Digestive System

1
Mariebs Human Anatomy and Physiology Ninth
Edition Marieb w Hoehn

• Chapter 23
• Digestive System
• Lectures 9 10
• Part 3 Stomach and Stomach Control

2
Esophagus
Veins drain into hepatic portal vein (via gastric
veins)
Lower esophageal (cardiac) sphincter prevents
reflux (backup) of stomach acid into the
esophagus.
Esophagus conveys food from pharynx to stomach by
peristalsis
3
Stomach
M
Stomach can hold about 1-1.5 liters of material
Gastricglands
MG cellsD cells
Greater curvature
Stomach Functions - Mixing - Reservoir -
Secretion of gastric juice - Digestion,
anti-bacterial action, facilitates absorption of
vitamin B12 - Secretion of gastrin, somatostatin
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

• 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

• pepsinogen
• from chief cells
• inactive form of pepsin

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

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

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

• INFANTS ONLY
• rennin (chymosin)
• gastric lipase

7
Secretion of H by Parietal Cells
Important functions of the stomach pH (1.5
2.0) - kills microorganisms - denatures
proteins - breaks down plant material and CT in
meats - activates pepsin
Figure from Martini, Anatomy Physiology,
Prentice Hall, 2001
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
Emotional states can exaggerate or inhibit this
phase
Figure from Martini, Anatomy Physiology,
Prentice Hall, 2001
10
Gastric Phase of Gastric Secretion
Proteins, alcohol, and caffeine can markedly
increase secretions by stimulating gastric
chemoreceptors Histamine stimulates acid
secretion by parietal cells
Figure from Martini, Anatomy Physiology,
Prentice Hall, 2001
11
Intestinal Phase of Gastric Secretion
Enterogastric reflex (inhibits gastric activity)
reduces gastric motility, stimulates contraction
of pyloric sphincter (pylorus)
Figure from Martini, Anatomy Physiology,
Prentice Hall, 2001
12
Overview of Gastric Control/Secretion
Key

Stimulation
-
Mucous Cells
Inhibition
Emptying of Stomach (? H )
Stomach Molility (Segmentation/Peristalsis)
ECL Cells Histamine
Endocrine Factor


Exocrine Factor


(cephalic/gastric phases)

D cells Somatostatin
Intrinsic Factor

B12
Parietal Cells
pH lt 3.0
-

H Cl-
HCO3- (alkaline tide)





(intestinal phase)
Stretch of stomach
pH gt 3.0(dilution of H)
Fats in Small Intestine
Chief Cells
Peptides
Pepsinogen
Pepsin
Protein Breakdown
Food in Stomach
Fat Breakdown
Lipases
13
Mixing and Emptying Actions
14
Gastric Absorption
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

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