By the end of this lesson
- Trace food through the alimentary canal in order.
- Identify functions of named organs.
- Distinguish canal organs from accessory organs.
BIG QUESTION
How does a meal become small soluble molecules that can enter the internal environment?
A continuous tube moves food while specialised regions digest, absorb water and nutrients, and remove undigested material.
The canal is a continuous pathway
Food passes through mouth → oesophagus → stomach → duodenum → ileum → colon → rectum → anus. The duodenum and ileum form the small intestine; the colon and rectum form most of the large intestine.
The sequence matters because each region creates conditions for the next.
Early stages process and digest
Teeth mechanically break food and saliva lubricates it; salivary amylase begins starch digestion. The oesophagus moves the bolus by peristalsis.
The stomach churns food, secretes acid and protease, and releases chyme gradually into the duodenum.
The small intestine completes digestion and absorbs
The duodenumThe first part of the small intestine, where pancreatic juice and bile are added to food from the stomach. receives pancreatic enzymes and bile. Digestion continues at the intestinal surface.
The ileumThe last part of the small intestine, where digested food molecules are absorbed into the blood and lacteals through the villi. provides a large adapted surface for absorption of digested nutrients.
The large intestine conserves water
The colonThe first part of the large intestine, where water and some ions are absorbed from undigested material. absorbs water and some ions from remaining contents. The rectum stores faeces before egestion through the anus.
Egestion is removal of undigested material; it is not excretion of metabolic waste.
Accessory organs add secretions
The pancreasAn organ that secretes digestive enzymes (amylase, protease and lipase) and alkaline fluid into the duodenum, and also releases the hormone insulin. secretes digestive enzymes and alkaline fluid into the duodenum. The liver produces bile and the gall bladder stores and concentrates it.
Food does not pass through these accessory organs.
Organ sequence and function
Trace one meal without skipping junctions.
| Region | Main role | Key event |
|---|---|---|
| Mouth | Ingestion and mechanical digestion. | Chewing; saliva starts starch digestion. |
| Oesophagus | Transport. | Peristalsis moves the bolus. |
| Stomach | Storage, churning and protein digestion. | Acidic chyme forms. |
| Duodenum | Chemical digestion. | Receives bile and pancreatic juice. |
| Ileum | Absorption. | Digested molecules cross villi. |
| Colon | Water absorption. | Remaining contents become more solid. |
| Rectum and anus | Storage and egestion. | Faeces leave the body. |
| Pancreas | Accessory secretion. | Enzymes and alkaline fluid enter duodenum. |
Language that earns precision
- alimentary canal
- The muscular tube through which food passes from mouth to anus.
- ingestion
- Taking food into the body.
- digestion
- Mechanical and chemical breakdown of food.
- absorption
- Movement of digested molecules into blood or lymph.
- egestion
- Removal of undigested material as faeces.
Common misconceptions
Claim: Food passes through the pancreas.
Correction: The pancreas is accessory and secretes into the duodenum.
Claim: Most nutrients are absorbed in the stomach.
Correction: Most absorption occurs across the small intestine.
Claim: Egestion is excretion.
Correction: Egestion removes undigested food; excretion removes metabolic waste.
Claim: The large intestine is longer than the small intestine.
Correction: It is wider but much shorter.
Match the answer to the command word
Use the full anatomical name and check the order from mouth to anus.
Separate movement, digestion and absorption.
Name both the secretion and where it enters the canal.
Recall, apply, then justify
Give the route taken by food from mouth to anus.
Hint
Include the two named parts of each intestine.
Suggested marking points
- Mouth → oesophagus → stomach.
- Stomach → duodenum → ileum.
- Ileum → colon → rectum.
- Rectum → anus.
Explain the roles of the stomach, duodenum and ileum.
Hint
Two precise points per region.
Suggested marking points
- Stomach churns food.
- Stomach acid/protease supports protein digestion.
- Duodenum receives bile and pancreatic enzymes.
- Chemical digestion continues in the duodenum.
- Ileum has an adapted surface with villi.
- Digested nutrients are absorbed in the ileum.
Distinguish egestion from excretion.
Hint
Undigested versus metabolic.
Suggested marking points
- Egestion removes undigested/unabsorbed material from the gut.
- Excretion removes metabolic waste produced by cells.
Predict the consequences of a blockage preventing pancreatic secretions reaching the duodenum.
Planning hint
Consider enzyme classes, pH and absorption.
What a strong design includes
- Less pancreatic amylase would reduce starch digestion.
- Less protease would reduce protein digestion.
- Less lipase would reduce lipid digestion.
- Reduced alkaline fluid would leave chyme more acidic.
- Fewer small soluble products would be available for absorption.
- Undigested nutrients could alter water movement and microbial activity in the gut.