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Breathing in the Air : The Lungs

Model of the Human Rib Cage © Shirley Burchill

The Human Breathing System

The lungs are surrounded by the rib cage which protects them. The ribs also support the lungs and help to pump air in and out when we breathe. We call this ventilation.

The lungs are surrounded by the rib cage which protects them. The ribs also support the lungs and help to pump air in and out when we breathe. We call this ventilation.

The other part of the thorax which helps to pump air in and out is the diaphragm. The diaphragm is a sheet of fibre and muscle which rises and falls as we breathe.

Model of the Thorax (rib cage removed) © Shirley Burchill


On average an adult human breathes in (inspires) and breathes out (expires) ten to fourteen times a minutes when resting. This can increase to 25 times a minute during heavy exercise.

Each time we breathe we inspire and expire five to seven dm3 of air per minute, this is about half a dm3. This, however, is when we are resting. If we begin to exercise, not only do we breathe faster but we also breathe deeper.

An adult human has an extra 2,5dm3 of breath to call upon if needed. So when taking heavy exercise a man could be breathing in and out up to 120dm3 of air each minute.

Inspired and Expired Air

The air that we inspire is a mixture of gases. The most important of these are nitrogen, oxygen, carbon dioxide and water vapour. The air that we expire is not the same.


Inspired air (%)

Expired air (%)







Carbon dioxide



Water vapour



The water vapour in the atmosphere varies a lot. If it is raining the air is saturated with water vapour. The air that we expire is always full of water vapour. This is why, on a cold day, we breathe out clouds of mist from our mouths and noses. When the warm, wet air from our lungs meets the cold air of the atmosphere the water vapour condenses into mist.

The temperature of the air may also change. As we breathe in cold air it is warmed up to body temperature so that it does not harm our lungs.

The Route taken by the Air

Normally we breathe in through the nose. This allows us to chew food and breathe at the same time because the nose is separated from the mouth by a bone called the palate.

Model showing the nose and nasal cavities © Shirley Burchill

It is interesting to note that reptiles and amphibians do not have a palate, so when they eat something they must hold their breath.

The Nose - An Air-conditioning Unit

The nose is a biological air conditioning unit. It contains hairs which traps large dust particles. If there is a lot of dust in the air we may also sneeze to remove it from the nostrils. If the air is cold when it enters the nose it is warmed by the blood in the many capillaries which are found close to the walls of the nasal cavity. These blood vessels also provide water which humidifies the air if it is too dry. We can feel the difference between cold, dry air which is breathed into the lungs via the nose and the same type of air breathed in via the mouth.

Temperature (°C)

Inspired air

Expired air

- 70

+ 37



+ 50

+ 37

The air then passes into the throat and down the windpipe (trachea). The entrance of the trachea is protected by a valve, the epiglottis, which closes when we swallow food or drink. In this way the air passage is only blocked for a second so we do not have to hold our breath.

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