Human Alimentary Canal

Chewing, swallowing, digestion.

What you need to know (from the curriculum)...

  • Define ingestion as taking substances (food, drink) into the body through the mouth

  • Define egestion as passing out food that has not been digested, as faeces, through the anus.

  • Identify the main regions/organs of the alimentary canal including mouth, salivary glands, oesphagus, stomach, small intestine (duodenum and ileum), pancreas, liver, gall bladder, large intestine (colon and rectum), anus.

  • Describe the functions of the organs/regions listed above in relation to ingestion, digestion, absorbtion, assimilation and egestion of food.

  • Define digestion as the break down of large insoluble molecules into small, water-soluble molecules using mechanical and chemical processes.

  • Describe the role of longitudinal and circular muscles in peristalsis

  • Outline the role of bile in emulsifying fats, to increase the surface area for the action of enzymes.

  • State where amylase, protease and lipase enzymes are secreted, and state the function listing the substrate and end product.

Lets see what you already know...

The basics of digestion

Watch this 3 minute video clip and answer the questions as you go....

Some key words that you need to know

The Mouth

  • Where ingestion happens.

  • The mouth contains a set of teeth to mechanically breakdown food into smaller pieces, this is to increase the surface area that enzymes can work on.

  • The tongue is very important in shaping the food into a ball called a bolus that will be passed to the oesophagus for swallowing.

  • Salivary glands secrete amylase enzymes into the mouth to begin the chemical digestion of starch (a type of carbohydrate).
Saliva does not just contains enzymes, but it covers the food in mucas which allows it pass smoothly through the digestive system.

Mouth function

Class experiment with crisps and amylase
We are going to recreate the mouth using scissors, amylase and ....


  • The oesphagus connects the mouth with the stomach

  • It is a muscular tube made from two types of muscle

  1. Longitudinal
  2. Circular

  • As seen in the first video (at the top) the bolus is moved down the oesaphagus by waves of muscular contraction.

  • Behind the bolus the circular muscle contracts and the longitudinal muscle relaxes.

  • In front of the bolus the longitudinal muscle contracts and the circular muscle relaxes

  • This wave of muscular contraction and relaxation is called peristalsis.

Oesophageal cancer

Did you know....

  • Smoking can cause cancer in organs of the digestive system?

  • This happens when the carcinogens (cancer causing chemicals) from cigarettes are swallowed (this happens a lot!)

  • During and after smoking a cigarette many of the carcinogens get mixed in with saliva and phlegm.

  • Cilia from the throat move the carcinogens up to the mouth where they are then swallowed and move through the entire digestive system, potentially causing cancer anywhere between the mouth and the anus.


Chemical and mechanical digestion

Small intestine

  • The first part of the small intestine is called the duodenum

  • The second part is called the illeum

  • At this point the food is a highly acidic watery liquid (called acid chyme). The pancreas is a very important organ that secretes alkali into the duodenum to help neutralise the acid chyme.

  • The pancreas also makes enzymes that are secreted into the duodenum

  1. Amylase (carbohydrase)
  2. Trypsin (protease)
  3. Lipase

  • As you can you on the above image, the liver and gall bladder also connect to the duodenum. The liver produces bile that is used to emulsify fat. Bile is stored in the gall bladder until it is needed.

  • The walls of the small intestines are covered in microscopic structures called villi, which have two great benefits for digestion:

  1. vastly increase the surface area for absorption and are
  2. they are only one cell thick to allow food molecules to pass through to the blood easily.

REMEMBER: Fibre does not get absorbed at any point and will continue its journey to be egested through the anus, cleaning the intestines as it moves.

Can you identify the villi from this image?

Processes of Small Intestine - Physiology

What happens if we have too many nutrients?

The Liver deals with excess....

  • Excess carbohydrates are turned into glycogen (a storage molecule that can be used later)
  • Excess amino acids are taken to the liver where deamination occurs.

Deamination is the removal of the amine group (NH2) molecule from an amino acid, this is converted into urea and released back into the blood where it is taken to the kidneys for excretion.

Large intestine


The main function is the reabsorption of water from food.

It is about 1.5m long

Processes of Large Intestine - Physiology


Undigested food leaving the body

Egestion occurs through the anus

Faces are formed in the large intestine and stored in the rectum before being egested through the anus.

Exam questions

Paper 1

Paper 3