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Chapter 63: Propulsion and Mixing
Of Food in the Alimentary Tract
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Ingestion of Food
• Mastication (Chewing)
a. Most of the chewing process is caused by a chewing
reflex
b. Chewing is important for the digestion of all foods,
but especially fruits and raw vegetables (cellulose)
c. 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
Ingestion of Food
• Swallowing (Deglutition)
a. Complicated because the pharynx subserves
respiration and swallowing; important that
respiration not be compromised because of
swallowing
b. Voluntary stage of swallowing-the food is rolled
posteriorly into the pharynx by pressure of the
tongue
Ingestion of Food
• Swallowing (Deglutition)
Fig. 63.1 Swallowing mechanism
Ingestion of Food
• Swallowing (Deglutition)
d. Pharyngeal Stage of Swallowing
1. Soft palate is pulled upward to close the posterior
nares
2. The trachea is closed (epiglottis covers the glottis)
3. Fast peristaltic wave is initiated by the nervous
system
4. Pharynx forces the bolus of food into the
esophagus
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
Ingestion of Food
• Motor Functions of the Stomach
a. Storage of large quantities of food until the food can
be processed in the stomach and intestinal tract
b. Mixing of the food with gastric secretions until it
becomes a semi-fluid mixture called chyme
c. Slow emptying of the chyme into the small intestine
at a rate suitable for proper digestion and absorption
Ingestion of Food
Fig. 63.2 Physiologic anatomy of the stomach
Ingestion of Food
• Storage Function of the Stomach
a. Mixing and propulsion of food in the stomachthe 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
Ingestion of Food
• Storage Function of the Stomach
b. Stomach emptying
1. Intense antral peristaltic contractions during
emptying-pyloric pump
2. Pyloric sphincter- usually open enough for water
and other fluids to empty but not for chyme
until it is the right consistency
Ingestion of Food
• Storage Function of the Stomach
c. Regulation of stomach emptying- rate at which the
stomach empties is dependent on signals from both
the stomach and the duodenum
1. Gastric factors that promote emptying- increased
food volume and some stretching
2. Effect of gastrin- causes secretion of gastric
juice; enhances the activity of the pyloric pump
Ingestion of Food
• Powerful Duodenal Factors that Inhibit Stomach
Emptying
a. Inhibitory effect of enterogastric nervous reflexes
from the duodenum; mediated by three routes:
1. Directly from the duodenum to the stomach
through the enteric nervous system
2. Through extrinsic nerves
3. Through the vagus nerve
All three inhibit the pyloric pump and increase
the tone of the pyloric sphincter
Ingestion of Food
• Powerful Duodenal Factors that Inhibit Stomach
Emptying
b. Types of factors that can initiate enterogastric
inhibitory reflexes:
1.
2.
3.
4.
5.
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
Ingestion of Food
• Powerful Duodenal Factors that Inhibit Stomach
Emptying
c. Hormonal feedback from the duodenum inhibits
gastric emptying
1.
2.
3.
4.
Fats
CCK
GIP
Glucose-dependent insulinotropic peptide
Movements of the Small Intestine
• Major Contractions (Segmentation Contractions)
Fig. 63.3 Segmentation movements of the small intestine
Movements of the Small Intestine
• Major Contractions (Segmentation Contractions)
1. Contractions “chop” the chyme 2-3 per minute but
can be as high as 12 per minute
2. Frequency is determined by the electrical slow
waves in the intestinal wall
Movements of the Small Intestine
• Propulsive Movements
1. Peristalsis in the small intestine-normally weak and
die out after 3-5 cm, rarely farther than 10 cm
2. Control of peristalsis by nervous and hormonal
agents (i.e. gastrin, CCK, insulin, motilin, serotonin).
3. Conversely, secretin and glucagon inhibit small
intestine motility
Function is to cause progression and the spreading out
of the chyme
Movements of the Small Intestine
• Propulsive Effect of the Segmentation Movements
1. 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
Movements of the Small Intestine
Fig. 63.4 Emptying of the ileocecal valve
Movements of the Colon
• Mixing Movements-Haustrations
• Propulsive Movements-Mass Movements
• Initiation of Mass Movements by Gastrocolic and
Duodenocolic Reflexes
Movements of the Colon
Fig. 63.5 Absorptive and storage functions of the large intestine
Movements of the Colon
• Defecation and Defecation Reflexes
Fig. 63.6 Afferent and efferent pathways of the parasympathetic
mechanism for enhancing the defecation reflex