Guyton Ch. 64 Propulsion and Mixing of Food in the Alimentary Tract

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What controls the timing of mixing and propulsion activities in the alimentary tract?

Automatic nervous mechanisms

Which chapter discusses the mechanisms of food ingestion, especially mastication and swallowing?

Chapter 64

What type of teeth provide a strong cutting action for chewing?

Incisors

What determines the amount of food that a person ingests?

Hunger

Which teeth provide a grinding action for chewing?

Molars

What is discussed in Chapter 72 in relation to the body's nutrition?

Mechanisms for maintaining an adequate nutritional supply

What is the primary function of the intense peristaltic contractions in the stomach during stomach emptying?

Forcing up to several milliliters of chyme into the duodenum

What is the role of the pyloric sphincter in stomach emptying?

To prevent passage of food particles until they become almost fluid consistency

Which factor promotes increased emptying from the stomach?

Stretching of the stomach wall

Which factor can initiate enterogastric inhibitory reflexes from the duodenum?

Presence of breakdown products of fats in the chyme

What is the effect of the hormone gastrin on stomach emptying?

Promotes stomach emptying

What happens if the pH of the chyme in the duodenum falls below about 3.5 to 4?

Stops further release of acidic stomach contents into the duodenum

What is the primary effect of enterogastric inhibitory reflexes?

Inhibit stomach emptying

What are the types of factors that are continually monitored in the duodenum to initiate enterogastric inhibitory reflexes?

Distention of the duodenum, presence of irritation of the duodenal mucosa, and acidity of the chyme

What is the role of the intense peristaltic contractions in causing stomach emptying?

Gradually pinching off food in the body of the stomach

What effect does increased food volume in the stomach have on stomach emptying?

It elicits local myenteric reflexes that accentuate activity

What is a significant effect of stretching of the stomach wall on stomach emptying?

It accentuates activity of the pyloric pump

Which factor elicits inhibitory enterogastric reflexes to slow or stop stomach emptying?

Breakdown products of fats

Which cranial nerve innervates most of the muscles of chewing?

Fifth cranial nerve

What force can all the jaw muscles working together close the molars with?

200 pounds

What initiates reflex inhibition of the muscles of mastication during chewing?

Presence of a bolus of food in the mouth

Why is chewing especially important for most fruits and raw vegetables?

They have indigestible cellulose membranes around their nutrient portions

What is the principal reason that grinding the food to a very fine particulate consistency aids digestion?

Increases surface area exposed to digestive secretions

What initiates a series of automatic pharyngeal muscle contractions during the involuntary pharyngeal stage of swallowing?

Bolus of food entering the posterior mouth and pharynx

What is the main role of the pharynx during swallowing?

Subserves respiration and swallowing

What prevents reflux of food into the nasal cavities during the involuntary pharyngeal stage of swallowing?

Soft palate is pulled upward to close the posterior nares

What is the function of the valvelike mechanism of the lower esophagus?

To prevent high intra-abdominal pressure from pushing stomach contents into the esophagus

What is the role of vagovagal reflex in the stomach?

To reduce the tone in the muscular wall of the stomach

What initiates the mixing waves in the stomach?

Basic electrical rhythm of the stomach wall

What is the main role of the peristaltic constrictor rings in the antrum of the stomach?

To squeeze antral contents upstream through retropulsion

What determines the degree of fluidity of chyme leaving the stomach?

The degree of digestion that has occurred

What causes hunger pangs in individuals after prolonged fasting?

Decreased levels of blood sugar

What happens when hunger contractions become extremely strong in the body of the stomach?

They fuse to cause a continuing tetanic contraction

How are hunger contractions affected by gastrointestinal tonus?

They are most intense in individuals with high gastrointestinal tonus

What promotes stomach emptying?

Intense peristaltic contractions in the antrum

What is the main role of cholecystokinin (CCK) in inhibiting gastric emptying?

Inhibiting the pyloric pump and increasing the strength of contraction of the pyloric sphincter

What stimulates the release of inhibitory hormones from the upper intestine?

Fats entering the duodenum

Which hormone has a general but weak effect of decreasing gastrointestinal motility?

Gastric inhibitory peptide (GIP)

What are the two main conditions under which the rate of stomach emptying is slowed by inhibitory feedback signals from the duodenum?

Too much chyme in the small intestine and excessively acidic chyme

What is the main effect of gastric inhibitory peptide (GIP) at physiological concentrations?

Stimulates secretion of insulin by the pancreas

What is the main role of secretin in relation to stomach emptying?

Inhibiting gastric emptying

What is one of the factors that can limit the rate of stomach emptying?

Inhibitory feedback signals from the duodenum

What is the primary function of the esophagus?

To conduct food rapidly from the pharynx to the stomach

Where are the most sensitive tactile areas for initiating the pharyngeal stage of swallowing located?

Around the pharyngeal opening, with greatest sensitivity on the tonsillar pillars

Which nerves transmit impulses from the sensitive areas of the mouth and pharynx to the medulla oblongata?

Trigeminal and glossopharyngeal nerves

What controls the orderly sequence of swallowing reflex?

Reticular substance of the medulla and lower portion of the pons

What term is used to collectively describe the areas in the medulla and lower pons that control swallowing?

Deglutition center

Which type of peristalsis results from distention of the esophagus by retained food?

Secondary peristalsis

What causes initiation of secondary peristaltic waves in the esophagus?

