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Lungs and gas exchange

Reflections and Exam tips


Animals need to take in oxygen for respiration. Respiration is a process during which oxygen reacts with glucose to release energy.

Single celled organisms like Amoeba, bacteria and Euglena can get their oxygen straight from the air by simple diffusion. They do not need specialised breathing systems because they have a large surface area to volume ratio.For them, being small is an advantage.

Humans and other animals have a small surface area to volume ratio. In simple terms, this means that there are a lot of cells hidden inside them, away from direct contact with oxygen in the air. This is not good! So they developed specialised breathing systems like gills and lungs.

The human breathing (respiratory) system is made of the nose (mouth as well), the nasal cavity (buccal cavity), the trachea (windpipe), which divides into 2 bronchi, which subdivide inside the left and right lungs to form smaller bronchioles which end in millions of tiny pockets called alveoli (alveolus for 1)

Alveoli are the site (place) where gas exchange takes place (happens). This is where oxygen goes into the blood and is carried away to muscles and other organs which need it. carbon dioxide (a waste product of respiration in muscles and organs diffuses from the blood into the alveoli and is breathed out.

Mechanism of breathing

Breathing in is also called inhaling (inspiring), while breathing out is exhaling (expiring). Inhaling and exhaling are brought about by the rib muscles (intercoastal muscles) and the diaphragm.

The changes in these parts of the rib cage change the pressure of the chest cavity compared to outside air pressure, causing air to either move in or out.

To understand this, look at the pressure vs volume diagram below:

As volume increases, pressure decreases. When volume decreases, pressure increases.


Practical Exercise:

Take a deep breath in and hold it. What do you notice about the movement of your chest cavity (rib cage)?

When inhaling:

  • The rib cage moves up and out.
  • The diaphragm (a muscle at the bottom of the rib cage) flattens.
  • All this increases the volume of the rib cage, including the lungs.
  • So pressure decreases (goes down) below atmospheric pressure.
  • Oxygen rich air from outside rushes in.


When breathing out, the opposite happens:

  • The rib cage moves down and in, the diaphragm moves up (to form a dome shape).
  • The volume of the chest cavity decreases, so the pressure inside the chest cavity increases forcing air out to lower pressure outside.
  • Carbon dioxide rich air is exhaled.

Comparing inhaled and exhaled air

Which gas goes down after exhaling? Why does it go down in exhaled air?

Which gas goes up after exhaling? Why does it go up in exhaled air?

Which gas remains the same? Why does it remain the same?