Chapter 21b

Chapter 21b

The Digestive System

Regulation of GI Function

• Long reflexes integrated in CNS• Short reflexes integrated in ENS• Reflexes involving GI peptides / Hormones


• 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


Figure 21-11 (1 of 3)

Sensoryreceptors

andneurons

Localstimulus

Neuronsof

myentericand

submucosalplexuses

Smoothmuscles

orsecretory

cells

Inter-neurons





Stimulus Receptor


Short reflexesTissueresponse

KEY


Figure 21-11 (2 of 3)

Sensoryreceptors

andneurons

Localstimulus

Neuronsof

myentericand

submucosalplexuses

Smoothmuscles

orsecretory

cells






Thecephalic

brain

Externalstimulus

(sight, smell,etc.)

Sensoryreceptors

Stimulus Receptor



KEY


Figure 21-11 (3 of 3)

Sensoryreceptors

andneurons

Localstimulus

Neuronsof

myentericand

submucosalplexuses

Smoothmuscles

orsecretory

cells

Inter-neurons






Thecephalic

brain

Externalstimulus

(sight, smell,etc.)

Sensoryreceptors

Stimulus Receptor


Short reflexes


KEY


• 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


• 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


• 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


• 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


• 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


• Triglycerides digest into monoglycerides and free fatty acids

Figure 21-18

Triglyceride

Monoglyceride Free fatty acids

+

Lipase, colipase


• 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


Interstitialfluid


Capillary


stomach

Lacteal

Emulsion

Bile saltsfrom liver Bile salts from liver

coat fat droplets.11


Figure 21-20, step 1


Interstitialfluid


Capillary


stomach

Lipaseand colipase

Lacteal

Emulsion




Micelles

2

1

2

1

2


Figure 21-20, steps 1–2


Interstitialfluid


Capillary


stomach

Lipaseand colipase

Lacteal

Emulsion

Bile saltsrecycle





Micelles

2

1

2

3a

1

2

3a


Figure 21-20, steps 1–3a


Interstitialfluid


Capillary


stomach

Lipaseand colipase

Lacteal

Emulsion

Bile saltsrecycle






Micelles

2

1

2

3a

3b

1

2

3a3b


Figure 21-20, steps 1–3b




Interstitialfluid


Capillary



stomach

Lipaseand colipase

Lacteal

Emulsion

Bile saltsrecycle

SmoothER

Chylomicron







Micelles

2

1

2

3a

3b

4

1

2

3a3b

4


Interstitialfluid


Capillary



stomach

Lipaseand colipase

Lymphto

venacava

Lacteal

Emulsion

Bile saltsrecycle

Golgiapparatus

SmoothER

Chylomicron




Chylomicrons are releasedinto the lymphatic system.




Micelles

2

1

2

3a

3b

4

5

1

2

3a3b

4

5




• 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


• NaCl reabsorption in the small intestine and colon

Figure 21-21

Cl–

Cl–

Cl–

K+

Na+

Na+

Na+

H+

HCO3–

Na+

reabsorbed.


or colon

Na+ enters cells bymultiple pathways.

The Na+-K+-ATPase pumpsNa+ into the ECF.

Intestinal cell Interstitialfluid

K+

Na+

ATP

12

1 2


• 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


PLAY Interactive Physiology® Animation: Digestive System: Digestion and Absorption

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Chapter 21b