The respiratory system consists of all structures which are used in the process of breathing and supplying our bodies with Oxygen. This is the path that air takes when we breathe it in.

  • Mouth or Nose - air is warmed and filtered and passed through the larynx and pharynx
  • Trachea - or windpipe carries air towards the lungs
  • Bronchi - the trachea splits into two bronchi, one leads to the left lung and the other to the right
  • Bronchioles - once within the lungs the bronchi continue to divide into these smaller tubes
  • Alveoli - the bronchioles end in small sacs called alveoli. This is where the gas exchange takes place

Gas Exchange

Gas exchange refers to the passing of Oxygen from the alveoli into the blood flow and the excess Carbon Dioxide (CO2) within the blood flow passing back into the alveoli to be breathed out. Each tiny alveoli is covered in a network of capillaries (tiny blood vessels) which make this process easier.

          • We breathe in air, containing 21% Oxygen
          • The air reaches the alveoli where the Oxygen passes through the alveoli walls and into the surrounding capillaries
          • Here oxygen enters the red blood cells where it combines with haemoglobin to form oxyhaemoglobin
          • It can now travel around the body to where it is needed, our organs and muscles
          • At the same time, Carbon Dioxide which is a waste product is collected from the muscles and organs, into the blood stream
          • When back at the lungs the CO2 passes out of the blood, into the alveoli to be breathed out
          • The cycle continues as more Oxygen is received into the blood flow.....

We know that the body uses Oxygen and creates waste Carbon Dioxide because of the volumes of both gases in the air we breath in and out:

Air breathed in
Air breathed out
Carbon Dioxide


This table shows that we use some of the Oxygen we breathe in, as less is breathed out. It also shows that we produce CO2 as there is more in the air we breathe out.


          • Breathing in is also known as inspiration
          • Breathing out is also known as expiration
          • The intercostal muscles are positioned inbetween our ribs
          • The Diaphragm is a sheet of muscle which sits under the ribs and lungs


In order to draw air into our lungs, the volume of the chest, or thoracic cavity must increase. This occurs because the Intercostal muscles and the diaphragm contract. The rib cage moves up and out and the diaphragm flattens to increase the space. This decreases the air pressure within our lungs, causing air to rush in from outside.


At the end of a breath, the intercostal muscles and diaphragm relax, returning to their starting position, which decreases the size of the thoracic cavity. The decreased space and increased air pressure in the lungs forces air out

Lung Capacity

Our lungs can hold varying amount of air, depending on how deeply and quickly we breathe. They are also never empty, even if you breathe out as far as you can. Here are some terms used to describe some of these volumes:

Tidal volume - The amount of air you breathe in or out with each breath

Inspiratory capacity - The maximum amount you can breathe in (after a normal breath out)

Expiratory reserve volume - After breathing out normally, this is the extra amount you can breathe out

Inspiratory reserve volume - After breathing in normally, this is the extra amount you can breathe in

Vital capacity - The maximum amount of air you could possibly breathe in or out in one breath

Residual volume - The amount of air left in your lungs after you have breathed out as much as possible

As we exercise, our need for Oxygen increases. This means that the amount we breathe in and pump around our bodies in the blood must change to keep up. To achieve this, we breathe faster and our heart pumps faster.

This increased oxygen uptake, is measure by your VO2, or the amount of oxygen your body uses in a minute. This can be used as a prediction of your fitness level. The maximum VO2 is called VO2 Max and the fitter you are the higher this is because your body is more effective at taking in and using oxygen.