Human defence systems: stomach.

The idea

The stomach is a muscular organ that receives food from the oesophagus. It churns food and mixes it with gastric juice, which contains hydrochloric acid and enzymes such as protease. The hydrochloric acid creates a very acidic pH, typically around pH 2, which kills many microorganisms swallowed with food or drink. This is a non-specific defence because the acid destroys a wide range of pathogens, not just one type. The low pH also provides the optimum conditions for protease enzymes to digest proteins. The stomach wall is protected from self-digestion by a layer of mucus. In the context of human defence systems, the stomach is part of the first line of defence, reducing the number of viable pathogens that reach the intestines.

For example: Drinking contaminated water may introduce bacteria, but the acidic environment of the stomach often destroys them before they can cause disease.

What earns the marks

• The stomach is a muscular organ that churns food and mixes it with gastric juice.
• Gastric juice contains hydrochloric acid, which gives the stomach a low pH and kills many ingested microorganisms.
• The acid environment is a non-specific defence because it destroys a wide range of pathogens.
• The stomach wall secretes mucus, which protects the lining from damage by acid and enzymes.
• The low pH also provides optimum conditions for protease enzymes that digest proteins.
• Pathogens that survive the stomach may reach the intestines and cause infection, so the stomach reduces the risk of disease.

Common mistakes and how to put them right

• Thinking that the stomach produces only enzymes.

  Put it right: State that gastric juice contains both acid and enzymes.

• Believing that the stomach is sterile.

  Put it right: Say that the acid kills many pathogens, reducing the risk of infection.

• Confusing the role of mucus in the stomach with mucus in the airways.

  Put it right: Specify the protective role of mucus in each location.

Exam tips

• Link the low pH of the stomach directly to the killing of microorganisms when explaining defence.
• Use the term 'non-specific' to describe the stomach acid defence, as it acts against many pathogens.
• If asked about enzymes, remember that protease enzymes work best in acidic conditions in the stomach.