chapter 21c

Chapter 21c

The Digestive System

The Cephalic Phase

• Chemical and mechanical digestion begins in the mouth

• Salivary secretion is under autonomic control• Softens and lubricates food

• Chemical digestion: Salivary amylase and some lipase

• Saliva also has a protective function• Chewing: mastication

Swallowing Reflex

Figure 21-24, step 1

Tongue pushes bolus against softpalate and back of mouth, triggeringswallowing reflex.

Hard palate

Tongue

Bolus

Epiglottis

Tonically contracted upperesophageal sphincter

Larynx

Soft palate

Glottis

1

Swallowing Reflex


Upper esophageal sphincter relaxeswhile epiglottis closes to keepswallowed material out of the airways.

Epiglottis

2

Swallowing Reflex


Food moves downward into theesophagus, propelled by peristalticwaves and aided by gravity.

3

The Gastric Phase

• Storage • Stomach

• Digestion • Stomach• Acid, enzymes, and signal molecules

• Protection

The Gastric Phase

• Activity of secretory cells of the gastric mucosa

Figure 21-25

Gastric mucosa

Openingof gastric

gland

Cell Types Substance Secreted Stimulus for Release Function of Secretion

Mucousneck cell

Mucus

Bicarbonate

Tonic secretion;with irritation ofmucosaSecreted withmucus

Physical barrier betweenlumen and epithelium

Buffers gastric acid toprevent damage toepithelium

Parietalcells

Gastric acid (HCl)

Intrinsic factorAcetylcholine,gastrin, histamine

Activates pepsin;kills bacteriaComplexes with vitaminB12 to permit absorption

Enterochromaffin-like cell Histamine Acetylcholine,

gastrinStimulates gastricacid secretion

Chief cellsPepsin(ogen)Gastric lipase

Acetylcholine; acid,secretin

Digests proteins

Digests fats

D cells Somatostatin Acid in the stomach Inhibits gastric acidsecretion

G cells GastrinAcetylcholine,peptidesand amino acids

Stimulates gastricacid secretion

The Gastric Phase

• Integration of cephalic and gastric phase secretion in the stomach

Figure 21-26

Lumen ofstomach

Gastric mucosa

Entericplexus

Chiefcell

Gastrin

Somatostatin

Amino acidsor peptides

Input viavagus nerve

Entericsensoryneuron


ECLcell

G cell

D cell

H+

Food

Histamine

Pepsin Pepsinogen

Parietalcell

Negative feedbackpathway

Food or cephalic reflexesinitiate gastric secretion.

Gastrin stimulates acidsecretion by direct actionon parietal cells or indirectlythrough histamine.

Acid stimulates short reflexsecretion of pepsinogen.

Somatostatin release by H+

is the negative feedbacksignal that modulates acidand pepsin release.

1

2

3

4

1

2

3

4

1

+

The Gastric Phase


Lumen ofstomach

Gastric mucosa

Entericplexus

Gastrin




G cell

Food


1

11

The Gastric Phase

Figure 21-26, steps 1–2

Lumen ofstomach

Gastric mucosa

Entericplexus

Gastrin




ECLcell

G cell

H+

Food

HistamineParietal

cell



1

2

1

2

1

The Gastric Phase


Lumen ofstomach

Gastric mucosa

Entericplexus

Chiefcell

Gastrin





ECLcell

G cell

H+

Food

Histamine

Pepsin Pepsinogen

Parietalcell




1

2

3

1

2

3

1

The Gastric Phase


Lumen ofstomach

Gastric mucosa

Entericplexus

Chiefcell

Gastrin

Somatostatin





ECLcell

G cell

D cell

H+

Food

Histamine

Pepsin Pepsinogen

Parietalcell

Negative feedbackpathway




Somatostatin release by H+

is the negative feedbacksignal that modulates acidand pepsin release.

1

2

3

4

1

2

3

4

1

+

The Gastric Phase

• The mucus-bicarbonate barrier of the gastric mucosa

Figure 21-27Capillary

Stomachlumen

Gastric juice pH ~ 2

Mucuslayer

Mucusdroplets

Gastricmucouscell

pH ~ 7 at cell surface

HCO3– HCO3

–

The mucus layer is a physical barrier.

