Structure of the digestive system (Nutrition, digestion and excretion)

 

Digestive system

The digestive system consists of key parts, each of which has a specific function. Most of these parts are part of the food’s journey from the mouth to the anus.

 

 

Diagram showing the parts of the digestive system

 

Parts of the digestive system and their function

 

• Mouth: The mouth begins the process of digestion. The teeth chop up the food and mix it with the enzymes in saliva.
• Oesophagus: This is a narrow tube that carries the food from the mouth to the stomach.
• Liver: This produces a chemical called bile that goes into the intestines. Bile helps to break down fats in the food.
• Stomach: This is a muscular sac that churns the food and drink with acid.
• Pancreas: This makes enzymes that go into the intestines and help to break down carbohydrates, protein and fats in food.
• Small intestine: Here, carbohydrates, proteins and fats are digested. The nutrients that are made are then absorbed into the blood.
• Large intestine: Food that cannot be digested - mainly fibre - moves into the large intestine. Water is taken out and goes into the blood.
• Rectum: Any undigested food stays in the rectum where it is stored as faeces.
• Anus: This is the opening at the very end of the digestive system where faeces leaves the body.

 

What are the processes involved in digestion?

 

1. Mechanical and chemical digestion

 

The mouth uses teeth to break down food mechanically, which means they crush it. Chemical digestion happens when enzymes change food into nutrients.
The liver makes bile and stores it in the gall bladder. This goes into the small intestine to split big molecules of fats into smaller ones. This is not digestion because it does not change fats into nutrients, just smaller drops. This is called emulsification and makes more surface area for enzymes to work on.

 

2. Movement of food through the digestive system

 

Muscles in the lining of the oesophagus, and small and large intestines push the digested food along by squeezing it in a process called peristalsis. It is like squeezing a tennis ball through a pair of tights.

 

3. Absorption of food and water

 

The intestines have many small folds that make a large surface area for taking in nutrients and water. In the small intestine there are also special cells called villi that have tiny hair-like structures sticking into the small intestine to take in digested food.

 

Summary:

 

• The blood absorbs tiny particles that the digestive system makes by breaking down food.
• These particles give the body energy to grow, heal itself and stay healthy.
• The body gets rid of food that cannot be broken down as faeces (poo).