Absorption and villi

Millions of tiny fingers

After digestion, the end products, amino acids, simple sugars, glycerol and fatty acids, move from the gut into the circulatory system.

The inner wall of the small intestine is highly folded, and the folds carry millions of finger-like villi. Together they give a very large surface. Food is absorbed by diffusion and active transport.

A villus wall is one cell thick. Inside are blood capillaries and a lymph vessel called a lacteal.

Folds, villi and a one-cell-thin wall all serve one goal: a huge surface with a short path, so nutrients pass quickly into blood or lymph.

Key terms

Villus (villi)

Increases the surface for absorption in the small intestine.

Lacteal

Takes in glycerol and fatty acids.

Hepatic portal vein

Carries amino acids and sugars from the villi to the liver.

Active transport

Moving nutrients into cells using energy, alongside diffusion.

Short questions with model answers

1. Q1. Write a brief note on the structure of villi. (Short Answer 5)

   • Shape and number.
   • Inside.

   Finger-like, one-cell-thick projections holding capillaries and a lacteal, giving a large surface for absorption.

2. Q2. Trace a glucose molecule from a villus to a muscle in your arm.

   • Into the blood.
   • Via the liver.

   Capillary, hepatic portal vein, liver, hepatic vein, heart, then out to the arm.

Common mistakes

• ✗ Fatty acids go straight into the blood capillaries.

  ✓ Glycerol and fatty acids enter the lacteal and reach the blood near the heart.

• ✗ Villi make digestive enzymes.

  ✓ Villi increase the surface for absorption (MCQ 10).

• ✗ Sugars from the gut go straight to the heart.

  ✓ They go to the liver first, by the hepatic portal vein.

MCQs

1. 1. Why does the small intestine have many folds and contain villi?

   1. (a) To slow down the passage of food
   2. (b) To promote food absorption
   3. (c) To prevent the backflow of chyme
   4. (d) To produce enzymes
   Show answer

   (b) Exercise MCQ 10 (p. 11). The item also names microvilli, which the chapter does not describe.

2. 2. Glycerol and fatty acids are absorbed into the:

   1. (a) Hepatic portal vein
   2. (b) Stomach wall
   3. (c) Lacteals
   4. (d) Gall bladder
   Show answer

   (c) p. 7.

3. 3. Amino acids and simple sugars are first carried to the:

   1. (a) Liver
   2. (b) Lungs
   3. (c) Kidneys
   4. (d) Large intestine
   Show answer

   (a) p. 7: by the hepatic portal vein.

4. 4. The wall of a villus is:

   1. (a) Made of bone
   2. (b) Many cells thick
   3. (c) Made of muscle only
   4. (d) A single layer of cells
   Show answer

   (d) p. 6.

5. 5. Food molecules cross the villus surface by:

   1. (a) Peristalsis only
   2. (b) Diffusion and active transport
   3. (c) Emulsification
   4. (d) Egestion
   Show answer

   (b) p. 6.

Quick revision

• Folds and millions of villi give the small intestine a huge absorbing surface.
• Glycerol and fatty acids go into lacteals; amino acids and sugars into capillaries.
• The hepatic portal vein takes sugars and amino acids to the liver first.