Chapter 21a

The Digestive System

About this Chapter

• Digestion function and processes• Anatomy of the digestive system• Motility• Secretion• Regulation of GI function• Digestion and absorption

About this Chapter

• The cephalic phase• The gastric phase• The intestinal phase• Immune functions of the GI tract

Activities of the Digestive System

• Ingestion• Digestion• Mechanical• Chemical / Secretion

• Motility• Peristaltic• Segmentation

• Absorption• Immunity• Elimination

Note:1. Which of these activities occur

at each region of the GI tract.2. Explain how these activities

occur, control (hormones or neural), enzymes and structural mechanisms, no need to name the transporters.

Figure 21-1

Digestive Function and Processes

• The volume of fluid entering the GI tract must equal the volume leaving 9 liters per day

7.5 L from smallintestine

1.4 L from largeintestine

0.1 L in feces

9.0 L removedfrom lumen

2.0 L food anddrink

1.5 L saliva(salivaryglands)

0.5 L bile(liver)

2.0 L gastricsecretions

1.5 L pancreaticsecretions

1.5 L intestinalsecretions

9.0 L Total inputinto lumen

Absorption

Excretion

Fluid input intodigestive system

Fluid removedfrom digestivesystem

Ingestion

Secretion

Figure 21-2

Four Basic Processes of the Digestive System

SECRETION

DIGESTION

ABSORPTION

MOTILITY

Lumen of digestive tract Wall Interstitialfluid

Blood

Food

Figure 21-3a

Digestive System Anatomy

• Trace through digestive system, not specific regions in addition to major parts. Also note accessory structures.

• Most simply:• Oral cavity pharynx

esophagus stomach small intestine large intestine rectum

• Accessory• Salivary glands• Pancreas• Liver and gall bladder

THE DIGESTIVE SYSTEM

Oral cavitySalivary

glandsEsophagus

Pancreas StomachSmall

intestineRectum

Largeintestine

(a)

Gallbladder Liver

ANATOMY SUMMARY

Digestive System Anatomy

• Stomach• Fundus body antrum (pylorus)

• Pyloric valve• Small intestine• Duodenum jejunum ileum

• Accessory organs: pancreas and liver• Large intestine: colon (ascending, transverse,

descending and sigmoid colons) and rectum• Anus

Figure 21-3a–b, d

Digestive System Anatomy

• A closer look at the structure of the stomach and small intestine

THE DIGESTIVE SYSTEM

Oral cavitySalivary

glandsEsophagus

Pancreas StomachSmall

intestineRectum

Largeintestine

(a)

Gallbladder Liver

ANATOMY SUMMARYSTRUCTURE OF THE STOMACH AND INTESTINE

Esophagus

Antrum

Pylorus

Rugae: Surface foldingincreases area

FundusDiaphragm

Body

(b) The stomach

Plica

Mesentery

VilliSubmucosalglandsSerosa

SubmucosaMucosa

(d) Structure of the small intestine

Circularmuscle

Longitudinalmuscle

Digestive System Anatomy

• Layers: same throughout, but modified for different functions

• Mucosa• Created from• Epithelial cells• Lamina propria• Muscularis mucosae

• Modifications increase surface area• Rugae / Plica and villi / crypts

• Submucosa• Muscularis externa• Serosa

Digestive System Anatomy

Figure 21-3c

SECTIONAL VIEW OF THE STOMACH

(c)

Epithelium

Lymph vessel

Myentericplexus

Arteryand vein

Lamina propria

Muscularis mucosae

Mucosa

Submucosa

Circular muscle

LongitudinalmuscleSerosa

Opening togastric gland

Oblique muscle

Muscularisexterna

Digestive System Anatomy

Figure 21-3e

(e)

Mucosa

Muscularismucosae

Submucosa

Circularmuscle

LongitudinalmuscleSerosa

Crypt

Peyer’s patch

Lymphvessel

Myentericplexus

Submucosalartery and vein

Submucosalplexus

Villi

Muscularisexterna

SECTIONAL VIEW OF THE SMALL INTESTINE

Digestive System Anatomy

PLAY Interactive Physiology® Animation: Digestive System: Anatomy Review: Overall Function of the GI System

Motility

• Tonic contractions • Sustained• Occur in smooth muscle sphincters and

stomach• Keep bolus from moving backwards

• Phasic contractions • Last a few seconds• Peristalsis moves bolus forward• Segmentation mixes

Figure 21-5a

Contractions in the GI Tract

• Peristalsis promotes forward movement

Figure 21-5b

Motility

• Segemental contractions promote mixing

PLAY Interactive Physiology® Animation: Digestive System: Motility

Figure 21-6

Secretion• 9 liters / day 7 of which from secretions• Parietal cells secrete hydrochloric acid into the lumen

of the stomach. Other sources include: duodenum / pancreas / salivary glands

Interstitialfluid

Lumen ofstomach

K+

H+

CO2

H2O

CA

CA = Carbonic anhydrase

ATP

Parietal cell

Cl–Cl– Cl–

HCO3– HCO3

–

H+ + OH–

K+

Cl–

Capillary

Figure 21-7

Secretion

• Anatomy of the exocrine and endocrine pancreas acini and islets

Duct cellssecrete NaHCO3 Pancreatic islet cells

Acinar cells

Pancreaticacini

Lumen

Figure 21-8

Secretion

• Bicarbonate secretion in the pancreas and duodenum

K+

2 Cl–Na+

Na+

K+

CFTRchannel

Pancreatic duct cellor duodenal cell

Interstitialfluid

Lumen ofpancreas or

intestine

K+

CO2H2O + CO2

H2O, Na+

CA

HCO3– HCO3

– + H+

ATPNa+

Cl–

Cl–

Capillary

Secretion

• Cl– secretion by intestinal and colonic crypt cells

Figure 21-9

K+

Na+

2 Cl–

Na+

K+

K+

Na+, K+ and Cl– enter bycotransport.

Cl– enters lumen throughCFTR channel.

Na+ is reabsorbed.3

Negative Cl– in lumen attractsNa+ by paracellular pathway.Water follows.

Interstitialfluid

Na+,H2O

Na+,H2O

Cl–Cl–

CFTR channel

Lumen

ATP

1

12

3

4

1

2

3

4

Secretion

• Digestive enzymes secreted into mouth, stomach and intestine

• Mucous cells in stomach and goblet cells in intestine

• Saliva is an exocrine secretion• Liver secretes bile

PLAY Interactive Physiology® Animation: Digestive System: Secretion