Regions of the Alimentary Canal
| Region | Function | Key details |
|---|---|---|
| Mouth | Mechanical digestion (teeth chew food); chemical digestion begins | Salivary glands produce saliva containing amylase (starch → maltose). Tongue shapes food into a bolus. |
| Oesophagus | Transports food from mouth to stomach | Peristalsis: waves of muscular contraction push the bolus down. No digestion occurs here. |
| Stomach | Protein digestion; kills bacteria | Produces HCl (pH 2) and pepsin (protease). Churns food mechanically, mixing it with gastric juice to form chyme. |
| Small intestine | Main site of chemical digestion and absorption | Duodenum receives bile and pancreatic juice. Ileum is lined with villi for absorption of digested nutrients. |
| Large intestine | Absorbs water from undigested food | Remaining material becomes more solid as water is absorbed. Contains bacteria that produce vitamin K. |
| Rectum | Stores faeces temporarily | Faeces = undigested food, dead cells, bacteria, fibre. |
| Anus | Egestion of faeces | A muscular sphincter controls the release of faeces. |
Associated Organs
Three organs are connected to the alimentary canal but food does not pass through them:
- Salivary glands: produce saliva containing salivary amylase and mucus. Amylase begins starch digestion in the mouth.
- Liver: produces bile, which is stored in the gall bladder. Bile emulsifies fats and neutralises stomach acid in the duodenum.
- Pancreas: produces pancreatic juice containing amylase, protease (trypsin) and lipase. These enzymes complete chemical digestion in the small intestine.
Exam note: When a question asks about "the digestive system", include these associated organs. When it asks about "the alimentary canal", only include the tube from mouth to anus.
Egestion vs Excretion
Do not confuse these terms:
- Egestion: the removal of undigested food from the body through the anus. The food was never absorbed into the body: it simply passed through the gut.
- Excretion: the removal of waste products of metabolism (chemical reactions in cells). Examples: CO₂ from respiration (exhaled), urea from deamination (excreted in urine).
Faeces contain undigested food, fibre, dead cells from the gut lining, bacteria, and mucus. Since these materials were never part of the body's metabolism, their removal is egestion, not excretion.