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Ch. 23 The Digestive System
Marieb - Human Anatomy & Physiology 11th Edition
Marieb, Hoehn11th EditionHuman Anatomy & PhysiologyISBN: 9780136874034Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoMarieb, Hoehn 11th EditionCh. 23 The Digestive SystemProblema 23
Capitolo 23, Problema 23

Describe the regulation of the cephalic, gastric, and intestinal phases of gastric secretion.

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Begin by understanding that gastric secretion is regulated in three overlapping phases: the cephalic, gastric, and intestinal phases, each triggered by different stimuli and involving neural and hormonal controls.
For the cephalic phase, explain that it is initiated by the sight, smell, taste, or thought of food, which activates the brain (specifically the hypothalamus and medulla oblongata). This leads to parasympathetic stimulation via the vagus nerve, promoting secretion of gastric juices even before food enters the stomach.
In the gastric phase, describe how the presence of food in the stomach stimulates gastric secretion. This occurs through two main mechanisms: distension of the stomach wall activating stretch receptors and chemical stimuli (like peptides and amino acids) activating chemoreceptors. These signals enhance secretion of gastric acid (HCl) and enzymes, primarily through local reflexes and release of the hormone gastrin from G cells.
For the intestinal phase, explain that it begins when chyme enters the small intestine. Initially, this phase can stimulate gastric secretion via intestinal gastrin release, but predominantly it inhibits gastric secretion to regulate the rate of stomach emptying. This inhibition is mediated by hormones such as secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP), as well as neural reflexes that reduce gastric motility and secretion.
Summarize by noting that these phases work together to optimize digestion: the cephalic phase prepares the stomach, the gastric phase maximizes secretion in response to food presence, and the intestinal phase modulates secretion to coordinate digestion and absorption in the intestines.

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Cephalic Phase of Gastric Secretion

The cephalic phase is the earliest phase of gastric secretion, triggered by the sight, smell, taste, or thought of food. It involves neural mechanisms, primarily the vagus nerve, which stimulates gastric glands to secrete acid and enzymes in preparation for digestion.
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Gastric Phase of Gastric Secretion

The gastric phase begins when food enters the stomach, causing distension and chemical stimulation. This phase involves both neural reflexes and hormonal signals, such as gastrin release, which enhance acid and enzyme secretion to aid in food breakdown.
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Intestinal Phase of Gastric Secretion

The intestinal phase occurs when chyme enters the small intestine, regulating gastric secretion through inhibitory signals. Hormones like secretin and cholecystokinin reduce acid production to prevent excessive acidity and coordinate digestion and nutrient absorption.
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Small Intestine Example 3
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