The small intestine

Where digestion is completed

The small intestine is a highly coiled tube nearly 7 m long. It has three parts: the duodenum (about 25 cm), the jejunum (about 2.5 m) and the ileum (about 4 m).

In the duodenum, bile from the liver arrives through the common bile duct. Bile salts break large fats into small droplets, making a milky fluid. This is emulsification.

Pancreatic juice enters through the pancreatic duct. Its trypsin, pancreatic amylase and lipase digest proteins, carbohydrates and lipids. Its sodium bicarbonate neutralises the acid chyme.

Bile has no enzymes. It only splits fat into tiny droplets, giving lipase far more surface to work on.

Key terms

Duodenum

Receives bile and pancreatic juice.

Bile

Its salts break fats into droplets; it has no enzymes.

Emulsification

Breaking large fats into small, separate droplets.

Pancreatic juice

Digests proteins, carbohydrates and lipids and neutralises acid chyme.

Short questions with model answers

1. Q1. State the role of pancreatic juice in digestion. (Short Answer 4)

   • Its enzymes.
   • Its bicarbonate.

   It digests all three main food types and neutralises the acid chyme.

2. Q2. A fried paratha reaches the duodenum. Trace what happens to its fat.

   • Bile first.
   • Then the enzyme.

   Emulsified by bile, then digested by lipase.

Common mistakes

• ✗ Bile is an enzyme that digests fat.

  ✓ Bile has no enzymes; its salts only emulsify fat. Lipase digests it.

• ✗ The order is ileum, jejunum, duodenum.

  ✓ Duodenum first, then jejunum, then ileum.

• ✗ Add 25 + 2.5 + 4 to get the length.

  ✓ 25 cm is 0.25 m; change it to metres before adding.

MCQs

1. 1. The three portions of the small intestine, in the correct order, are:

   1. (a) Duodenum, jejunum, ileum
   2. (b) Ileum, duodenum, jejunum
   3. (c) Colon, caecum, rectum
   4. (d) Caecum, colon, rectum
   Show answer

   (a) Exercise MCQ 1 (p. 10).

2. 2. The pancreas produces digestive enzymes and releases them into the:

   1. (a) Colon
   2. (b) Gall bladder
   3. (c) Liver
   4. (d) Duodenum
   Show answer

   (d) Exercise MCQ 5 (p. 10).

3. 3. The enzyme trypsin is found in:

   1. (a) Bile
   2. (b) Pancreatic juice
   3. (c) Gastric juice
   4. (d) Saliva
   Show answer

   (b) Exercise MCQ 8 (p. 11).

4. 4. The common bile duct carries bile from the gall bladder to the:

   1. (a) Stomach
   2. (b) Colon
   3. (c) Duodenum
   4. (d) Pancreas
   Show answer

   (c) Exercise MCQ 9 (p. 11).

5. 5. What neutralises the acidity of chyme in the duodenum?

   1. (a) Sodium bicarbonate in pancreatic juice
   2. (b) Pepsin
   3. (c) HCl
   4. (d) Salivary amylase
   Show answer

   (a) p. 5.

Quick revision

• The small intestine is nearly 7 m: duodenum, jejunum, ileum.
• Bile emulsifies fats; it has no enzymes.
• Pancreatic juice digests proteins, carbohydrates and lipids and neutralises acid chyme.