MAVO Year 4 · Nutrition and digestion · Section 12.4

The digestive system

What is digestion?

Digestion breaks large food molecules, such as proteins, fats and starch, into smaller molecules suitable for absorption. Glucose, water, mineral ions and many vitamins do not need this breakdown first. Most absorbed nutrients enter the blood, while much dietary fat initially enters lymphatic vessels.

The digestive tract and its secretions

Food passes through the mouth, oesophagus, stomach, duodenum, remaining small intestine, large intestine, rectum and anus. The duodenum is the first part of the small intestine, not a separate organ after it. Along the route, food mixes with digestive secretions, including saliva, gastric juice, bile and pancreatic juice. Many contain enzymes; bile helps fat digestion without containing digestive enzymes itself.

Digestive glands

Salivary glands, glands in the stomach and intestinal lining, the liver and the pancreas produce substances supporting digestion. Different secretions act on different components or provide suitable conditions for enzymes. Their combined action makes nutrients available for absorption.

Peristalsis

Coordinated contractions of circular and longitudinal muscles in the gut wall move and mix its contents. Peristalsis propels food along; other muscular movements help mix it with secretions. The process does not depend simply on gravity, so swallowed food can still move towards the stomach in different body positions.

Bowel function

Dietary fibre influences stool bulk, water retention and gut microorganisms, helping normal bowel movements. Sufficient fluids and regular activity also contribute. The remaining unabsorbed material passes towards the rectum for elimination.