Chapter 63: Propulsion and Mixing Of Food in the Alimentary Tract

Guyton and Hall, Textbook of Medical Physiology, 12 th edition

Ingestion of Food

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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

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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

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Swallowing (Deglutition)

Fig. 63.1 Swallowing mechanism

Ingestion of Food

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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

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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

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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

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Storage Function of the Stomach

a. 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

Ingestion of Food

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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

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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

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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

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Powerful Duodenal Factors that Inhibit Stomach Emptying

b. Types of factors that can initiate enterogastric inhibitory reflexes: 1. Degree of distension of the duodenum 2. Irritation of the duodenal mucosa 3. Degree of acidity of the duodenal chyme 4. Degree of osmolality of the chyme 5. Presence of breakdown products of proteins and fats in the chyme

Ingestion of Food

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Powerful Duodenal Factors that Inhibit Stomach Emptying

c. Hormonal feedback from the duodenum inhibits gastric emptying 1. Fats 2. CCK 3. GIP 4. Glucose-dependent insulinotropic peptide

Movements of the Small Intestine

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Major Contractions (Segmentation Contractions)

Fig. 63.3 Segmentation movements of the small intestine

Movements of the Small Intestine

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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

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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

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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

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Mixing Movements-Haustrations

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Propulsive Movements-Mass Movements

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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

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Defecation and Defecation Reflexes

Fig. 63.6 Afferent and efferent pathways of the parasympathetic mechanism for enhancing the defecation reflex