1. Unit 2: Digestion in The Stomach

2. (1) Stomach Structure
The stomach can contain up to 1.5L of food comfortably.
The stomach is composed of folds of excess tissue called “rugae”.
Rugae in the stomach allow it to:
Expand and enlarge as it fills with food and liquid.
Secrete digestive solutions for food metabolism.

3. (2) Digestion in the Stomach
Food is broken down both mechanically and chemically in the stomach.
Mechanical Digestion:
Stomach muscles use peristalsis to break down bolus.
Peristalsis is triggered by the release of a neurotransmitter called serotonin (from within the rugae).
Chemical Digestion:
Gastric Juice is secreted from the rugae, to break down the bolus using enzymes and acid.

5. (3) Gastric Juice Composition
Hydrochloric Acid
Pepsin (an Enzyme that metabolizes Protein)
Rennin (an Enzyme that metabolizes Protein)
Muccin (an Enzyme that lubricates the food mass)

6. (4) Gastric Juice Secretion
The hormone Gastrin is released from the G-Cells in between the Rugae folds:
Triggered by smell and taste.
Triggered by the increase in pH when food drops into the stomach.
Gastrin stimulates the release of Hydrochloric Acid from the rugae in the stomach:
HCl decreases the pH in the stomach.
HCl metabolizes proteins and carbohydrates.
Gastrin stimulates peristaltic movement in the stomach, and small and large intestine.

7. (5) Protein Metabolism
Proteins are complex molecules which require a lot to be broken down.
When proteins are metabolized, a large concentration of Hydrogen ions are released (which decreases the pH of the stomach).
The decrease in pH signals to the brain that the food is digested, and the G-Cells in the stomach stop releasing gastrin.

8. (6) Leaving the Stomach
Once food is digested, the stomach stops churning and secreting gastric juice.
Food them moves into small intestine.
The liquefied mass leaving the stomach is called “Chyme”.
Chyme exits out the Pyloric Sphincter into the small intestine.