chapter 21b
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Chapter 21b
The Digestive System
Regulation of GI Function
• Long reflexes integrated in CNS• Short reflexes integrated in ENS• Reflexes involving GI peptides / Hormones
Regulation of GI Function
• Integration of long and short reflexes in the digestive system
Figure 21-11
Sensoryreceptors
andneurons
Localstimulus
Neuronsof
myentericand
submucosalplexuses
Smoothmuscles
orsecretory
cells
Inter-neurons
Sympathetic and parasympathetic neurons
Enteric nervous system“The little brain” Effectors
Muscle contraction/relaxation orExocrine secretion (enzymes, mucus,acid, bicarbonate), paracrines orEndocrine secretions
Digestive system responses
(pH, stretch,osmolarity,products ofdigestion)
Thecephalic
brain
Externalstimulus
(sight, smell,etc.)
Sensoryreceptors
Stimulus Receptor
Integratingcenter Efferent path
Short reflexes
Long reflexesTissueresponse
KEY
Regulation of GI Function
Figure 21-11 (1 of 3)
Sensoryreceptors
andneurons
Localstimulus
Neuronsof
myentericand
submucosalplexuses
Smoothmuscles
orsecretory
cells
Inter-neurons
Enteric nervous system“The little brain” Effectors
Muscle contraction/relaxation orExocrine secretion (enzymes, mucus,acid, bicarbonate), paracrines orEndocrine secretions
Digestive system responses
(pH, stretch,osmolarity,products ofdigestion)
Stimulus Receptor
Integratingcenter Efferent path
Short reflexesTissueresponse
KEY
Regulation of GI Function
Figure 21-11 (2 of 3)
Sensoryreceptors
andneurons
Localstimulus
Neuronsof
myentericand
submucosalplexuses
Smoothmuscles
orsecretory
cells
Sympathetic and parasympathetic neurons
Enteric nervous system“The little brain” Effectors
Muscle contraction/relaxation orExocrine secretion (enzymes, mucus,acid, bicarbonate), paracrines orEndocrine secretions
Digestive system responses
(pH, stretch,osmolarity,products ofdigestion)
Thecephalic
brain
Externalstimulus
(sight, smell,etc.)
Sensoryreceptors
Stimulus Receptor
Integratingcenter Efferent path
Long reflexesTissueresponse
KEY
Regulation of GI Function
Figure 21-11 (3 of 3)
Sensoryreceptors
andneurons
Localstimulus
Neuronsof
myentericand
submucosalplexuses
Smoothmuscles
orsecretory
cells
Inter-neurons
Sympathetic and parasympathetic neurons
Enteric nervous system“The little brain” Effectors
Muscle contraction/relaxation orExocrine secretion (enzymes, mucus,acid, bicarbonate), paracrines orEndocrine secretions
Digestive system responses
(pH, stretch,osmolarity,products ofdigestion)
Thecephalic
brain
Externalstimulus
(sight, smell,etc.)
Sensoryreceptors
Stimulus Receptor
Integratingcenter Efferent path
Short reflexes
Long reflexesTissueresponse
KEY
Regulation of GI Function
• Reflexes involving GI Peptides
Figure 21-12
Cephalic phase of digestion (feedforward)DistensionAcidPresence of food
Secretory cells of thestomach and small intestine
GI peptides
GI smooth muscleExocrine cells of stomach, pancreas, intestineOther endocrine cellsNervous system
Changes in GI motilityRelease of bile and pancreatic secretionsEnzyme, acid, and bicarbonate synthesis/releaseHunger/satiety
Stimuli
Receptor/integrating
center
Efferentpathway
Effectors
Responses
Overview of GI Reflexes
ENS Shares Features with CNS
• Intrinsic neurons• Neurotransmitters and neuromodulators• Support cells• Diffusion barrier• Integrating center
Digestive Hormones
• Gastrin family• Gastrin and CCK
• Secretin family• Secretin, VIP, GIP, and GLP-1
• Others • Motilin
Secretin
CCK
Fats and Proteins in SI
BilePancreatic enzymes• Trypsin• Chymotrypsin• ProcarboxypepsidaseBicarbonate ions
Control of the Digestive System
PLAY Interactive Physiology® Animation: Digestive System: Control of the Digestive System
Digestion and Absorption
• A villus and a crypt in the small intestine
Figure 21-13
Enterocytes
Capillaries
Goblet cells
Lacteals
Endocrine cellsMuscularis mucosae
Microvilli
Lamina propria
Brush border
Enterocyte
Cryptlumen
Figure 21-14
Digestion and Absorption
• Carbohydrates break down into monosaccharides
Starch, glycogen
SucroseMaltose
Maltase
2 glucose 1 glucose +1 fructose
1 glucose +1 galactose
Sucrase Lactase
Lactose
Glucose polymers
DisaccharidesAmylase
digest to
Monosaccharides
Figure 21-15
Digestion and Absorption
• Carbohydrate absorption in the small intestine
Glucose orgalactose Fructose
Na+
K+
Na+Lumen ofintestine
Intestinalmucosa
SGLTGLUT2GLUT5
Glucose enters the cell with Na+ on theSGLT symporter and exits on GLUT2.Fructose enters on GLUT5 and exitson GLUT2.
