1. Unit Twelve: Gastrointestinal Physiology
Chapter 63: Propulsion and Mixing
Of Food in the Alimentary Tract
Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. Ingestion of Food
Mastication (Chewing)
Most of the chewing process is caused by a chewing
reflex
Chewing is important for the digestion of all foods,
but especially fruits and raw vegetables (cellulose)
Digestive enzymes act only on the surface of food
particles so it the rate of digestion is dependent on
the total surface area exposed to the digestive
secretions

3. Ingestion of Food
Swallowing (Deglutition)
Complicated because the pharynx subserves
respiration and swallowing; important that
respiration not be compromised because of
swallowing
Voluntary stage of swallowing-the food is rolled
posteriorly into the pharynx by pressure of the
tongue

4. Swallowing (Deglutition)
Ingestion of Food
Swallowing (Deglutition)
Fig Swallowing mechanism

5. Ingestion of Food
Swallowing (Deglutition)
d. Pharyngeal Stage of Swallowing
Soft palate is pulled upward to close the posterior
nares
The trachea is closed (epiglottis covers the glottis)
Fast peristaltic wave is initiated by the nervous
system
Pharynx forces the bolus of food into the
esophagus

6. Ingestion of Food
Swallowing (Deglutition)
e. Swallowing center inhibits the respiratory center of
of the medulla; halting respiration at any point
f. Esophageal Stage- primary and secondary peristalsis
g. Receptive relaxation of the stomach-as the wave
approaches the stomach
h. Function of the lower esophageal sphincter-there is a
“receptive” relaxation ahead of the peristaltic wave

7. Ingestion of Food
Motor Functions of the Stomach
Storage of large quantities of food until the food can
be processed in the stomach and intestinal tract
Mixing of the food with gastric secretions until it
becomes a semi-fluid mixture called chyme
Slow emptying of the chyme into the small intestine
at a rate suitable for proper digestion and absorption

8. Ingestion of Food
Fig Physiologic anatomy of the stomach

9. Ingestion of Food
Storage Function of the Stomach
Mixing and propulsion of food in the stomach-
the basic electrical rhythm of the stomach wall
1. Chyme-degree of fluidity depends on the amount
of food, water, and stomach secretions-pasty
2. Hunger contractions-rhythmic peristaltic
contractions of the body of the stomach after the
stomach has been empty for several hours

10. Ingestion of Food
Storage Function of the Stomach
b. Stomach emptying
Intense antral peristaltic contractions during
emptying-pyloric pump
Pyloric sphincter- usually open enough for water
and other fluids to empty but not for chyme
until it is the right consistency

11. Ingestion of Food
Storage Function of the Stomach
Regulation of stomach emptying- rate at which the
stomach empties is dependent on signals from both
the stomach and the duodenum
Gastric factors that promote emptying- increased
food volume and some stretching
Effect of gastrin- causes secretion of gastric
juice; enhances the activity of the pyloric pump

12. Ingestion of Food
Powerful Duodenal Factors that Inhibit Stomach
Emptying
Inhibitory effect of enterogastric nervous reflexes
from the duodenum; mediated by three routes:
Directly from the duodenum to the stomach
through the enteric nervous system
Through extrinsic nerves
Through the vagus nerve
All three inhibit the pyloric pump and increase
the tone of the pyloric sphincter

13. Ingestion of Food
Powerful Duodenal Factors that Inhibit Stomach
Emptying
Types of factors that can initiate enterogastric
inhibitory reflexes:
Degree of distension of the duodenum
Irritation of the duodenal mucosa
Degree of acidity of the duodenal chyme
Degree of osmolality of the chyme
Presence of breakdown products of proteins
and fats in the chyme

14. Ingestion of Food
Powerful Duodenal Factors that Inhibit Stomach
Emptying
Hormonal feedback from the duodenum inhibits
gastric emptying
Fats
CCK
GIP
Glucose-dependent insulinotropic peptide

15. Movements of the Small Intestine
Major Contractions (Segmentation Contractions)
Fig Segmentation movements of the small intestine

16. Movements of the Small Intestine
Major Contractions (Segmentation Contractions)
Contractions “chop” the chyme 2-3 per minute but
can be as high as 12 per minute
Frequency is determined by the electrical slow
waves in the intestinal wall

17. Movements of the Small Intestine
Propulsive Movements
Peristalsis in the small intestine-normally weak and
die out after 3-5 cm, rarely farther than 10 cm
Control of peristalsis by nervous and hormonal
agents (i.e. gastrin, CCK, insulin, motilin, serotonin).
Conversely, secretin and glucagon inhibit small
intestine motility
Function is to cause progression and the spreading out
of the chyme

18. Movements of the Small Intestine
Propulsive Effect of the Segmentation Movements
Peristaltic rush- intense irritation of the mucosa, as
occurs in infectious diarrhea, can cause rapid and
powerful peristalsis
Function of the Ileocecal Valve-prevent backflow
of cecal contents

19. Movements of the Small Intestine
Fig Emptying of the ileocecal valve

20. Movements of the Colon
Mixing Movements-Haustrations
Propulsive Movements-Mass Movements
Initiation of Mass Movements by Gastrocolic and
Duodenocolic Reflexes

21. Movements of the Colon
Fig Absorptive and storage functions of the large intestine

22. Defecation and Defecation Reflexes
Movements of the Colon
Defecation and Defecation Reflexes
Fig Afferent and efferent pathways of the parasympathetic
mechanism for enhancing the defecation reflex