Under normal conditions, which substance protects the inner lining of the human stomach from the action of hydrochloric acid present in gastric juice?

Introduction / Context:

The human stomach secretes gastric juice, which contains hydrochloric acid and digestive enzymes. Hydrochloric acid creates a strongly acidic environment that helps in digestion and kills many harmful microbes. However, this acid could also damage the stomach itself if the inner lining were not properly protected. Understanding how the stomach protects its own tissues is important in basic human physiology and in understanding conditions like acidity and ulcers. This question asks which substance normally protects the stomach lining from the acid it secretes.

Given Data / Assumptions:

• The inner lining of the stomach is exposed to hydrochloric acid present in gastric juice.
• Under normal conditions, the lining is not digested or burned by this acid.
• Options name villi, mucus, saliva, digestive juices, and bile salts.
• We assume a standard healthy stomach without ulcers or damage.

Concept / Approach:

The mucosa of the stomach is lined by cells that secrete mucus, a thick, slippery substance composed mainly of glycoproteins and water. This mucus forms a protective barrier over the epithelial cells, trapping bicarbonate ions and helping to neutralise acid at the surface. As a result, the strong hydrochloric acid in the lumen does not directly attack the cells of the stomach wall. Villi are finger like projections found in the small intestine, not in the stomach. Saliva is produced in the mouth, not by the stomach lining. Digestive juices like pepsin actually contribute to protein digestion and could harm unprotected tissues, and bile salts are produced by the liver and stored in the gallbladder, acting in the small intestine rather than in the stomach. Therefore, mucus is the correct protective substance.

Step-by-Step Solution:

Step 1: Recall that the stomach secretes hydrochloric acid to assist in digestion and maintain an acidic pH. Step 2: Recognise that this acid is strong enough to damage tissues if no protection exists. Step 3: Identify mucus as the thick secretion that forms a coating over the stomach lining and helps neutralise acid at the surface. Step 4: Distinguish mucus from other secretions such as saliva or bile, which are produced in different organs and act elsewhere in the digestive tract. Step 5: Conclude that mucus is the substance that normally protects the stomach lining from hydrochloric acid.

Verification / Alternative check:

Textbooks describing the structure of the stomach mention three main types of secretory cells in the gastric glands: parietal cells that produce hydrochloric acid, chief cells that secrete pepsinogen, and mucous cells that release mucus. They emphasise that mucus and bicarbonate form a protective barrier against acid and enzymes. Discussions about peptic ulcers often note that damage or reduction of this mucous barrier exposes the stomach lining to acid, leading to pain and ulcers. This information supports mucus as the correct answer.

Why Other Options Are Wrong:

Option A, Villi, are structures of the small intestine and are not present in the stomach. Option C, Saliva, is produced in salivary glands and acts in the mouth, not inside the stomach. Option D, Digestive juices such as pepsin, help digest proteins and can damage tissues if not controlled, so they are not protective. Option E, Bile salts, act in the small intestine to emulsify fats and do not line the stomach.

Common Pitfalls:

Students may see the word digestive juices and assume that any secretion of the stomach is protective, overlooking the fact that acid and enzymes can be harmful without a barrier. Others may confuse the role of structures like villi, which increase surface area for absorption, with protective roles. To avoid such mistakes, learners should clearly remember that mucus is the protective coating in the stomach, just as skin is the protective covering of the body surface.

Final Answer:

The inner lining of the stomach is protected from hydrochloric acid under normal conditions by a layer of Mucus.