AQA GCSE Biology

Revision Notes

2.3.1 The Lungs

The Lungs

Adaptations for gas exchange

  • All gas exchange surfaces have features to increase the efficiency of gas exchange including:
    • Large surface area to allow faster diffusion of gases across the surface
    • Thin walls to ensure diffusion distances remain short
    • Good ventilation with air so that diffusion gradients can be maintained
    • Good blood supply (dense capillary network) to maintain a high concentration gradient so diffusion occurs faster
  • Remember that gas exchange occurs by the process of diffusion; breathing is essential in maintaining high concentration gradients between the air in the alveoli and the gases dissolved in the blood
    • In particular, breathing keeps the oxygen level in the alveoli high and the carbon dioxide level low

Structures & functions of the lungs

Structures in the human breathing system, IGCSE & GCSE Biology revision notes

Each lung contains around 250 – 300 million alveoli, with the total surface area of each lung being around 70m2

Structure & function lungs table

The Lungs_1, downloadable IGCSE & GCSE Biology revision notes

Ventilation of the lungs

  • Air passes through the following structures when we breathe in:
    1. Trachea
    2. Bronchus (plural: bronchi)
    3. Bronchiole
    4. Alveoli (the site of gas exchange where oxygen diffuses into the blood and carbon dioxide diffuses out)
  • The diaphragm is a thin sheet of muscle that separates the chest cavity from the abdomen; it is ultimately responsible for controlling ventilation in the lungs
    • When the diaphragm contracts it flattens and this increases the volume of the chest cavity (thorax), which causes a decrease in air pressure inside the lungs relative to outside the body, drawing air in
    • When the diaphragm relaxes it moves upwards back into its domed shape and this decreases the volume of the chest cavity (thorax), which causes an increase in air pressure inside the lungs relative to outside the body, forcing air out
  • The external and internal intercostal muscles work as antagonistic pairs (meaning they work in different directions to each other)
  • During inhalation the external set of intercostal muscles contract to pull the ribs up and out:
    • This also increases the volume of the chest cavity (thorax), decreasing air pressure, drawing air in
  • During exhalation, the external set of intercostal muscles relax so the ribs drop down and in:
    • This decreases the volume of the chest cavity (thorax) increasing air pressure, forcing air out
  • When we need to increase the rate of gas exchange (for example during strenuous activity) the internal intercostal muscles will also work to pull the ribs down and in to decrease the volume of the thorax more, forcing air out more forcefully and quickly – this is called forced exhalation
    • There is actually a greater need to rid the body of increased levels of carbon dioxide produced during strenuous activity!
  • This allows a greater volume of gases to be exchanged

Inhalation, IGCSE & GCSE Biology revision notesExhalation, IGCSE & GCSE Biology revision notes

Exam Tip

You may see the terms inhalation OR inspiration (breathing in), and exhalation OR expiration (breathing out). Both sets of terms mean exactly the same thing, so don’t let them confuse you!

You should be able to explain how breathing occurs, referring to the intercostal muscles, the rib cage and the diaphragm, and the effect they all have on volume and pressure in the lungs. Remember, if you learn one, the other is almost exactly the opposite.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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