Chapter 23Digestive SystemLectures 9 & 10

Part 3: Stomach and Stomach Control

Marieb’s HumanAnatomy and

PhysiologyNinth Edition

Marieb Hoehn

2

Esophagus

Lower esophageal (cardiac) sphincter prevents reflux (backup) of stomach acid into the esophagus.

Veins drain into hepatic portal vein (via gastric veins)

Esophagus conveys food from pharynx to stomach by peristalsis

3

Stomach

Greater curvature

Stomach Functions: - Mixing - Reservoir - Secretion of gastric juice - Digestion, anti-bacterial action, facilitates absorption of vitamin B12

- Secretion of gastrin, somatostatin

Stomach can hold about 1-1.5 liters of material

M

Gastricglands

MG cellsD cells

Rugae flatten as stomach fills

4

Blood Supply and Drainage of Stomach

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

5

Lining and Gastric Glands of Stomach

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

6

Gastric Secretions

• pepsinogen• from chief cells• inactive form of pepsin

• pepsin• from pepsinogen in presence of HCl• protein splitting enzyme

• hydrochloric acid• from parietal cells• needed to convert pepsinogen to pepsin• ‘p’ in parietal and ‘p’ in pH

• mucus (cardia)• from goblet cells and mucous glands• protective to stomach wall

• intrinsic factor• from parietal cells• required for vitamin B12 absorption

• INFANTS ONLY• rennin (chymosin)• gastric lipase

• mucus, gastrin, somatostatin• from pyloric glands• protective to stomach wall• gastrin and somatostatin are hormones

7

Secretion of H+ by Parietal Cells

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Important functions of the stomach pH (1.5 – 2.0)

- kills microorganisms

- denatures proteins

- breaks down plant material and CT in meats

- activates pepsin

8

Three Phases of Stomach Control

• Cephalic phase• triggered by smell, taste, sight, or thought of food• begin secretion and digestion

• Gastric phase• triggered by distension, presence of food, and rise in pH in stomach• enhances secretion and digestion

• Intestinal phase • triggered by distension of small intestine and pH change• controls rate of gastric emptying; may slow emptying

NOTE that all these phases control activity in the STOMACH

Know what each phase does (shown in red)

9

Cephalic Phase of Gastric Secretion

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Emotional states can exaggerate or inhibit this phase

10

Gastric Phase of Gastric Secretion

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Proteins, alcohol, and caffeine can markedly increase secretions by stimulating gastric chemoreceptors

Histamine stimulates acid secretion by parietal cells

11

Intestinal Phase of Gastric Secretion

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Enterogastric reflex (inhibits gastric activity) –reduces gastric motility, stimulates contraction of pyloric sphincter (pylorus)

12

+ +

Parasympathetic NS

G cells

Gastrin

+

+

Both

Mucous Cells

ECL Cells

Histamine

Parietal CellsH+ + Cl-

HCO3- (alkaline tide)

Intrinsic Factor

+

+

+

Chief Cells

Pepsinogen Pepsin

Protein Breakdown

Peptides

+

Stretch of

stomach

pH > 3.0(dilution of H+)

Food in Stomach

++ +

D cells

Somatostatin

pH < 3.0+

Emptying of Stomach

( [H+ ])

-

Overview of Gastric Control/Secretion

Fats in Small

Intestine

+

(cephalic/gastric phases)

(intestinal phase)

+

Key

Stimulation

-Inhibition

Endocrine Factor

Exocrine Factor

LipasesFat

Breakdown

B12

Stomach Molility (Segmentation/Peristalsis)

+

13

Mixing and Emptying Actions

14

Gastric Absorption

• some water• certain salts• certain lipid-soluble drugs, e.g., aspirin• alcohol (slowed by presence of fats)

Gastric absorption is very limited due to:

- blanket of mucus covering cells - tight junctions between adjacent epithelial cells - epithelial cells lack specialized transport mechanisms - gastric lining is relatively impermeable to water - chyme usually contains only partially digested material

Some substances can be absorbed by the stomach: