Stomach Functions and Gastric Processes

Stomach Functions

• Stomach volume can change significantly from 50 ml when empty to 1500 ml when full
• Gastric folds, known as rugae, flatten with food intake and stomach relaxation
• Receptive relaxation: reflex relaxation triggered by eating and mediated by the vagus nerve
• Stomach's ability to accommodate large volume changes is essential for digestion

Gastric Filling & Receptive Relaxation

• Receptive relaxation is a reflex response that allows the stomach to expand to accommodate food intake
• This relaxation is triggered by the presence of food in the mouth and esophagus
• It is mediated by the vagus nerve, which carries signals from the brain to the stomach muscles
• This reflex allows for a gradual and comfortable filling of the stomach, preventing sudden distension

Gastric Emptying

• Rate of gastric emptying is influenced by the factors in both the stomach and the duodenum.
• Factors in stomach:
  • Amount of chyme: Rate of stomach emptying is proportional to the volume of chyme present
  • Stomach distension: Leads to increased gastric motility, directly stimulating smooth muscle and involving intrinsic plexuses and the vagus nerve, and via gastrin release.
  • Chyme fluidity:
    • Higher fluidity results in faster emptying
    • Lower fluidity results in slower emptying
• Emptying of gastric contents into the duodenum can take several hours

Factors Affecting Gastric Emptying: Duodenal Influences

• Fat in the duodenum: Inhibits gastric emptying through the release of cholecystokinin (CCK) by I cells
• Acid in the duodenum:
  • Inhibits gastric emptying through reflexes in the enteric nervous system.
  • Stomach secretes HCl, creating acidic chyme, neutralized by NaHCO3 from the pancreas
  • Unneutralized H+ in the duodenum inhibits gastric emptying, lasting until neutralization is complete
  • Unneutralized H+ also inactivates pancreatic enzymes
• Hypertonicity in the duodenum:
  • Occurs if absorption rate of amino acids and glucose lags behind digestion
  • Causes an increase in osmolarity in the duodenum, pulling water from the surrounding tissues
  • Reflexly inhibits gastric emptying to allow time for the duodenum to catch up
• Distension of the duodenum:
  • Inhibits gastric emptying
  • Allows the duodenum time to process the chyme

Gastric Digestive Juice

• Gastric juice is secreted by cells in the gastric mucosa.
• Two main areas of secretion:
  • Oxyntic mucosa: Found in the body and fundus of the stomach.
  • Pyloric gland area: Located in the antrum.
• Gastric pits are lined with gastric glands.
  • Gastric pits are openings in the surface of the gastric mucosa.
  • Gastric glands are found at the base of gastric pits and contain various cell types.

Gastric Secretory Cells

• Oxyntic glands: Found in the body and fundus
  • Surface epithelial cells: Cover the gastric mucosa.
  • Mucous cells: Line the gastric pits and entrance of glands, secrete thin, watery mucus.
  • Chief cells: Line the deeper parts of glands, secrete pepsinogen.
  • Parietal cells: Line deeper parts of glands, secrete HCl.
  • ECL cells: Located in gastric glands, secrete histamine.
• Pyloric gland area: Located in the antrum
  • G cells: Secrete gastrin in response to eating

Functions of Gastric Secretory Cells:

• Gastrin: A hormone produced by G cells in response to eating.
  • Stimulates parietal cells to secrete H+, and promotes gastric growth
• HCl: Secreted by parietal cells, plays a crucial role in:
  • Activating pepsinogen: Convert pepsinogen into active pepsin for protein digestion.
  • Killing bacteria: Helps destroy harmful microorganisms present in consumed food.
  • Breaking down food: Helps mechanically breakdown food and mixes it with gastric juices.
• Pepsinogen: Secreted by chief cells.
  • Inactive form of pepsin, activated by HCl in the stomach.
  • Starts the digestion of proteins.

Control of Gastric Secretion:

• Cephalic phase:
  • Initiated by the sight, smell, taste, or thought of food.
  • Signals are sent from the brain via the vagus nerve to stimulate gastric secretion.
• Gastric phase:
  • Triggered by the presence of food in the stomach.
  • Distention of the stomach walls and the presence of peptides, amino acids, and caffeine encourage gastrin release, promoting gastric secretion.
• Intestinal phase:
  • Occurs when chyme enters the duodenum.
  • The presence of fats, amino acids, and acid in the duodenum elicits the release of hormones and reflexes that inhibit gastric secretion.

Factors Decreasing Gastric Secretion:

• Food moving from the stomach to the duodenum: Removes the major stimulus for gastric secretion
• Accumulation of gastric juices: Decreases gastric pH, triggering somatostatin release from D cells in the antrum, inhibits gastric secretion
• Fats, acid, and duodenal distension: These influences also decrease gastric secretion.

Mucus: A Protective Layer

• Mucus secreted by surface epithelial cells and mucus cells forms a protective layer on the gastric mucosa.
• Functions of mucus:
  • Protection against mechanical injury: Covers the surface of the gastric mucosa.
  • Protection against self-digestion: Prevents pepsin from digesting the stomach wall.
  • Protection against acid injury: Neutralizes HCl near the gastric lining, preventing damage.

Peptic Ulcers

• Breakdowns in the gastric mucosal barrier can lead to peptic ulcers.
• Causes:
  • Helicobacter pylori: A bacterium responsible for >80% of peptic ulcers.
  • Reflux in the esophagus: Excessive acidic gastric contents back up into the esophagus.
  • Dumping of excessive acidic gastric contents into the duodenum: Can damage the duodenal lining.
  • Contributing Factors:
    • Ethyl alcohol
    • Nonsteroidal anti-inflammatory drugs (NSAIDs)
    • Stress
    • Smoking

True/False Statements:

• The stomach can accommodate an increased volume: True. Receptive relaxation allows for rugae to flatten, enabling this increase without pressure change.
• The presence of acid in the duodenum speeds up gastric emptying: False. Acid slows down gastric emptying to allow time for the duodenum to neutralize the acid.
• Gastrin is the only agonist to stimulate parietal cell H+ secretion: False. Histamine, gastrin, and acetylcholine all stimulate H+ secretion.
• Gastric contractions that propel chyme against a closed pyloric sphincter result in retropulsion: True. This action helps mix chyme and break down food into smaller particles.
• Pepsinogen is in its inactive form when secreted: True. Pepsinogen needs to be converted to active pepsin by HCl.
• Most gastric secretion occurs during the intestinal phase: False. The majority of gastric secretion happens during the gastric phase.
• PUs can be caused by a broken GMB due to excessive NSAID use: True.