Syllabus Edition

First teaching 2023

First exams 2025

|

Mechanism of Ventilation (HL IB Biology)

Revision Note

Marlene

Author

Marlene

Last updated

Ventilation: Mechanism

  • Ventilation is essential for the effective exchange of gases in the lungs
    • It replaces older air in the lungs with fresh air from the external environment
    • This helps to maintain the concentration gradient of oxygen and carbon dioxide between the alveoli and blood
  • Ventilation involves inspiration (breathing in) and expiration (breathing out)

Inspiration

  • The breathing-in, or inspiration, process causes the volume of the chest to increase and the air pressure to decrease until it is lower than the atmospheric pressure
    • When gas is in a large volume container that allows the gas particles to spread out, the pressure exerted by the gas on the walls of the container is low
  • As a result, air moves down the pressure gradient and rushes into the lungs
    • A gas will always move down a pressure gradient from an area of high pressure to an area of low pressure
  • The inspiration process
    • The diaphragm contracts and flattens, increasing chest volume
    • In addition to the flattening of the diaphragm the external intercostal muscles contract, causing the ribcage to move upwards and outwards; this also increases chest volume

Inhalation, IGCSE & GCSE Biology revision notes

The process of inspiration

Expiration

  • Breathing out, or expiration, occurs mostly due to the recoil of the lungs after they have been stretched by the inspiration process, and is therefore a mainly passive process
  • Volume of the chest decreases and pressure increases, causing air to be forced out down its pressure gradient
    • When gas is in a low volume container it is compressed, causing the gas particles to exert more pressure on the walls of the container
  • The passive expiration process
    • External intercostal muscles relax, allowing the ribcage to move down and in
    • Diaphragm relaxes and becomes dome-shaped
    • The recoil of elastic fibres in the alveoli walls reduces the volume of the lungs
  • The expiration process can be active when there is a need to expel excess air from the lungs e.g. when blowing out a candle
  • The active expiration process
    • Internal intercostal muscles contract to pull the ribs down and in
    • Abdominal muscles contract to push organs upwards against the diaphragm, decreasing the volume of the chest cavity
    • This causes forced exhalation

Exhalation, IGCSE & GCSE Biology revision notes

The process of passive expiration

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.