KEY
Figure 21-16
Digestion and Absorption
• Enzymes for protein digestion
COOH
COOH
COOHCOOH
H2N
H2N
H2NH2N
+H2O
+H2O +H2O
Aminoacids
Carboxy-terminal end
Amino-terminal end
Aminopeptidase Carboxypeptidase
Exopeptidase digests terminal peptidebonds to release amino acids.
Endopeptidasedigests internalpeptide bonds.
2 smaller peptides
COOH
COOH
H2N
H2N
H2N
COOH
COOH
H2N
(a) Peptide structure
(b)
Peptidebonds
Amino acid Amino acid
Peptide(c)
Figure 21-17
Digestion and Absorption
• Peptide absorption
Di- and tripeptidescotransport with H+.
Amino acidscotransportwith Na+.
Small peptidesare carried intactacross the cell bytranscytosis.
Peptides
Proteins
Na+
Na+
H+H+
Peptidases
Na+Na+
K+
H+
To the liverBlood
ATP
Digestion and Absorption
• Triglycerides digest into monoglycerides and free fatty acids
Figure 21-18
Triglyceride
Monoglyceride Free fatty acids
+
Lipase, colipase
Digestion and Absorption
• Bile salts emulsify fats
Figure 21-19
Hydrophobic sideassociateswith lipids.
Bile salt-coatedlipid droplet
Bilesalt
Bilesalt
DiglyceridePhospholipids
Polar side chains(hydrophilic sideassociates with
water)
Monoglyceride
Free fatty acids
Water
(a)
(b)
Cholesterol
Figure 21-20
Digestion and Absorption of Fats
Cells ofsmallintestine
Interstitialfluid
Lumen ofsmall intestine
Capillary
Triglycerides + cholesterol + protein
Large fatdroplets from
stomach
Lipaseand colipase
Lymphto
venacava
Lacteal
Emulsion
Bile saltsrecycle
Golgiapparatus
SmoothER
Chylomicron
Bile saltsfrom liver
Monoglycerides and fattyacids move out of micellesand enter cells by diffusion.
Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles.
Chylomicrons are releasedinto the lymphatic system.
Absorbed fats combine withcholesterol and proteins inthe intestinal cells to formchylomicrons.
Cholesterol is transportedinto cells by a membranetransporter.
Bile salts from livercoat fat droplets.
Micelles
2
1
2
3a
3b
4
5
1
2
3a3b
4
5
Cells ofsmallintestine
Interstitialfluid
Lumen ofsmall intestine
Capillary
Large fatdroplets from
stomach
Lacteal
Emulsion
Bile saltsfrom liver Bile salts from liver
coat fat droplets.11
Digestion and Absorption of Fats
Figure 21-20, step 1
Cells ofsmallintestine
Interstitialfluid
Lumen ofsmall intestine
Capillary
Large fatdroplets from
stomach
Lipaseand colipase
Lacteal
Emulsion
Bile saltsfrom liver
Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles.
Bile salts from livercoat fat droplets.
Micelles
2
1
2
1
2
Digestion and Absorption of Fats
Figure 21-20, steps 1–2
Cells ofsmallintestine
Interstitialfluid
Lumen ofsmall intestine
Capillary
Large fatdroplets from
stomach
Lipaseand colipase
Lacteal
Emulsion
Bile saltsrecycle
Bile saltsfrom liver
Monoglycerides and fattyacids move out of micellesand enter cells by diffusion.
Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles.
Bile salts from livercoat fat droplets.
Micelles
2
1
2
3a
1
2
3a
Digestion and Absorption of Fats
Figure 21-20, steps 1–3a
Cells ofsmallintestine
Interstitialfluid
Lumen ofsmall intestine
Capillary
Large fatdroplets from
stomach
Lipaseand colipase
Lacteal
Emulsion
Bile saltsrecycle
Bile saltsfrom liver
Monoglycerides and fattyacids move out of micellesand enter cells by diffusion.
Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles.
Cholesterol is transportedinto cells by a membranetransporter.
Bile salts from livercoat fat droplets.
Micelles
2
1
2
3a
3b
1
2
3a3b
Digestion and Absorption of Fats
Figure 21-20, steps 1–3b
Digestion and Absorption of Fats
Figure 21-20, steps 1–4
Cells ofsmallintestine
Interstitialfluid
Lumen ofsmall intestine
Capillary
Triglycerides + cholesterol + protein
Large fatdroplets from
stomach
Lipaseand colipase
Lacteal
Emulsion
Bile saltsrecycle
SmoothER
Chylomicron
Bile saltsfrom liver
Monoglycerides and fattyacids move out of micellesand enter cells by diffusion.
Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles.
Absorbed fats combine withcholesterol and proteins inthe intestinal cells to formchylomicrons.
Cholesterol is transportedinto cells by a membranetransporter.
Bile salts from livercoat fat droplets.
Micelles
2
1
2
3a
3b
4
1
2
3a3b
4
Cells ofsmallintestine
Interstitialfluid
Lumen ofsmall intestine
Capillary
Triglycerides + cholesterol + protein
Large fatdroplets from
stomach
Lipaseand colipase
Lymphto
venacava
Lacteal
Emulsion
Bile saltsrecycle
Golgiapparatus
SmoothER
Chylomicron
Bile saltsfrom liver
Monoglycerides and fattyacids move out of micellesand enter cells by diffusion.
Pancreatic lipase and coli-pase break down fats into monoglycerides and fatty acids stored in micelles.
Chylomicrons are releasedinto the lymphatic system.
Absorbed fats combine withcholesterol and proteins inthe intestinal cells to formchylomicrons.
Cholesterol is transportedinto cells by a membranetransporter.
Bile salts from livercoat fat droplets.
Micelles
2
1
2
3a
3b
4
5
1
2
3a3b
4
5
Digestion and Absorption of Fats
Figure 21-20, steps 1–5
Digestion and Absorption
• Nucleic acids are digested into nitrogenous bases and monosaccharides
• Intestine absorbs vitamins and minerals• Fat-soluble absorbed with fats• Water-soluble by mediated transport• Vitamin B12 when complexed to intrinsic factor and
absorbed in ileum• Mineral by active transport
Digestion and Absorption
• NaCl reabsorption in the small intestine and colon
Figure 21-21
Cl–
Cl–
Cl–
K+
Na+
Na+
Na+
H+
HCO3–
Na+
reabsorbed.
Lumen ofsmall intestine
or colon
Na+ enters cells bymultiple pathways.
The Na+-K+-ATPase pumpsNa+ into the ECF.
Intestinal cell Interstitialfluid
K+
Na+
ATP
12
1 2
Digestion and Absorption
• Summary of motility, secretion, digestion, and absorption in different regions of the digestive system
Figure 21-22
LiverGallbladder
Pylorus
Pancreas
Ileocecal valve
Rectum
Anal sphincters
Salivary gland
Esophagus
Upper esophagealsphincter
Lower esophagealsphincter
M: swallowing, chewingS:saliva (salivary glands), lipaseD:carbohydrates, fats (minimal)A:none
ORAL CAVITY AND ESOPHAGUS
STOMACHM:peristaltic mixing and propulsionS:HCl (parietal cells); pepsinogen and gastric lipase(chief cells); mucus and HCO3
– (surface mucous cells);gastrin (G cells); histamine (EC cells)D:proteins, fatsA:lipid-soluble substances such as alcohol and aspirin
M: motilityS:secretionD:digestionA:absorption
KEY
M:segmental mixing; mass movement for propulsionS:mucus (goblet cells)D:none (except by bacteria)A:ions, water, minerals, vitamins, and small organicmolecles produced by bacteria
LARGE INTESTINE
M:mixing and propulsion primarily by segmentationS:enzymes; HCO3
– and enzymes (pancreas); bile (liver);mucus (goblet cells); hormones: CCK, secretin, GIP,and other hormonesD:carbohydrates, fats, polypeptides, nucleic acidsA:peptides by active transport; amino acids, glucose,and fructose by secondary active transport; fats by simple diffusion; water by osmosis ions,minerals, and vitamins by active transport
SMALL INTESTINE
Digestion and Absorption
PLAY Interactive Physiology® Animation: Digestive System: Digestion and Absorption
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