The Alimentary Canal: The Small Intestine

1. Mouth
2. Oesophagus
3. Stomach
4. Small intestine
5. Liver
6. Pancreas
7. Large intestine

4. Small Intestine

The small intestine is a 6-meter-long tube with a small diameter, divided into two main parts:

8. Duodenum
9. Ileum

Duodenum (D for Duodenum, D for Digestion)

• The first 25 cm of the small intestine.
• Food is forced backwards and forwards by peristalsis to ensure digestion takes place.
• Cells lining the duodenum produce digestive enzymes.
• The liver and pancreas release substances (e.g., bile, pancreatic juice) into the duodenum to assist in digestion.

Ileum

• The ileum is the second part of the small intestine.
• The function of the ileum is absorption.
• The ileum has numerous infoldings called villi. These increase the surface area for absorption.
• Each villus contains a lacteal, which is filled with lymph.
• Fatty acids and glycerol are absorbed into the lacteal and transported to the bloodstream near the neck.

Adaptations of the Small Intestine for Absorption

1. Villi increase surface area, improving nutrient absorption.
2. Villi have a rich capillary blood supply. This assists the transport of absorbed nutrients.
3. Lacteals in villi absorb fatty acids and glycerol.
4. Length of the small intestine allows sufficient time for absorption.