Ch23 pt2

Hormones Secreted by the Stomach

Hormonal Functions

Gastrin

• Production Site: Stomach mucosa, mainly G cells of the pyloric antrum.

• Production Stimulus: Presence of peptides and amino acids in the stomach.

• Target Organs & Actions:

  • Stomach: Increases secretion by gastric glands; promotes gastric emptying.

  • Small Intestine: Promotes intestinal muscle contraction.

  • Ileocecal Valve: Relaxes valve.

  • Large Intestine: Triggers mass movements.

Ghrelin

• Production Site: Stomach mucosa, mainly fundus.

• Production Stimulus: Fasting state (levels increase just prior to meals).

• Function: Regulates food intake by stimulating appetite and promoting fat storage.

Histamine

• Production Site: Stomach mucosa.

• Production Stimulus: Presence of food in the stomach.

• Target Organ: Hypothalamus.

• Function: Stimulates parietal cells to release HCl, enhancing the acidic environment necessary for digestion and pathogen defense.

Serotonin

• Production Site: Stomach mucosa.

• Production Stimulus: Presence of food in the stomach.

• Target Organ: Stomach.

• Function: Contracts stomach muscle to aid in the mechanical breakdown of food and regulate gastric motility.

Somatostatin

• Production Site: Mucosa of stomach, especially pyloric antrum; also found in the duodenum.

• Production Stimulus: Presence of food in the stomach; sympathetic axon stimulation.

• Target Organs & Actions:

  • Stomach: Restricts all gastric secretions, gastric motility, and emptying, acting as a brake to prevent excessive gastric activity.

  • Pancreas: Restricts pancreatic secretions to moderate digestive enzyme activity.

  • Small Intestine: Reduces intestinal absorption by decreasing blood flow, thus regulating nutrient uptake.

Gastric Secretion Control

• Gastric juice secretion is finely controlled by nerves and hormones, activated by stimuli from the brain, stomach, and small intestine.

• Three Phases of Gastric Secretion: Cephalic, gastric, intestinal.

  • All three phases can occur simultaneously after secretion begins, ensuring an efficient digestive process.

Phases of Gastric Secretion

Cephalic Phase

• Triggered by the sight, thought, taste, or smell of food.

• Involves the cerebral cortex and vagus nerve; stimulates gastric secretory activity.

• Loss of appetite or depression can inhibit this reflex, affecting digestion.

Gastric Phase

• Lasts 3-4 hours after food enters the stomach.

• Triggered by local neural and hormonal mechanisms from food entry causing stomach distension: Stretch receptors activate.

• G cells secrete gastrin in response to food chemicals and rising pH.

• Excessive acidity (< pH 2) leads to decreased gastrin secretion.

• Release of gastric acids and enzymes enhances digestion.

Intestinal Phase

• Involves both excitatory and inhibitory signals to modulate gastric activity.

• Triggered by the presence of food in the duodenum.

• Initial gastrin release stimulates gastric secretion; however, distension of the duodenum inhibits further gastric activity to prevent overload.

• Local reflexes and intestinal hormones (like secretin and CCK) play a critical role in regulating this phase.

Mucosal Barrier and Ulcers

Mucosal Barrier Functions

• Protects the stomach lining from self-digestion due to its own gastric juices.

• Composed of:

  • Bicarbonate-rich mucus forming a physical barrier.

  • Tight junctions preventing gastric juice penetration into the epithelial cells.

  • Rapid replacement of epithelial mucosal cells through stem cells, maintaining integrity against wear.

Ulcers

• Occur when the mucosal barrier breaks down, leading to erosions and deeper ulcers within the gastric wall.

• Commonly caused by excessive NSAID intake or Helicobacter pylori infection, compromising the mucosal barrier.

• Symptoms include burning pain and indigestion; treatable with antacids and antibiotics.

• Complications like perforated ulcers pose serious risks and may require surgical intervention.

Digestive Functions of the Stomach

• Participates in nearly all digestive functions except ingestion and defecation.

• Absorbs some drugs, notably alcohol and aspirin, directly through the gastric mucosa.

Mechanical Digestion

• Occurs through peristalsis and mixing waves within the stomach (approximately every 20 seconds).

• Formation of chyme is achieved through the thorough mixing of ingested food with gastric juices.

• Pylorus filters chyme, permitting only small particles to enter the duodenum, thus regulating gastric emptying.

Chemical Digestion

• Activates enzymes, primarily pepsin, to effectively begin the process of protein digestion.

• Production of intrinsic factor is crucial for vitamin B12 absorption, linking stomach function with overall nutrient status.

Small Intestine and Large Intestine Overview

Small Intestine

• Main site for digestion and absorption, approximately 3-5 meters long.

• Divided into three regions:

  • Duodenum: receives bile and pancreatic juices.

  • Jejunum: primary site for nutrient absorption.

  • Ileum: absorbs vitamin B12 and bile acids.

• Adaptations for absorption:

  • Circular folds, villi, and microvilli significantly increase the surface area available for nutrient uptake.

Large Intestine

• Structure composed of four regions: cecum, colon, rectum, anus.

• Main functions include absorbing water, synthesizing vitamins (like vitamin K), and forming feces.

• Unique features include teniae coli (longitudinal muscle bands), haustra (pouches), and epiploic appendages (fat-filled pouches).

Accessory Organs in Digestion

Liver

• Produces bile, which is essential for lipid digestion and emulsification; also plays a critical role in nutrient processing and metabolism.

Pancreas

• Produces pancreatic juice rich in bicarbonate and digestive enzymes needed for the digestion of all nutrient types (carbohydrates, proteins, and fats).

• Involves in regulating blood glucose levels through insulin and glucagon production.

Gallbladder

• Stores and concentrates bile produced by the liver; releases bile into the small intestine as needed to aid in fat digestion.