7.2 Digestive System

The digestive system is a group of organs that work together to perform a particular function. The human digestive system includes the alimentary canal and also the liver and pancreas. All of these organs work together to break down the food that we eat, so that it can be absorbed into the blood and delivered to all body cells.

The digestive system deals with food through a series of processes:

Ingestion is the taking of food and drink into the body. We do this using the lips, teeth and tongue.

Digestion is the breakdown of food. The food that we eat contains large pieces and large molecules. These must be broken down:

• Physical digestion: the breakdown of food into smaller pieces, without making any chemical changes to the molecules in the food
• Chemical digestion: the breakdown of large molecules in food into smaller molecules, so that they can be absorbed

The small molecules can then get through the walls of the intestines and into the blood.

Absorption is the movement of nutrients from the alimentary canal into the blood. This occurs primarily in the small intestine.

Assimilation is the uptake and use of nutrients by cells. This is what happens to the nutrients as they become part of the body. They are absorbed by individual cells and used for energy or to make new substances.

Egestion is the removal of undigested food from the body as faeces. There is always some material in our food that we cannot digest. Much of this is fibre. This is not absorbed. It remains in the intestines and eventually passes out as faeces.