Gaseous Exchange

What are my objectives while studying this topic?

By studying this topic you’ll learn about the alveoli and health issues relating to the rate of diffusion.

Why is this important?

The rate of diffusion is related to asthma, emphysema and the treatment of premature babies.

Can I get an introduction to this topic?

Yes, why not have a look at the following image:


Electron micrograph of an alveolus and capillaries.

So, let’s get to it: what is gaseous exchange?

The main function of the respiratory system is gaseous exchange. This is when oxygen and carbon dioxide moves between the lungs and blood.

Diffusion occurs when molecules move from an area of high concentration (of that molecule) to an area of low concentration. This occurs as the blood in the capillaries surrounding the alveoli has a lower oxygen concentration of Oxygen than the air in the alveoli which has just been inhaled.

Both alveoli and capillaries have thin walls (only one cell thick) and allow gases to diffuse across them. This also happens with carbon dioxide (CO2). The blood in the surrounding capillaries has a higher concentration of CO2 than the inspired air because it is a waste product of energy production. Therefore CO2 diffuses the other way, from the capillaries, into the alveoli where it is exhaled.

What will I be expected to be able to do after studying this topic?

  • Know and be able to describe the structure of alveoli

  • Be able to explain the adaptations of the alveoli that maximize the rate of diffusion

  • Be able to relate the factors affecting the rate of diffusion to pulmonary disorders and the treatment of premature babies

What are the key words to learn and use?


  • Surfactant

  • Asthma

  • Emphysema

How long will this topic take to study?

On average you’ll be looking at about three hours of study time.

What are the main activities on this topic?


Determine the rate of diffusion using pH paper threaded on a string at different intervals inside a tube. The tube will be fitted with bungs and a cotton wool ball soaked either in conc. acid or ammonia. Repetition of the experiment with dampened pH paper may show a different result. Relate this experiment to the theory of gaseous exchange.


Find out how exercise affects the ventilation rate, and how this is regulated in response to changing carbon dioxide levels in the blood. You should be able to recognise and describe what these traces show about an individual’s pulmonary function.

Discuss the diagnostic capability of spirometry measurements for pulmonary problems using the analysis of data showing the typical values as given in the specification: ‘Normal values for some physiological measurements’, page 15, table 1.


Through discussion or a ‘Market-Place’ activity, produce a mind map or a table summarising gaseous exchange. You will need to take the following into consideration:

  • Maintaining a moist alveolar surface

  • The transport mediums for the gases

  • The composition of alveolar air

  • Maximising transfer through efficient transport, large surface area and both capillary walls. These walls are very thin as they are made up of just one layer of flattened cells, epithelium cells, and the narrow lumen of the capillaries.

Additionally, consider the changes to the gaseous exchange rate in asthmatics through restriction in the bronchioles and fluid/mucus-filled alveoli, people with emphysema due to the breakdown of the thin walls of the alveoli, and new born premature babies and their lack of surfactant.

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