The Respiratory System

Aerobic respiration requires gaseous exchange and we can break this down into two parts. Gaseous is related to gases and exchange is related to giving and taking (a transfer).

The two gases exchanged in the respiratory system are oxygen and carbon dioxide. It contains organs that allow us to get the oxygen we need and It removes carbon dioxide as waste that we don’t need.

The respiratory system contains:

  • Two lungs
  • Tubes that lead from the entrance (mouth and nose) to the lungs
  • Different structures in the chest that allow air to move in and out of the lungs

Air Travelling Through the Respiratory System

1. We take air in through our nose and mouth

2. This air goes down the trachea (also known as the windpipe), towards the lungs

3. Once the air reaches the lungs, it moves into two smaller tubes called bronchi

  • There is one bronchus in each lung

4. The bronchi then split into smaller tubes called bronchioles, which the air moves into

5. At the end of each bronchiole, the air ends up in microscopic air sacs called alveoli

  • Each lung contains hundreds of millions of alveoli


The process of ventilation (breathing), involves the movement of:

  • Ribs – The bones protecting the respiratory system
  • Intercostal muscles – Muscles between the ribs
  • The diaphragm – Smooth muscle at the bottom of the ribcage

Inhalation (Breathing in)

When we inhale, the diaphragm and intercostal muscles contract, causing the ribcage to move outwards and upwards. This increases the volume of the lungs, which decreases the pressure of the lungs.

As the pressure inside the lungs is lower than outside the body, air is drawn into the lungs. This equalised the pressure.

Exhalation (Breathing out)

When we exhale air, the diaphragm and intercostal muscles relax, causing the ribcage to move downwards and inwards. The volume of the lungs decreases, which increases the pressure of the lungs.

As the pressure in the lungs is greater than outside the body, air is forced out of the lungs. This equalises the pressure.

Measuring Lung Volume

To measure lung volume using a measuring cylinder, marker pen, plastic or rubber tubing, bowl of water, and large plastic container:

1. Connect one end of the tubing to the mouth and the other end to the measuring cylinder.

2. Fill the large plastic container with water and place the measuring cylinder in it.

3. Take a deep breath in and exhale forcefully through the tubing, pushing the water out of the measuring cylinder and into the bowl.

4. Use the marker pen to mark the level of the water in the measuring cylinder after the exhalation.

5. The volume of air exhaled can be calculated by subtracting the initial volume of water in the measuring cylinder from the final volume of water, as marked on the cylinder.

6. This value is equal to the volume of air in the lungs.