Human Anatomy: The Stomach

The Stomach

• The cardia surrounds the lower esophageal sphincter.
• The fundus is the rounded portion above and to the left of the cardia.
• The body is the large, central portion of the stomach.
• The pylorus is the narrow inferior region that connects to the duodenum via the pyloric sphincter.

The Stomach Linings

• The mucosa forms folds called rugae when the stomach is empty.
• The mucosa is simple columnar epithelium with pits that extend into the lamina propria.
• The pits in the mucosa are gastric glands.

Gastric Glands

• Gastric glands contain several types of secreting cells:
  • Zymogenic (peptic) cells secrete pepsinogen (a precursor to pepsin).
  • Parietal (oxyntic) cells secrete hydrochloric acid and intrinsic factor.
  • Mucous cells secrete mucus.
  • Enteroendocrine cells secrete stomach gastrin.

The Muscularis

• The stomach muscularis has 3 layers of smooth muscle:
  • An outer longitudinal layer.
  • A middle circular layer.
  • An inner oblique layer.
• This arrangement allows the stomach to churn food, mix it with gastric juice, and pass it into the duodenum.

Gastric Juice

• Gastric juice is a collection of secretions from several cell types.
• Zymogenic (peptic) cells secrete enzymes, like pepsin.
• Parietal cells secrete HCl.
• Mucous cells secrete mucus.
• 2500ml of gastric juice is produced daily.

Composition of Gastric Juice

• Gastric juice contains various cations (Na+, K+, Mg2+, H+) and anions (Cl-, HPO42-, SO42-).
• It also includes enzymes like pepsin I-III and gelatinase.
• Intrinsic factor is involved in vitamin B12 absorption.
• Water is present in the gastric juice.
• Hydrochloric acid is crucial for converting pepsinogen to pepsin.

Regulation of Gastric Secretion

• Gastric secretion is regulated by both nervous and hormonal mechanisms.
• Parasympathetic impulses stimulate gastric secretion.
• Several hormones also regulate gastric secretion. Emotions (like anger, fear, and anxiety) slow digestion through the sympathetic nervous system.
• Gastric secretion has cephalic, gastric, and intestinal phases.

Cephalic Phase

• Sight, smell, and taste of food trigger the cephalic phase.
• Signals from the cerebral cortex or hypothalamus travel to the medulla.
• The medulla sends impulses via the vagus nerve to the gastric glands, stimulating gastric secretions.

Gastric Phase

• Distension of the stomach initiates the gastric phase.
• Receptors detect the presence of proteins and alcohol and stimulate the production of stomach gastrin.
• Stomach gastrin increases gastric juice production and the motility of the digestive tract.
• Pyloric mucosa responds to protein and alcohol, releasing stomach gastrin and regulating the chyme flow.

Intestinal Phase

• Partially digested proteins entering the duodenum trigger the release of enterogastric gastrin.
• This phase regulates gastric secretion through nerve signals and the release of hormones like secretin, CCK, and GIP.
• The duodenum's distension, the presence of acid, protein, and fats cause inhibition of gastric secretions.

Hormonal Inhibition of Gastric Secretion

• Intestinal mucosa secretes hormones that inhibit gastric secretion and decrease motility.
• Secretin, CCK, and GIP inhibit gastric secretion.

Digestion in the Stomach

• Mechanical digestion occurs via mixing waves, breaking food into smaller pieces.
• Chemical digestion involves the activation of pepsinogen to pepsin and the beginning of protein digestion.

Chemical Digestion

• Pepsin is the main enzyme for protein digestion, working best in the acidic environment of the stomach (pH 2).
• Pepsinogen is converted to pepsin by HCl. Mucus protects stomach lining from pepsin.
• Gastric lipase is another enzyme present in the stomach, crucial for the limited fat digestion. Rennin helps in milk digestion in infants.

Regulation of Gastric Emptying

• Gastric emptying is regulated by nerve impulses in response to stomach distension and hormone release (stomach gastrin).
• Different food types (e.g., carbohydrates, proteins, fats) have varying emptying speeds.
• Stimulation: distension of the stomach, and products within the stomach like proteins and alcohol.
• Inhibition: the enterogastric reflex, the presence of chime within the duodenum (fatty acids, highly acidic, hyper/hypotonic), hormones like secretin, CCK, and GIP.

Pancreas, Liver, and Gallbladder

• The pancreas is a gland with both endocrine and exocrine functions. Pancreatic juice contains enzymes for carbohydrate, protein, and fat digestion. Bicarbonate ions raise the pH to neutralize gastric acid for optimal enzyme action.
• The liver produces bile, which emulsifies fats, and performs many other important functions concerning metabolism, storage and detoxification..
• The gallbladder stores and concentrates bile until needed by the small intestine.

Intestinal Juice

• A clear yellow fluid is secreted by the small intestine's glands.
• Contains water, mucus, and other substances.

Passage of Digested Nutrients

• Absorption occurs through the villi and microvilli in the small intestine.
• Methods of absorption include diffusion, facilitated diffusion, osmosis, and active transport.

The Large Intestine

• The large intestine completes absorption and forms feces.
• Bacteria in the large intestine aid in the digestion of some remaining nutrients via fermentation.
• Vitamins and electrolytes are reabsorbed.
• Water is absorbed.

Defecation

• Mass peristaltic movements push feces toward the rectum.
• The external anal sphincter controls the defecation process by allowing or preventing the expulsion of feces.