Revision Notes

11.1 The Breathing System

Features of Gas Exchange Surfaces

  • The surfaces where gas exchange occurs in an organism are very different and different organisms have evolved different mechanisms for getting the gases to the gas exchange surface depending on size, where they live etc.
  • All gas exchange surfaces have features in common
  • These features allow the maximum amount of gases to be exchanged across the surface in the smallest amount of time
  • They include:
    • 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 to maintain a high concentration gradient so diffusion occurs faster


The alveolus is the gas exchange surface in humans, IGCSE & GCSE Biology revision notesThe alveolus is the gas exchange surface in humans

Exam Tip

You may notice that several of the features of alveoli that make them suited to their function are the same as those that make villi suited to their function; or root hair cells suited to their function – the reason for this is because all of these structures are involved in transporting substances across their surfaces – by diffusion, active transport, osmosis or a combination.

So if you learn the features for one, you also know many of the features of the others!

Structure of the Breathing System

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

The Breathing System table, IGCSE & GCSE Biology revision notes

Extended Only

The Intercostal Muscles

  • Muscles are only able to pull on bones, not push on them
  • This means that there must be two sets of intercostal muscles; one to pull the rib cage up and another set to pull it down
  • One set of intercostal muscles is found on the outside of the ribcage (the external intercostal muscles)
  • The other set is found on the inside of the rib cage (the internal intercostal muscles)


The lungs, IGCSE & GCSE Biology revision notes

There are 2 sets of intercostal muscles: the external, on the outside of the rib cage, and the internal, on the inside of the rib cage

Extended Only

The Trachea

  • Rings of cartilage surround the trachea (and bronchi)
  • The function of the cartilage is to support the airways and keep them open during breathing
  • If they were not present then the sides could collapse inwards when the air pressure inside the tubes drops
Extended Only

The Function of Cilia & Mucus

  • The passages down to the lungs are lined with ciliated epithelial cells
  • Cilia comes from the Latin for eyelash, so unsurprisingly these cells have tiny hairs on the end of them that beat and push mucus up the passages towards the nose and throat where it can be removed
  • The mucus is made by special mucus-producing cells called goblet cells because they are shaped like a goblet, or cup
  • The mucus traps particles, pathogens like bacteria or viruses, and dust and prevents them getting into the lungs and damaging the cells there


Ciliated Cells, IGCSE & GCSE Biology revision notesMucus traps particles, dust and pathogens and cilia beat and push it up and away from the lungs

Exam Tip

The function of cilia and mucus is often a 3-mark question on the extended paper.

The examiners are looking for you to state the following:

  1. The mucus is produced by goblet cells and traps bacteria, dust, particles
  2. The cilia beat
  3. And push the mucus away from the lungs towards the throat

This is quite simple, but often marks are lost as students haven’t been precise enough with their explanations!

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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