Stomach

Breaks up particles, liquifies foods, chemical digestion of proteins and fats.

Chyme - Acidic, soupy, pasty mix of semidigested food

Gross Anatomy

1. 4 stomach regions

   1. Cardiac

      1. Small area close to cardiac orifice

      2. Esophagus enters

   2. Fundic Region/Fundus

      1. Dome superior to esophageal attachment

   3. Body

      1. Distal to cardiac orifice

      2. Largest part of stomach

   4. Pyloric Part

      1. Narrow pouch at inferior end

      2. Subdivided into antrum and pyloric canal

   5. Pylorus

      1. From pyloric canal into duodenum

      2. Surrounded by pyloric sphincter

   6. Pyloric Sphincter

      1. Thick smooth muscle ring

      2. Regulates chyme passage into duodenum

   7. Greater curvature

      1. Long way around surface of inferolateral surface

      2. Greater omentum hangs from here

   8. Lesser Curvature

      1. Shorter distance form esophagus to duodenum

Microscopic Anatomy

1. Stomach wall mucosa has simple columnar glandular epithelium

   1. Apical regions of these cells are full of mucin

   2. Which swells with water after secreted and becomes mucous

2. Mucous and submucosa become flat and smooth when stomach is full

   1. When stomach empties, gastric rugae form

   2. Gastric Rugae - longitudinal wrinkles

3. Lamina propria filled with tubular glands

4. Muscularis Externis has three layers

   1. Outer longitudinal, middle circular, inner oblique

5. Gastric Mucosa

   1. Full of gastric pits - depressions with same epithelium

   2. 2 to 3 tubular glands open in these pits and span the lamina propria

   3. Called cardiac glands and pyloric glands in the cardiac region and pyloric regions

   4. Mucous cells

      1. Secrete mucous mainly in cardiac and pyloric regions

      2. Mucous neck cells in gastric glands

      3. Concentrated in neck of glands where it opens into a gastric pit

   5. Regenerative Stem Cells

      1. In the base of pit and neck of gland

      2. Divide rapidly for continuous new cells

      3. New cells go upward to gastric pits and downwards to replace dead cells in glands

   6. Parietal Cells

      1. In upper half of of mainly gastric glands, occasionally pyloric glands

      2. Secrete hydrochloric acid, intrinsic factor, and ghrelin (appetite-stimulator)

   7. Chief Cells

      1. Most numerous

      2. Secrete enzymes gastric lipase and pepsinogen

      3. Lower half of gastric glands, none in cardiac or pyloric\

   8. Enteroendocrine Cells

      1. All stomach regions, mostly cardiac and pyloric, in lower end of a gland

      2. Secrete hormones and paracrine messengers for digestion regulation.

Gastric Secretions

1. Glands produce about 2 to 3 L of gastric juice per day

   1. Composed mostly of water hydrochloric acid, and pepsin

Stomach Acid Functions

1. Activates pepsin and lingual lipase

2. Break up CT and plant cell walls to help liquefy food and form chyme

3. Converts ferric ions to ferrous ions

   1. Iron that can be absorbed and used for hemoglobin synthesis

4. Contributes to innate immunity by destroying most ingested pathogens

Pepsin

1. Zymogens - inactive proteins

   1. Later converted to active enzymes by removing some of their amino acids

2. Pepsinogen - A zymogen secreted by chief cells in the stomach

   1. Hydrochloric acid removes some amino acids and converts it to pepsin

3. Pepsin - digests protein and has an autocatalytic effect

   1. As pepsin is formed, it converts pepsinogen into more pepsin

   2. Digest dietary protein into smaller peptide chains which goes into the small intestine for digestion completion.

Gastric Lipase

1. Secreted by chief cells

2. Digests 10% to 15% of dietary fats in stomach

Intrinsic Factor

1. A glycoprotein secreted by parietal cells

2. Responsible for small intestine’s absorption of vitamin B12

   1. Binds B12 to intestinal cells to be absorbed by receptor-mediated endocytosis

   2. No B12 = No hemoglobin = anemia

3. Less is produced as we age due to gastric mucosa atrophy

   1. Higher risk of anemia = pernicious anemia

Chemical Messengers

1. Neuroendocrine cells can produce as many as 20 hormones and paracrine signals

2. Some are gut-brain peptides - they play a crucial role in regulating gastrointestinal function, including appetite, motility, and secretion.

   1. Substance P

   2. Vasoactive intestinal peptide (VIP)

   3. Secretin

   4. Gastric Inhibitory Peptide (GIP)

   5. Cholecystokinin

   6. Neuropeptide Y

Digestion and Absorption

1. Most digestion and all nutrient absorption occur after chyme has passed into the small intestine.

2. Stomach mainly absorbs aspirin and some lipid-soluble drugs, but no major nutrients

3. Alcohol is absorbed by small intestine, but intoxication is based partially on how quickly the stomach empties

Gastrin

1. Hormone

2. Produced by enteroendocrine G cells in pyloric glands

3. Used to signal parietal cells to secrete hyaluronic acid and intrinsic factors

4. Also signals chief cells to secrete pepsinogen

Enterogastric Reflex - A neural response that inhibits gastric motility and secretion when the duodenum is distended or acidic.

1. Duodenum sends inhibitory signals to the medulla

   1. Reduce vagal stimulation of the stomach

   2. Stimulate sympathetic neurons - sends inhibitory signals to the stomach

2. Chyme stimulates duodenal endocrine cells to release secretin and cholecystokinin