Ch 23 Digestive System Functional Anatomy Pt II

Lumen

Central opening where food passes from the oral cavity to the stomach for chemical digestion

Mucosa (Esophagus)

Stratified squamous epithelium, allows for friction resistance while food passes.

Submucosa (esophagus)

connective tissue with modest secretions aiding the movement of food through the esophagus.

Muscularis Externa (esophagus)

Upper region consists of skeletal muscle for swallowing, transitioning to smooth muscle (circular and longitudinal layers).

Circular layer

squeezes the lumen to push food down

Longitudinal layer

Shortens the esophagus and opens the lumen.

Adventita

Dense, irregular connective tissue surrounding muscularis externa; important since the esophagus is largely outside the abdomen

Stomach

Temporarily holds food and mixes it with digestive juices, can hold about 50 mL when empty, food is converted to chyme by acids and enzymes

Sympathetic control

Inhibits

Parasympathetic control

stimulates

Modified muscularis layers

contains inner most oblique layer of smooth muscle, Allows physical mixing, churning of food

Modified mucosa layer

Simple columnar epithelium entirely foveolar cells, Produce HCO3 containing mucus coat, Joined by tight junctions, Surface epithelium renowned every 3-6 days (stem cells in the gastric pits)

Cardia

near the esophagus

Fundus

dome-like portion under the diaphragm

Body

The largest segment of the stomach

Pyloric Antrum

leads to pyloric canal and pyloric sphincter

Regions of the stomach

Cardia, Fundus, Body, Pyloric Antrum

Pyloric Sphincter

Regulates exit from stomach to duodenum of the small intestine, prevents backflow.

Cardia Sphincter

Controls entry of food and gastric juices into the esophagus, preventing acid reflux

Muscularis Externa (stomach)

1. Contains three layers: longitudinal, circular, and an additional oblique layer for churning and peristalsis, mixing food with gastric secretions to create chyme.

2. Function: Churning and propulsion of food towards pyloric canal for digestion

Gastric functionality

Chemical Digestion and Rugae

Chemical digestion

stomach plays a crucial role in digesting proteins through pepsinogen (inactive enzyme) activated to pepsin by acid, breaking down proteins into smaller peptides.

Rugae

Folds in the stomach lining that allow for expansion and increased surface area for contact with secretions; they do not decrease the effective surface area during contraction.

Curvatures of the stomach

Lesser curvature, Greater curvature

Lesser curvature

Smaller side adjacent to the esophagus

Greater curvature

Larger side opposite the lesser curvature

Pepsinogen to Pepsin

Stomach secretes pepsinogen (inactive enzyme), activated by Hydrochloric acid, forming pepsin, which breaks down proteins into smaller peptides

Role of hydrochloric acid

Hydrochloric acid facilitates protein breakdown and activates pepsinogen.

It creates an acidic environment necessary for digestion

Churning action of the stomach

Churning mixes food with enzymes and acid, forming chyme, a semi-liquid mixture essential for digestion.

Chyme consists of ingested food combined with gastric secretions.

Nervous system influence

stomach activity influenced by autonomic nervous system

stress inhibits stomach activity , while relaxation stimulates contraction and secretions that assist digestion

Stomach Mucosa

Surface epithelium, Lamina Propria

Surface epithelium

Composed of simple columnar epithelium, known as foveolar cells. Gastric pits located here

Lamina Propia

Contains glands responsible for secreting mucus and digestive enzymes.

Gastric glands

Located in the surface epithelium: Mucous neck cells, parietal cells, chief cells, enteroendocrine cells

Mucous neck cells

Secrete mucus for protection against acid

Parietal cells

Secrete hydrochloric acid and intrinsic factor (essential for Vitamin B12 absorption).

Chief cells

secrete pepsinogen

Enteroendocrine cells

Release signaling molecules like gastrin, histamine, and serotonin into the bloodstream

Protection mechanisms in the stomach

Mucus and Bicarbonate secretion

Mucus secretion

Mucus coats the stomach lining, protecting it from acidity and enzyme degradation.

Bicarbonate secretion

Bicarbonate neutralizes excess acid, preventing damage to the stomach lining

Cell structure and replacement

Foveolar cells connected by tight junctions prevent acid from leaking into intercellular spaces.

Stomach lining is replaced every 3 to 6 days to maintain integrity and functionality.

Gastrin, Histamine, Endorphins, Serotonin

Hormones and signals produced by enteroendocrine cells, influencing gastric functions