Reflexes initiated by vagal afferent fibers to the esophagus

Which nerves control peristaltic waves in the upper third of the esophagus?

Glossopharyngeal and vagus nerves

What happens when vagus nerves to the esophagus are cut?

The myenteric nerve plexus becomes excitable enough to cause strong secondary peristaltic waves without support from vagal reflexes

What is receptive relaxation of the stomach?

'Digestive relaxation' that prepares the stomach for receiving food from the esophagus

What is another name for the lower esophageal sphincter?

'Cardiac sphincter'

What is the main role of segmentation contractions in the small intestine?

To divide the intestine into spaced segments

What determines the maximum frequency of segmentation contractions in the small intestine?

Frequency of electrical slow waves in the intestinal wall

Under what conditions does the maximum frequency of the segmentation contractions occur?

When extreme stimulation is exerted on the intestinal wall

What is the role of electrical slow waves in the intestinal wall?

To determine the frequency of segmentation contractions

What effect does stretching of the intestinal wall have on segmentation contractions?

Initiates a new set of segmentation contractions

How do segmentation contractions contribute to food mixing with secretions in the small intestine?

By dividing the intestine into spaced segments

What is the velocity of peristaltic waves as they move toward the anus in the small intestine?

0.5 to 2.0 cm/sec

What is the main cause of the increased peristaltic activity in the small intestine after a meal?

Beginning entry of chyme into the duodenum

What is the role of serotonin in relation to small intestinal motility?

Enhances intestinal motility

What is the primary function of the peristaltic waves in the small intestine?

Cause progression of chyme toward the ileocecal valve and spread out the chyme along the intestinal mucosa

What initiates intense peristaltic contractions called 'peristaltic rush' in the small intestine?

Intense irritation of the intestinal mucosa

What is the role of muscularis mucosae in the small intestine?

Cause short folds to appear in the intestinal mucosa and increase absorption

What is the function of the ileocecal valve?

Prevent backflow from the colon to the small intestine

What is the resistance capability of the ileocecal valve against reverse pressure?

>50 to 60 centimeters of water

What causes resistance to emptying at the ileocecal valve?

Ileocecal sphincter constriction

What is the typical rate of travel for net movement along the small intestine?

~1 cm/min

'Peristaltic rush' is initiated partly by intrinsic enhancement of which reflexes?

(Myenteric plexus) reflexes in the gut wall

What is a significant effect of stretching of the stomach wall on stomach emptying?

Delay stomach emptying

What prevents continual dribble of fecal matter through the anus?

Conscious signals inhibiting the external sphincter constriction

What converts the intrinsic myenteric defecation reflex into a powerful process of defecation?

Parasympathetic defecation reflex involving the sacral segments of the spinal cord

What causes automatic emptying of the lower bowel in newborns and some people with transected spinal cords?

Lack of conscious control exercised through voluntary contraction or relaxation of the external anal sphincter

What is the role of the peritoneointestinal reflex?

Strongly inhibits excitatory enteric nerves, causing intestinal paralysis

Which reflex inhibits intestinal activity as a result of kidney irritation?

Renointestinal reflex

What initiates enterogastric inhibitory reflexes to slow or stop stomach emptying?

Stretching of the stomach wall

What is one of the functions of vagovagal reflex in the stomach?

Inhibition of excitatory enteric nerves

What is one effect of increased food volume in the stomach on stomach emptying?

Slowing down of stomach emptying

What is the role of the cecum in controlling the ileocecal sphincter and ileal peristalsis?

Intensifying contraction of the ileocecal sphincter

Which factor promotes increased emptying from the stomach?

Stimulation of intense peristaltic contractions

How are reflexes from the cecum to the ileocecal sphincter and ileum mediated?

By both the myenteric plexus and extrinsic autonomic nerves

What happens when vagus nerves to the esophagus are cut?

Loss of involuntary control over swallowing

What characterizes the movements of the colon during absorption and storage functions?

Slow and sluggish movements

What is the main effect of gastric inhibitory peptide (GIP) at physiological concentrations?

Slowing down stomach emptying

What can be observed during mixing movements in the large intestine?

Baglike sacs called haustrations due to combined circular and longitudinal muscle contractions

What is one function of peristaltic constrictor rings in the antrum of the stomach?

Initiation of intense peristaltic contractions in the stomach

What is a characteristic of propulsive movements in the colon?

A series of mass movements persisting for 60-90 minutes

What initiates mass movements in the colon after meals?

Distention of the stomach and duodenum

What contributes to maintaining an empty rectum most of the time?

  • Presence of a weak functional sphincter near the sigmoid colon-rectum juncture

What effect does a mass movement have in causing the desire for defecation?

  • Forcing feces into the rectum

What stimulates mass movements after meals?

Constriction at points distended or irritated in the colon

What contributes to progressive absorption of fluid and dissolved substances in the large intestine?

Haustral contractions rolling over fecal material

What is a key characteristic of propulsive movements known as mass movements?

Contraction force developing progressively for about 30 seconds

What leads to defecation when a mass movement forces feces into the rectum?

Relaxation of anal sphincters in response to rectal distention

Test your knowledge on the valvelike closure mechanism of the lower esophageal sphincter and the factors preventing reflux of stomach contents into the esophagus. Explore how increased intra-abdominal pressure affects the esophagus to prevent reflux.

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