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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Alimentary Canal Structure
The alimentary canal (also called the digestive tract or gut) is a long tube that runs from your mouth to your anus. It's where food is broken down into tiny pieces that your body can use. Think of it as a food processing factory inside your body!
Key Definitions:
The digestive system is made up of the alimentary canal plus accessory organs like the liver, pancreas and gallbladder. Together they work to break down food into nutrients your body can use for energy, growth and cell repair.
The food we eat is too big to be absorbed directly into our bloodstream. Digestion breaks down large food molecules (like proteins, carbohydrates and fats) into smaller ones that can pass through the walls of the small intestine and into our blood.
The alimentary canal is about 9 metres long and includes several different parts, each with its own special job. Let's explore each section from start to finish!
Your digestive journey begins in your mouth, where both mechanical and chemical digestion start.
Your teeth physically break down food by chewing (mastication). This increases the surface area for enzymes to work on.
Types of teeth:
Salivary glands release saliva containing the enzyme amylase, which begins breaking down starch into maltose (a sugar).
Saliva also contains mucus to lubricate food and make it easier to swallow.
After you swallow, food passes through the oesophagus (food pipe), which connects your mouth to your stomach.
The oesophagus uses wave-like muscle contractions called peristalsis to push food down to your stomach. These rhythmic contractions move food even if you're standing on your head!
A ring of muscle called the cardiac sphincter at the bottom of the oesophagus acts like a one-way valve. It relaxes to let food into the stomach but stays closed to prevent stomach acid from flowing back up.
The stomach is a muscular bag that churns and mixes food with digestive juices.
Contains hydrochloric acid (pH 1-2) which kills bacteria and denatures proteins.
Protease enzymes (like pepsin) begin breaking down proteins into smaller peptides.
After 2-3 hours, food is mixed with stomach secretions to form a semi-liquid called chyme.
Your stomach can stretch to hold about 1-1.5 litres of food! The inner lining of your stomach is replaced every 3-4 days because stomach acid is so strong it would digest the stomach itself without this constant renewal.
This is where most digestion and absorption happens. The small intestine is about 6-7 metres long but only about 2.5cm wide!
The inner lining of the small intestine is covered in tiny finger-like projections called villi. Each villus is covered with even smaller projections called microvilli (forming a "brush border").
This structure increases the surface area for absorption by about 600 times! More surface area means more nutrients can be absorbed at once.
Each villus contains:
The thin walls of villi (just one cell thick) allow for rapid diffusion of nutrients.
After the small intestine has absorbed most nutrients, the remaining material passes into the large intestine (colon).
The main job of the large intestine is to absorb water and mineral salts from undigested food. This turns the liquid waste into semi-solid faeces.
The large intestine contains billions of helpful bacteria that:
The rectum is the final section of the large intestine where faeces are stored before being eliminated through the anus. The anus contains two sphincter muscles (one voluntary and one involuntary) that control when you go to the toilet.
These organs aren't part of the alimentary canal itself but play crucial roles in digestion:
Produces bile, which is stored in the gallbladder. Bile emulsifies fats (breaks them into tiny droplets) to increase the surface area for fat-digesting enzymes.
Produces pancreatic juice containing enzymes that digest carbohydrates, proteins and fats. Also produces bicarbonate to neutralize stomach acid.
Stores and concentrates bile produced by the liver, releasing it into the duodenum when fatty food is present.
Lactose intolerance affects about 65% of the global population. People with this condition don't produce enough of the enzyme lactase, which breaks down lactose (milk sugar). When they consume dairy products, undigested lactose reaches the large intestine where bacteria ferment it, causing symptoms like bloating, gas and diarrhoea. This highlights the importance of specific enzymes in the digestive process.
Let's follow a cheese sandwich through your digestive system:
The whole journey takes about 24-72 hours, with most nutrients being absorbed within the first few hours.
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