Untitled Quiz

Gastric Digestion

• Gastric digestion involves the breakdown of food in the stomach.
• The stomach's main function is to store, digest, and regulate the emptying of food.
• Phases of gastric function include filling, storage, mixing, and emptying.

Gastric Phase of the Response to a Meal

• The gastric phase is one of the three phases of gastric secretion.
• The gastric phase begins after swallowing and involves multiple factors.
• A critical figure in the study of gastric function is Dr. William Beaumont and his 1825 study of Alexis St. Martin.

Regions of the Stomach

• The stomach is divided into regions with specific functions.
• The "muscular pump" (antrum) is responsible for mixing food with gastric juice.
• The fundus acts as a storage pouch.

Enteroendocrine Cells

• Enteroendocrine cells produce and release hormones that regulate gastric function.
• G cells secrete gastrin, stimulating acid secretion and motility.
• D cells secrete somatostatin to inhibit gastrin and acid secretion.

Stomach as an Endocrine Organ

• The stomach releases numerous hormones that affect the rest of the digestive tract and have other functions beyond digestion.
• Examples of hormones include gastrin, histamine, serotonin, somatostatin, melatonin, ghrelin, obestatin, peptide YY, ANP, leptin, neuropeptide W, nesfatin-1 , and estrogen.

The Stomach

• The stomach stores food, aids digestion via enzymes (HCl, Lipase, pepsin), and protects its walls through mucus and bicarbonate.

Stomach Motility

• Key functions of stomach motility include filling, storage, mixing, and emptying.

Gastric Filling

• Receptive relaxation allows the stomach to expand without a dramatic change in pressure.
• The lower esophageal sphincter (LES) relaxes to allow food entry.

Gastric Storage

• The fundic area acts as a pocket for gas and food that is stored before mixing.
• The mixing process is largely absent during the storage phase.

Gastric Mixing

• The pyloric sphincter plays a role in rhythmic contractions (peristaltic waves) of the antrum, which mix the ingested material with gastric secretions to form chyme.
• Chyme is a mixture of ingested food and gastric secretions that is progressively propelled towards the duodenum.

Gastric Emptying

• Gastric emptying involves the controlled release of chyme into the duodenum and depends on forces such as peristaltic contractions.

Gastric Emptying Regulation

• Variables such as peptides and amino acids, fat, acid, and hypertonicity regulate gastric emptying.
• Mechanisms that control gastric emptying include enterogastric reflex, hormones (e.g., CCK), and osmotic pressure of duodenal contents.

Acid in the Duodenum

• Acid entering the duodenum triggers secretin release.
• Secretin inhibits gastric emptying.
• Secretin stimulates the pancreas to secrete bicarbonate that neutralizes the acid.
• Acid in the duodenum signals the slowing of gastric emptying.

Fat Digestion Products in the Duodenum and Jejunum

• Fat digestion products decrease the rate of gastric emptying through cholecystokinin (CCK) and gastric inhibitory peptide actions.

Osmotic Pressure of Duodenal Contents

• Hyperosmotic duodenal and jejunal content slows gastric emptying by increasing gastric inhibitory responses
• These responses can involve either nervous or hormonal systems for inhibitory actions.

Peptides and Amino Acids in the Duodenum

• Peptides and amino acids in the duodenum trigger gastrin release,
• Gastrin stimulates parietal and chief cells.
• This process leads to increased antral contractions and pyloric sphincter constriction, decreasing the rate of gastric emptying.

Distension

• Overfilling (distension) of the duodenum inhibits further gastric emptying.

Storage Function

• The stomach stores food and its contents must not exceed the absorptive capacity of the small intestine.

In Summary

• The central nervous system, sympathetic and parasympathetic systems, and hormones regulate the rate of gastric emptying.
• The major regulatory factors include acid, fats, and hypertonicity/distension.

Emotions

• Emotions influence gastric motility.
• Sadness and fear decrease motility.
• Anger and aggression increase motility, as does intense pain.

