7.5 Absorption

2026 Syllabus Objectives

  1. State that the small intestine is the region where nutrients are absorbed
  2. State that most water is absorbed from the small intestine but that some is also absorbed from the colon
  3. Explain the significance of villi and microvilli in increasing the internal surface area of the small intestine
  4. Describe the structure of a villus
  5. Describe the roles of capillaries and lacteals in villi

What is Absorption? 🔑

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By the time food has passed through the duodenum, most of the large molecules have been fully digested into small ones through chemical digestion:

  • Carbohydrates → broken down to glucose
  • Proteins → broken down to amino acids
  • Fats → broken down to fatty acids and glycerol

Absorption is the process by which these small, soluble molecules pass through the walls of the small intestine and into the blood.

🔑 Key Definition: Absorption is the process by which small, soluble molecules pass through the walls of the small intestine and into the blood.

Where Does Absorption Occur? 📌

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The Small Intestine - Primary Site of Absorption

The small intestine is the region where nutrients are absorbed. This is where the digested products of carbohydrates, proteins, and fats enter the bloodstream.

Substances Absorbed in the Small Intestine

The following substances are absorbed into the blood from the small intestine:

  • Glucose (from carbohydrate digestion)
  • Amino acids (from protein digestion)
  • Fatty acids and glycerol (from fat digestion)
  • Vitamins
  • Mineral ions (including iron and calcium ions)
  • Water (most water absorption occurs here)

Water Absorption

Most water is absorbed from the small intestine, but a smaller quantity of water is also absorbed in the colon (part of the large intestine).

Structure of the Small Intestine: Villi and Microvilli

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What are Villi? ⚡

The inner wall of the small intestine is covered with tiny finger-like projections called villi (singular: villus).

Characteristics of villi:

  • Each villus is approximately 1 mm long
  • Thousands of villi line the small intestine
  • They create a greatly increased internal surface area

🔑 Key Definition: Villi are very small finger-like projections that line the inner surface of the small intestine, greatly increasing its surface area.

What are Microvilli?

The cell membrane on the surface of each villus is folded to form many tiny projections called microvilli.

🔑 Key Definition: Microvilli are tiny folds on the surfaces of the cells of the epithelium of the villi in the small intestine.

Functions of microvilli:

  • Further increase the surface area for absorption
  • The enzyme maltase acts on these membranes, breaking down maltose into glucose molecules
  • Through these membranes, nutrients are absorbed (glucose, amino acids, fatty acids, glycerol, vitamins, mineral ions, and water)

Significance of Villi and Microvilli 🔑

The villi and microvilli hugely increase the internal surface area of the small intestine. This adaptation is critical for efficient absorption because:

  1. Villi increase the surface area of the intestinal lining
  2. Each villus is covered with thousands of microvilli, increasing the total surface area even more
  3. Increased surface area greatly speeds up the rate at which absorption happens

This ensures that digested nutrients can be absorbed quickly and efficiently into the bloodstream before the food passes further along the digestive system.

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