Edexcel iGCSE Biology-2.32 small intestine- Study Notes- New Syllabus

Small Intestine – Adaptations for Absorption

📝 Introduction

• After digestion, food is broken into small soluble molecules like glucose, amino acids, fatty acids, and glycerol.
• These nutrients are absorbed mainly in the small intestine, especially the ileum.
• The small intestine is specially adapted to absorb nutrients quickly and efficiently.
• Its structure increases surface area and reduces the distance for diffusion.
• It is one of the most important regions for getting nutrients into the blood and lymph.

⚙️ Adaptations of the Small Intestine

Very Long Structure

• The small intestine is around 6 metres long.
• This long length provides a very large internal surface area.
• More surface area means more space for absorption to take place.
• It also allows food to stay longer, giving more time for absorption.

Villi – Finger-like Projections

• The inner lining of the small intestine has many villi (singular: villus).
• Villi are tiny finger-shaped structures projecting into the intestinal space.
• They massively increase the surface area for absorption.
• Each villus is adapted to absorb nutrients quickly into the body.

Microvilli – Even More Surface Area

• The surface of epithelial cells in villi has tiny folds called microvilli.
• These form a “brush border” appearance.
• Microvilli further increase the surface area for absorption.
• This ensures maximum uptake of nutrients from digested food.

Thin Wall Structure

• The wall of each villus is only one cell thick.
• This reduces the distance nutrients must travel during absorption.
• Short diffusion distance means faster movement of substances.
• It improves the rate at which nutrients enter the blood.

Rich Blood Supply

• Each villus contains a dense network of blood capillaries.
• These capillaries quickly carry away absorbed nutrients.
• This maintains a steep concentration gradient.
• The gradient ensures continuous diffusion of glucose and amino acids into the blood.

Presence of Lacteal

• Each villus contains a central lymph vessel called a lacteal.
• Lacteals absorb fatty acids and glycerol.
• These fats are transported through the lymph system before entering the bloodstream.
• This prevents fats from blocking blood capillaries and ensures smooth transport.

Movement of Villi

• Villi show slight rhythmic movement inside the intestine.
• This movement mixes the digested food with intestinal juices.
• It brings fresh nutrients into contact with the villi surface.
• This improves the efficiency of absorption.

📌 Key Parts and Their Functions

📊 How Absorption Happens

Glucose and Amino Acids

• Glucose and amino acids pass through villus walls into blood capillaries.
• They are then carried to the liver and body cells via the bloodstream.
• These nutrients are mainly used for energy, growth, and repair.

Fatty Acids and Glycerol

• Fatty acids and glycerol enter the lacteal instead of blood capillaries.
• They are transported through the lymph system first.
• Later, they enter the bloodstream for use or storage.

⚡ Why the Small Intestine is Highly Efficient

• Huge surface area due to length, villi, and microvilli.
• Very thin walls reduce diffusion distance.
• Rich blood supply maintains concentration gradient.
• Lacteals help in fat transport.
• Continuous movement ensures better contact with nutrients.