Gastric Secretions - Mucosal Gland Cells in Gastric Pits

• The stomach secretes gastric juices that play a crucial role in digestion.
• Gastric juice primarily includes HCl and pepsinogen.

Gastric Mucosa (Corpus & Antrum)

• Both the corpus and antrum contain various cells involved in gastric function.
• Parietal cells secrete HCl.
• Chief cells secrete pepsinogen.
• G cells secrete gastrin.
• D cells secrete somatostatin.
• ECL cells secrete histamine

Mucus

• Mucus protects the stomach wall from its own acid and enzymes, acting as a physical barrier.
• The mucus layer also lubricates and protects, preventing digestion.

Hydrochloric Acid

• Parietal cells actively transport H+ against a substantial concentration gradient into the gastric lumen.
• Cl- is also actively transported into the lumen.
• The creation of HCl is critical for the stomach's function.

Parietal Cell Ultrastructure

• Parietal cells contain abundant mitochondria to power the high-energy HCl transport.

Gastric Acid Secretion

• Steps in gastric acid secretion involve the exchange of bicarbonate for chloride, which results in the production of HCl.

Gastrin Secretion

• Gastrin is secreted into the blood from G cells.
• Gastrin stimulates the parietal and chief cells to secrete acid.

Histamine

• Histamine is a powerful stimulator of acid secretion.

Somatostatin-Paracrine Effect

• Somatostatin is a paracrine regulator that inhibits gastrin release from G cells when the antrum contains acid.

Control of Acid Secretion

• Factors influencing gastric secretion involve both neural and hormonal mechanisms.

Intrinsic Factor

• Parietal cells in the stomach secrete intrinsic factor (IF) that binds to Vitamin B12 in the small intestine to allow its absorption.

Pepsinogen Secretion

• Pepsinogen, secreted by chief cells, is converted to pepsin by HCl, performing protein breakdown in the acidic gastric environment.

Stimulation of Chief Cells in Response to a Meal

• Acetylcholine, gastrin, and histamine stimulate pepsinogen secretion.

Other Enzymes (Lipase)

• Lipase in the stomach is not essential for lipid breakdown but participates in breaking down fatty acids and monoglycerides to aid digestion.

The Mucus-Bicarbonate Barrier of the Gastric Mucosa

• A protective layer of mucus and bicarbonate protects the gastric mucosa from damage caused by acid and enzymes.

The Stomach Lining

• The stomach lining constantly regenerates, preventing damage from gastric secretions.

H. pylori

• H. pylori can live in the acidic stomach environment due to its motility, location in the antrum, urease production, and toxin secretion.

Helicobacter Pylori

• One of the major discoveries in bacterial gastritis is Helicobacter pylori.
• Their work in conjunction helped win a Nobel prize in physiology.

Acid Inhibitory Therapy

• Proton pump inhibitors (PPIs) target the parietal cells to reduce acid production, playing an essential role in treating many gastric disorders.

The Stomach and Absorption

• The stomach does not absorb food or water under normal conditions.

Ethyl Alcohol and Aspirin

• Unlike other drugs, ethyl alcohol and aspirin are directly absorbed by the stomach.

Vomiting (Emesis)

• Vomiting is a forceful expulsion of gastric contents.
• It generally relies on reverse peristalsis and is controlled by the nervous system.
• Various factors such as chemical agents, odors, smells, touch, pain and elevated intracranial pressure can trigger vomiting.

Causes of Vomiting

• Many triggers can induce vomiting, including chemical agents, noxious stimuli in the digestive tract, emotional factors, or physical stimulation.
• Elevated intracranial pressure, head trauma, and intense pain also induce vomiting.

Excessive Vomiting

• Excessive vomiting leads to a decrease in plasma volume, dehydration, and circulatory problems, often resulting in metabolic alkalosis.

Beneficial Effect of Vomiting

• Vomiting can be beneficial in cases of accidental ingestion (poison) where emetics can remove noxious materials.