9 Stomach

Stomach overview:

After exiting the esophagus food enters the stomach, a J-shaped organ which lies between the esophagus and the entry of the small intestine. It spans the epigastric and left hypochondriac regions.

The stomach receives the food bolus from the esophagus which enters through the lower esophageal sphincter and acts like a blender, grinding down ingredients into a smaller, more fluid liquid called chyme.

The stomach has several identifiable features, containing several folds of the mucosa layer, called rugae. Further, the stomach can be divided into 5 areas:

• Cardia: The top section, located closest to the heart
• Fundus: The rounded section, located under the diaphragm
• Body: The largest section, located just below the fundus
• Antrum: An intermediate section, located beneath the body
• Pylorus: The bottom and most distal part of the stomach, which contains the pyloric sphincter

fix x. Anterior View: Stomach Components

The stomach’s main job is to break down bolus mechanically through involuntary muscular contraction (peristalsis) and chemically through enzymes.

Stomach function

The stomach’s main job is to break down food through two ways: mechanically, through involuntary muscular contraction and chemically through enzymes and acids.

Mechanically, the stomach breaks down food through rhythmic contractions known as peristaltic contractions. This is facilitated from the smooth muscle from the funfundus dus to the pylorus.

Chemically, the stomach produces several secretions from several types of cells to aid in digestion:

1. Parietal cells: produce gastric acid (including hydrochloric acid or HCL) to break down organic matter
2. G-cells: produce gastrin to stimulate acid secretion in the stomach
3. Chief cells: produce pepsinogen to break down proteins
4. Goblet cells: produce mucus, acting like a protective raincoat to shield the stomach lining from the acid

fix x. Anterior View: Stomach cells

Note how there are no enzymes specific to breaking down carbohydrates. The process of breaking down carbs initiates in the mouth and continues in the small intestine. Yet, another enzyme released in the stomach is gastric lipase which breaks down lipids.

Stomach Digestion:

Like the esophagus, the stomach also facilitates digestion via peristalsis as depicted below. The stomach’s smooth musculature aids in this rhythmic contraction to churn food into chyme:

fix x. Stomach: Peristalsis

By the time food exits the stomach, it leaves as a thick, soupy mixture, known as chyme.

The cells of the stomach are very important!

Hint: Another often overlooked aspect of the digestive system are the roles of parietal, chief, and goblet cells which play key roles in digestion within the stomach. Don’t forget to review these cells and their functions with respect to digestion.