietal cells
Receptors
Histamine (H2)
ACh
Gastrin
H
H
Lumen
GI-S-13
24
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
25
(No Transcript)
26
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
27
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
28
Protection of the epithelial lining of the stomach
pH 2
Gastric lumen
Mucus layer
HCO3-
HCO3-
pH 7
Mucus
GI-S-19
29
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
30
(No Transcript)
31
The gastrointestinal system
Small Intestine Digestion Absorption
Secretion Motility
32
Pancreatic Secretion
Aqueous Component (ductule cells)
water
Function acid neutralization bicarbonate E
nzymatic Component (acinar cells) proteases
Function digestion lipases sacchari
dases
33
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
34
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
35
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
36
Organic components of bile
  • Bile acids / bile salts 70
  • Phospholipids 20
  • Cholesterol 5
  • Bilirubin 1
  • Everything else 4

GI-SILPGB-40
37
Components of bile
38
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
39
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40
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
41
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
42
(No Transcript)
43
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
44
Important Principle
The absorption of water is dependent on and
proportional to the absorption of solutes
GI-LI-25
45
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
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