The anatomy and physiology of the stomach including the small and large intestine. It also includes the organs of the pancreas and the liver. Describing the histology of the stomach such as the mucosa. And includes the gastric regulation phases: cephalic, gastric, and intestinal phases
Functions of the Stomach:
- Storage of food until it can be emptied into the small intestine for digestion & meter the
rate at which it leaves
- Secretion of HCl and enzymes to begin protein (and some fat) digestion
- Pulverize and mix food with gastric secretions to produce chyme to be delivered to
duodenum
Anatomy of the Stomach:
- Cardia: Prevents the contents of the stomach from backflowing into the esophagus
- Opening between cardia and esophagus is called cardiac orifice
- Fundus: Lies above cardiac orifice and has Smooth muscle is relatively thin
- Body: Main part of the stomach
- Smooth muscle layer is thin
- Pylorus: Terminal end of the stomach
- Thick layer of smooth muscle
- Opening to duodenum is called pyloric orifice surrounded by smooth muscle
sphincter called pyloric sphincter
- Gastric folds (rugae): The longitudinal folds of the stomach mucosa
- Allow the stomach to expand when it fills with food and drink
- Lesser curvature: Smaller, concave superomedial surface
- Greater curvature: Larger, convex inferolateral surface
Histology of the Stomach:
- Mucosa: Lined by simple columnar epithelium
- Indented with numerous gastric pits
- Several gastric glands extend deep into the mucosa from the base of each gastric
pit
- Muscularis mucosae partially surround the gastric glands to help expel the gastric
secretions
- Muscularis: Composed of three layers of smooth muscle
- Outer longitudinal, Middle circular, and Inner oblique
- Assists the continual churning of the foodstuffs
- Increases in thickness as it progresses from the body to pyloris
Gastric secretions:
- Surface mucous cells: Line the stomach lumen and extend into gastric pits
- Continuously secrete a viscous, alkaline mucus
- Prevents ulceration of the stomach lining
- Mucous neck cells: Found in the neck of the gastric pit and interspersed with parietal
cells
- Produce a watery, acidic mucus
- Helps to maintain acidic conditions of stomach
, - Mucus is protective: Lubricating properties protect stomach lining from mechanical
damage
- Protects the stomach wall from self-digestion
- Inhibits pepsin activity
- Neutralizes HCl due to its alkaline nature
- Parietal cells (onxytic cells): Produce intrinsic factor
- Glycoprotein required for the absorption of Vit B12 needed for
erythropoiesis
- Produce hydrochloric acid
- Not produced as HCl, H+ and Cl- ions are secreted
- Responsible for low pH of stomach
- Functions of HCl: Does not actually digest anything
- Activates pepsinogen to pepsin and provides an optimal environment for its
activity
- Kills most microorganisms
- Aids the breakdown of connective tissue, muscle fibers, and plant cell walls
- Denatures proteins via low pH and promotes chemical digestion
- Chief cells (Most numerous): Secrete zymogenic packets containing pepsinogen
- Inactive precursor to the proteolytic enzyme pepsin
- Converted to pepsin in the stomach by HCl and other active pepsin molecules
- HCl cleaves off a small fragment activating the enzyme
- Pepsin (active form) is autocatalytic – It activates other pepsinogen to the
active form
- Cleaves specific amino acid sequences of proteins to produce smaller
peptide
- Produce gastric lipase
- Limited role in fat digestion
- G-cells (enteroendocrine cells): Secrete gastrin into bloodstream
- Paracrine hormone
- Stimulates stomach secretions and stomach motility
Gastric Motility:
- Mixing: Form of mechanical digestion that convert the bolus into chyme
- Strong peristaltic contractions mix the bolus with gastric secretions to produce
chyme
- Results in the reduction in size of the food particles
- Emptying: Movement of acidic chyme through the pyloric sphincter into the duodenum
- Thickening of muscularis results in stronger contractions
- Creates a pressure gradient to drive chyme (~3 mL) through pyloric
sphincter
- Retropulsion
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