The digestive system breaks down large, insoluble food molecules into small, soluble ones that body cells can use. In humans it consists of a long muscular tube called the alimentary canal together with several accessory organs that release useful secretions into the canal.
The alimentary canal
Food passes in one direction through: mouth → oesophagus → stomach → small intestine (duodenum and ileum) → large intestine (colon and rectum) → anus. The wall of the canal has layers of circular and longitudinal muscle. Their alternate contraction and relaxation produce waves called peristalsis that push food along, even against gravity.
Accessory organs
- Salivary glands — release saliva containing salivary amylase.
- Liver — produces bile, which emulsifies fats.
- Gall bladder — stores bile before it flows into the duodenum.
- Pancreas — secretes pancreatic juice with several digestive enzymes.
These organs do not have food passing through them, yet their secretions are essential for digestion. Each region of the canal is also specialised: the mouth has teeth for chewing and glands for saliva, the stomach has a thick muscular wall that churns food and cells that make acid, and the small intestine has a folded inner surface for absorbing nutrients. This division of labour lets each part carry out its job efficiently.
Key idea
Nutrition in humans involves five processes: ingestion (taking in food), digestion (breaking it down), absorption (taking nutrients into blood), assimilation (using nutrients in cells) and defaecation (removing undigested matter).
Why break food down?
Large molecules such as starch and protein cannot cross the intestinal wall. Digestion converts them into small molecules like glucose and amino acids that are small enough to be absorbed and transported to cells.
Example
When you swallow a mouthful of rice, peristalsis in the oesophagus carries it to the stomach in a few seconds. Salivary amylase from an accessory gland has already begun digesting the starch even before swallowing.
Remember
The alimentary canal is the tube food travels through; accessory organs sit beside it and pour in secretions. Peristalsis, not gravity, moves food.