The Stomach Anatomy and Functions

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 stomach wall has the same four basic layers as the rest of the GI tract, with modifications.
• The mucosa forms folds called rugae when empty.
• Simple columnar epithelium lines the mucosa, containing gastric pits that extend into the lamina propria.
• Gastric glands are located within these pits.

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 three layers of smooth muscle: an outer longitudinal layer, a middle circular layer, and an inner oblique layer.
• This multi-layered arrangement allows for a variety of contractions to churn food.
• This churning mixes food with gastric juice, breaking it into smaller particles before sending it to the duodenum.

Gastric Juice

• Gastric juice is a mixture of secretions from zymogenic cells, parietal cells, and mucous cells.
• Daily production of gastric juice is approximately 2500 ml.

Composition of Gastric Juice

• Gastric juice contains various cations (e.g., Na+, K+, Mg2+, H+) and anions (e.g., Cl-, HPO42-, SO42-).
• It also contains pepsin (I-III), gelatinase, mucus, intrinsic factor, water, and hydrochloric acid.
• Hydrochloric acid converts pepsinogen to pepsin, essential for protein digestion, along with killing ingested bacteria and aiding protein digestion.
• Intrinsic factor is essential for vitamin B12 absorption.

Regulation of Gastric Secretion

• Gastric secretion is regulated by both nervous and hormonal mechanisms.
• Parasympathetic stimulation from nuclei in the medulla travels via the vagus nerve to stimulate gastric glands.
• Emotions like anger and fear can slow down digestion by stimulating the sympathetic nervous system.

Cephalic, Gastric , and Intestinal Phases

• Gastric secretion is regulated in three phases (Cephalic, Gastric, and Intestinal).
• The Cephalic phase involves sight, smell, or taste of food triggering the medulla to stimulate gastric glands.
• The Gastric phase involves food stretching the stomach, stimulating acid and gastrin release, and increasing gastric motility.
• The Intestinal phase involves partially digested proteins in the duodenum triggering the release of enterogastrin, which further regulates secretion.

Inhibition of Gastric Secretion

• Presence of acid, partially digested proteins, irritation of the duodenum mucose, presence of fats, and other substances, also inhibit gastric secretion.
• Hormones such as secretin, CCK, and GIP also slow down or inhibit the gastric juice secretion by inhibiting sympathetic stimulation.

Mechanical Digestion

• The stomach mixes food with gastric juice through mixing waves (peristaltic movements)
• This process reduces food to a thin liquid called chyme.

Chemical Digestion

• The primary chemical activity of the stomach is protein digestion by pepsin.
• The acidic environment (pH 2) is optimum for pepsin to function.
• Pepsin breaks down certain peptide bonds between amino acids in proteins.
• The zymogenic cells secrete pepsinogen, which is converted to pepsin by hydrochloric acid from parietal cells.
• Mucus protects the stomach lining from digestion by pepsin.
• Gastric lipase is present in the stomach for fats.
• Infants secrete rennin which acts on milk casein.

Regulation of Gastric Emptying

• Gastric emptying is stimulated by nerve impulses in response to distension and the release of stomach gastrin, as well as food types
• The rate of emptying is influenced by the type of food.
• The stomach empties contents into the duodenum within 2-6 hours after ingestion.
• Carbohydrates remain in the stomach the shortest time, followed by protein and fat.
• Stomach emptying is inhibited by the enterogastric reflex and hormones (secretin, CCK, and GIP).

The Small Intestine

• The mucosa contains intestinal glands (crypts of Lieberkühn) that secrete intestinal juice.
• Duodenal glands secrete alkaline mucus.
• Mucus protects the wall of the small intestine from acidity.
• Goblet cells add additional mucus.
• Solitary and Peyer's patches increase immunity.

Intestinal Juice

• A clear yellow fluid; about 2-3 litres per day.
• pH is 7.6, with water and mucus.

Absorption in the Small Intestine

• Digested nutrients are absorbed from the alimentary canal into the blood and lymph.
• This process, called absorption, accounts for approximately 90% in the small intestine.

The Large Intestine

• The large intestine completes the absorption process.
• It also manufactures certain vitamins (by bacteria), forms feces, and expels the feces.
• Bacteria in the large intestine break down remaining carbohydrates;
• forming gases as a result.
• It absorbs water, salts, and some electrolytes.