The Alimentary Canal: The Stomach

1. Mouth
2. Oesophagus
3. Stomach
4. Small intestine
5. Liver
6. Pancreas
7. Large intestine

3. The Stomach

• The stomach is a J-shaped sac that stores and digests food.
• Food enters the stomach through the cardiac sphincter muscle at the top of the stomach.
• A sphincter muscle is a circular muscle that can open and close.

Components of the Stomach

1. Mucous: Produced by goblet cells lining the stomach. It protects the stomach wall from being digested by its own enzymes and acid (prevents self-digestion).
2. Hydrochloric acid (HCl): Provides a pH of 1-2, kills bacteria, and activates pepsinogen. HCl is not an enzyme.
3. Pepsin: An enzyme that breaks down proteins into peptides.

Chemical Digestion

• Enzymes carry out chemical digestion.
• Pepsinogen is an inactive enzyme produced by glands lining the stomach.
• It is activated by HCl to form pepsin.
• Pepsin digests proteins into smaller units called peptides.
• The stomach is protected from digestion by a mucous layer.
• Pepsin only becomes active when mixed with stomach acid.
• An ulcer can form if there is too much acid in the stomach.

Physical Digestion

• The stomach churns food and mixes it with gastric juices to form a liquid called chyme.
• This increases the surface area of the food, allowing enzymes to digest the food more quickly.