Breathing System

The function of the breathing system is to take in oxygen from the air and get rid of carbon dioxide. This is called gas exchange. 

The diagram below shows the structure of the breathing system.

The lungs are found in the upper part of the body called the thorax.

They are protected by the ribcage and separated from the abdomen below by a sheet of muscle called the diaphragm.

Gas exchange occurs across the walls of the alveoli. They are adapted for this function in several ways:

• There are many of them, so they provide a very large surface area
• They have a moist surface, so oxygen can dissolve before diffusing into the blood
• They are surrounded by capillaries, so they have a good blood supply to carry the gases

Oxygen diffuses into the blood from the air in the alveoli and attaches to red blood cells to be transported around the body.

Carbon dioxide diffuses out of the blood plasma into the air in the lungs to be breathed out.

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Breathing in is called inhalation.

• The muscles between the ribs contract pulling the ribcage up and outwards
• At the same time, the diaphragm contracts and flattens
• These movements increase the volume of the thorax
• The pressure inside the thorax decreases so air is forced into the lungs

Breathing out is called exhalation.

• The muscles between the ribs relax so the ribcage moves down and in
• At the same time, the diaphragm relaxes and becomes dome-shaped
• These movements decrease the volume of the thorax
• The pressure inside the thorax increases so air is forced out of the lungs
  

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