The stomach

A muscular, acidic bag

The J-shaped stomach lies in the upper left of the abdomen, below the diaphragm. It stores food and does mechanical and partial chemical digestion.

Its thick smooth muscles contract hard and churn the food into smaller particles. The heat from churning helps melt lipids.

Gastric glands secrete gastric juice: hydrochloric acid, the inactive enzyme pepsinogen, and mucus. After three to four hours the food becomes a soup-like chyme.

HCl switches pepsinogen on to pepsin, and pepsin cuts proteins into peptides. Mucus keeps the acid away from the stomach's own wall.

Key terms

Gastric juice

The juice made by gastric glands in the stomach wall.

Pepsin

Breaks large proteins into shorter chains called peptides.

Chyme

Partly digested food leaving the stomach.

Sphincter

A muscle ring that opens and closes a passage.

Short questions with model answers

1. Q1. What are the functions of HCl in gastric juice? (Short Answer 3)

   • Activates the enzyme.
   • Protects the body.

   HCl activates pepsin and kills harmful bacteria.

2. Q2. A meal leaves the stomach 5 to 15 mL at a time. Which structure controls this, and why is it useful?

   • The gate.
   • Small amounts.

   The pyloric sphincter; small portions can be handled fully by the intestine.

Common mistakes

• ✗ HCl itself digests the protein.

  ✓ Pepsin digests protein; HCl only activates pepsin and kills bacteria.

• ✗ Gastric glands release active pepsin.

  ✓ They release inactive pepsinogen, which HCl converts to pepsin.

• ✗ The stomach completes the digestion of food.

  ✓ It does only partial chemical digestion; the small intestine completes it.

MCQs

1. 1. In the stomach, pepsinogen is converted into pepsin by the action of:

   1. (a) Bile salts
   2. (b) Hormones
   3. (c) HCl
   4. (d) Bicarbonate
   Show answer

   (c) Exercise MCQ 6 (p. 10). Your book labels both of the first two options "d"; the answer is HCl.

2. 2. Pepsin breaks proteins into:

   1. (a) Peptides
   2. (b) Maltose
   3. (c) Fatty acids
   4. (d) Glucose
   Show answer

   (a) p. 4.

3. 3. Which prevents food flowing back from the stomach into the oesophagus?

   1. (a) Pyloric sphincter
   2. (b) Epiglottis
   3. (c) Gastric glands
   4. (d) Cardiac sphincter
   Show answer

   (d) p. 4: the cardiac (lower oesophageal) sphincter.

4. 4. Why doesn't the stomach digest its own wall?

   1. (a) The wall is made of bone
   2. (b) Mucus lines the wall and neutralises HCl there
   3. (c) Pepsin works only in the mouth
   4. (d) There is no acid in the stomach
   Show answer

   (b) pp. 4–5: so pepsinogen is not activated at the wall.

5. 5. Food usually stays in the stomach for about:

   1. (a) 3 to 4 minutes
   2. (b) 3 to 4 days
   3. (c) 3 to 4 hours
   4. (d) 30 seconds
   Show answer

   (c) p. 5.

Quick revision

• The stomach stores food, churns it, and partly digests it into chyme in 3 to 4 hours.
• HCl activates pepsin and kills bacteria; pepsin breaks proteins into peptides; mucus protects the wall.
• The cardiac sphincter stops backflow; the pyloric sphincter releases chyme in small amounts.