Gastrointestinal Physiology Quiz

Digestive System - The Stomach

• The stomach is a J-shaped, sac-like organ located between the esophagus and small intestine.
• It's divided into three sections: fundus, body, and antrum.
• The stomach has three main functions:
  • Storing ingested food until it can be emptied into the small intestine.
  • Secreting hydrochloric acid (HCl) and enzymes that begin protein digestion.
  • Mixing movements that convert pulverized food into chyme.
• The pyloric sphincter is a crucial barrier between the stomach and the upper part of the small intestine.

Stomach Anatomy

• Esophagus: The tube connecting the mouth to the stomach.
• Gastroesophageal sphincter: Controls the passage of food from the esophagus to the stomach.
• Body: Main part of the stomach.
• Fundus: The upper, rounded part of the stomach.
• Antrum: The lower portion of the stomach, leading to the pyloric sphincter.
• Pyloric sphincter: Connects the stomach to the duodenum, regulating the release of chyme.
• Pyloric gland area (PGA): The portion of the stomach that secretes gastric juice.
• Oxyntic mucosa: Secretes stomach acid and digestive enzymes (chief and parietal cells); located in the body and fundus.
• Smooth muscle: The muscular layer in the stomach wall responsible for mixing and emptying.
• Gastric folds (rugae): Folds in the stomach mucosa that allow the stomach to expand.
• Duodenum: The first part of the small intestine, following the stomach.

Stomach Mucosa

• The stomach lining is composed of a simple columnar epithelium.
• Gastric pits: Indentations in the stomach lining.
• Gastric glands: Located at the base of the gastric pits.
• Surface mucous cells: Secrete mucus to protect the stomach lining from the acidic digestive juices.
• Mucous neck cells: Secrete mucus, but their mucus is alkaline.
• Parietal cells: Secrete hydrochloric acid (HCl) and intrinsic factor.
• Chief cells: Secrete pepsinogen, an inactive enzyme precursor converted to pepsin in the presence of HCl.
• Enteroendocrine cells: Secrete hormones like gastrin. Different glands contain different populations of these cells (e.g. cardiac glands, pyloric glands, oxyntic glands)

Gastric Juice Contents

• HCl
• Pepsinogen
• Electrolytes
• Intrinsic factor
• Mucus (mucus gel layer)

Gastric Mucosal Barrier

• Protects the stomach lining from HCl damage.
• The luminal membranes of the gastric cells are impermeable to H+.
• The cells are tightly joined by junctions.
• A mucus coating protects the mucosa.
• Prevents the acid from damaging the stomach.

Gastric Motility

• Functions:
  • Serve as a reservoir
  • Breaks food into smaller particles and mixes it with gastric juice
  • Empties gastric contents at a controlled rate
• Four aspects:
  • Filling (involves receptive relaxation – fundus)
  • Storage
  • Mixing (takes place in the antrum)
  • Emptying (largely controlled by factors in the duodenum)
• Different regions have different types of contractions (tonic or phasic).
• The reservoir part (fundus and upper body) has tonic contractions, while the antral pump ( lower body and antrum) has phasic contractions.

Factors Influencing Gastric Emptying (from duodenum perspective)

• Fat: Delay emptying due to fat digestion/absorption being primarily in the duodenum.
• Acid: Unneutralized acid in the duodenum inhibits further emptying.
• Hypertonicity: High osmolarity in the duodenum inhibits emptying.
• Distension: Too much chyme in the duodenum inhibits further emptying.

Gastric Emptying and Mixing

• Gastric contents can remain unmixed for about one hour in the stomach. Liquids empty faster than solids, and fats take longer to empty than liquids.
• Mixing happens in the antrum via retropulsion (tossing chyme back into the stomach for further mixing).
• A peristaltic contraction from the fundus pushes chyme toward the opening of the pyloric sphincter and into the duodenum.
• A portion of the chyme can get tossed back into the stomach if the pyloric sphincter is closed.

Regulation of Gastric Acid Secretion (stomach perspective)

• Gastrin: Released by G cells in response to food presence & stimulates parietal cell & ECL secretion.
• Histamine: Released by ECL cells, stimulates parietal cells.
• ACh (acetylcholine): Stimulates parietal cells via vagal nerves (Cephalic and Gastric phase) and potentially enteric nerves, enhances HCl secretion.

Control of Pyloric Sphincter

• Hormones (CCK, Secretin, Gastrin, GIP) promote constriction.
• Sympathetic innervation.

Phases of Gastric Secretion

• Cephalic phase: Increased secretion of acid and pepsinogen before food reaches the stomach (e.g. smell, taste conditioning).
• Gastric phase: Food reaching the stomach triggers secretion of gastric juices, particularly in response to protein presence.
• Intestinal phase: Helps shut off gastric juice flow once chyme moves into small intestine.

Disorders of Gastric Emptying

• Gastroparesis: Delayed emptying of the stomach.

Types Of Cells Of The Stomach

• Parietal cells: Secrete HCl, intrinsic factor
• Chief cells: Secrete pepsinogen
• Mucus neck cells: Secrete mucus
• G Cells: Gastrin
• D Cells: Somatostatin
• ECL cells: Histamine

Additional factors

• Emotions (sadness, fear decrease gastric motility, anger, aggression increase, intense pain inhibits motility)
• Presence of food & acid in the duodenum delays gastric emptying

Conclusion

• This provides an overview of the stomach's structure, function, and regulation. Further study into specific mechanisms and regulatory pathways is recommended.

Test your knowledge on gastric motility and its phases with this quiz. Explore concepts such as receptive relaxation, gastric emptying regulation, and the stomach's role as a reservoir. Perfect for students in physiology or medical courses.