Bicarbonate is a chemicalbarrier that neutralizes acid.

Reflexes of Cephalic and Gastric Phases

• Long and short reflexes of the cephalic and gastric phases of digestion

Figure 12-23

The Intestinal Phase

• Chyme in the small intestine inhibits gastric motility and secretion

Figure 21-28

Acid secretion

Gastric motility

Pepsin and lipase secretion

Entericnervoussystem

AcidHyper-

osmoticsolution

CarbohydratesFats,

proteins

? Endocrine cell

GIP GLP-1 CCK Secretin

Pancreaticbicarbonate

secretion

Pancreaticenzyme

secretion

Insulinsecretion

Foodinto

stomach

Chymeinto smallintestine

STOMACH

SMALLINTESTINE

PANCREAS


• Bicarbonate neutralizes gastric acid• Goblet cells secrete mucus for protection and

lubrication• Bile • Fat digestion

• Digestive enzymes• Enteropeptidase


• Activation of pancreatic zymogens

Figure 21-29

Lumen of small intestine

Intestinalmucosa

Pancreatic duct

• Chymotrypsinogen• Procarboxypeptidase• Procolipase• Prophospholipase

• Chymotrypsin• Carboxypeptidase• Colipase• Phospholipase

Pancreatic secretions(include inactive

zymogens)

Trypsinogen

Trypsin

Enteropeptidasein brush border

activates trypsin.

ZYMOGENS

ACTIVATED ENZYMES

activates

Hepatic Portal System

• Most fluid is absorbed in the small intestine

Figure 21-30

Aorta

Hepaticvein

Inferiorvena cava

Digestivetract arteries

Hepaticportal vein

Hepatic artery

Capillariesof liver

Capillaries of digestivetract: stomach, intestines,

pancreas, and spleen

Liver


• Most digestion occurs in small intestine• Large intestine concentrates waste for

excretion• Motility in large intestine• Mass movement triggers defecation• Defecation reflex

• Digestion and absorption in large intestine• Diarrhea can cause dehydration

The Intestinal Phase: Anatomy of the Large Intestine

Figure 21-31

RectumRectum

Internal anal sphincter

AnusExternal anal sphincter

Cecum

Appendix

IleumIleocecal valve

Ascendingcolon

Transverse colon

AortaHepatic portal vein

Inferior vena cava

Descendingcolon

Sigmoid colon

Haustra

Tenia coli

Circular muscle

Longitudinal layer(tenia coli)

Intestinalglands

Lymphoidnodule

Submucosa

Muscularisexterna

Muscularis mucosae


Figure 21-31 (1 of 3)


Figure 21-31 (2 of 3)


Figure 21-31 (3 of 3)

Immune Function

• M cells sample the contents of the gut• Immune cells secrete cytokines• Cytokines trigger inflammatory response• Increase in Cl–, fluid, and mucus secretion

• Vomiting is a protective reflex

Summary

• Function and processes• Digestion, absorption, motility, secretion, and

GALT• Anatomy• Motility• Tonic and phasic contractions• Slow wave potentials, interstitial cells of Cajal,

migrating motor complex, peristaltic contractions, and segmental contractions

Summary

• Secretion• Parietal cells, CFTR chloride channel, mucous

cells, goblet cells, saliva, and bile salts• Regulation• Short reflexes, ENS, long reflexes, intrinsic

neurons, gastrin family, secretin family, and other hormones

Summary

• Digestion and absorption• Amylase, disaccharidases, endopeptidases,

exopeptidases, lipase, colipase, micelles, chylomicrons, and intrinsic factor

• The cephalic phase• The gastric phase• Mucous cells, chief cells, D cells, ECL cells, and

G cells

Summary

• The intestinal phase• Brush border, mass movement, and defecation

reflex• Immune functions• M cells and vomiting

chapter 21c

Documents

gastric phasefigure

gastric mucosafigure

gastric phase secretion

mouthsalivary secretion

direct actionon parietal

indirectlythrough histamine

mechanical digestion

somatostatin release