Regulation of Digestion and Hormonal Control

Chapter 24: The Digestive System (Part 9)

Regulation of Digestion

• For digestion to occur, enzymes must be present in the right place at the right time.

• Enzymes should not be released until food is present.

• The release of digestive secretions is orchestrated by:

  • Nerves: Control quick or anticipatory processes.

  • Hormones: Control processes that take more time.

Regulation of Pancreatic and Gallbladder Secretions

• Acidic chyme from the stomach stimulates S cells in the duodenal mucosa to release the hormone secretin.

  • Definition: Secretin is sometimes referred to as "nature’s antacid."

  • Question: Does pH go up or down when secretin is released?

Cholecystokinin (CCK)

• Fatty acids (FA’s) and triglycerides in chyme stimulate CCK cells (enteroendocrine cells) in the duodenal mucosa to secrete cholecystokinin (CCK).

  • The process is as follows:

  1. Chyme containing fatty acids and triglycerides enters the duodenum.

  2. The intestinal mucosa releases CCK into the bloodstream.

  3. CCK stimulates the pancreas.

  4. The pancreas secretes juice rich in lipase.

  5. CCK stimulates the gallbladder to expel bile and relaxes muscle to open the hepatopancreatic sphincter.

  • Note: Bile breaks lipid clusters into smaller lipid droplets to facilitate the action of lipase.

Gastrin

• Functions of Gastrin:

  1. Stimulates the secretion of large amounts of gastric juice.

  2. Strengthens the contraction of the lower esophageal sphincter.

  3. Secretion is stimulated by low pH and inhibited by high pH.

  • Answer Options for a Question:

    • a) 1 only

    • b) 2 only

    • c) 3 only

    • d) 1 and 2 are correct

    • e) 1, 2, and 3 are correct

Major Hormones That Control Digestion

• TABLE 24.8: Major Hormones That Control Digestion

  • Hormones:

  • Gastrin

    • Stimulus and Site of Secretion: Distension of the stomach, partially digested proteins, caffeine, and high pH of stomach chyme stimulate gastrin secretion by enteroendocrine G cells in the pyloric antrum of the stomach.

    • Actions:

    • Major: Promotes secretion of gastric juice, increases gastric motility, promotes growth of gastric mucosa.

    • Minor: Constricts lower esophageal sphincter, relaxes pyloric sphincter.

  • Secretin

    • Stimulus and Site of Secretion: Acidic (high H+ level) chyme that enters the small intestine stimulates secretion of secretin by enteroendocrine S cells in the duodenum.

    • Actions:

    • Major: Stimulates secretion of pancreatic juice and bile rich in HCO₃ (bicarbonate ions).

    • Minor: Inhibits secretion of gastric juice, promotes growth and maintenance of the pancreas, enhances effects of CCK.

  • Cholecystokinin (CCK)

    • Stimulus and Site of Secretion: Partially digested proteins (amino acids), triglycerides, and fatty acids that enter the small intestine stimulate secretion of CCK by enteroendocrine CCK cells in the mucosa of the small intestine and the brain.

    • Actions:

    • Major: Stimulates secretion of pancreatic juice rich in digestive enzymes, causes ejection of bile from the gallbladder, opens the sphincter of the hepatopancreatic ampulla (sphincter of Oddi), induces satiety.

    • Minor: Inhibits gastric emptying, promotes growth and maintenance of the pancreas, enhances effects of secretin.

Phases of Digestion

Cephalic Phase

• Definition: This phase involves receptors for smell, sight, thought, or initial taste of food activating neural centers, preparing the mouth and stomach for food.

• Outputs: Stimulates gastric secretions and gastric motility.

Gastric Phase

• Receptors: Chemoreceptors and stretch receptors in the stomach detect an increase in pH and distension.

• Input: Neural and hormonal mechanisms regulate the pH of gastric juice and gastric motility.

• Control Center: Submucosal plexus.

• Output: Nerve impulses (parasympathetic).

• Effectors:

  • Parietal cells (secrete gastric acid).

  • Smooth muscle in the stomach wall (involved in peristalsis).

• Homeostasis: Achieved when pH of gastric juice and distension of stomach walls return to normal (pre-eating status).

Intestinal Phase

• Definition: Begins when food enters the small intestine.

• Effects:

  • Slows the exit of chyme from the stomach.

  • Stimulates the flow of bile and pancreatic juice.

Summary of Control of Gastric Motility

Absorptive State

• Definition: The period of time when cell metabolism is primarily fueled by nutrients derived from the most recent meal.

• Characteristics:

  • Anabolic processes exceed catabolic processes.

  • Glycogen and fat are stored in the liver and muscles, and adipose tissue, respectively.

Postabsorptive State

• Definition: The period of time when cell metabolism is primarily fueled by nutrients stored from previous meals.

• Characteristics:

  • Synthesis of glycogen, fat, and protein ceases.

  • Glycogenolysis: Hydrolysis of glycogen and liver stores.

  • Lipolysis: Catabolism of triglycerides.