Why are the lungs good at their job? (Biology AS-level)

Sometimes when understanding a subject, it helps to know what the examiners want you to understand about it. For the lungs, it’s very clear what AQA and OCR want you to know: what the lungs need to have to do their job, and how they tick all those boxes in flesh and blood. So this revision guide will go through the lungs from that angle: what do the lungs need to be like to do their job, and how do they tick those boxes?

The lungs: good thing or bad thing?

Good. (That’s a joke, not one of those silly arts-subject ‘on the one hand, yes…but on the other hand, no…’ questions.) The lungs give your body oxygen and flush out carbon dioxide. You need that for respiration.

Skip this if you’re fine with this topic, but respiration is getting energy out of food, and the main type of it your body does, aerobic respiration, uses oxygen:
Respiration 1(Energy isn’t on the diagram because it’s not a chemical.)

So the lungs need to be good at getting oxygen in and carbon dioxide out.

Diffusion

For the lungs to work, oxygen needs to diffuse in from them to the blood, and carbon dioxide needs to diffuse out of the blood into the lungs. So what’s diffusion?

  • The movement of a chemical in gas or liquid
  • From high concentration to low concentration     
  • Down a concentration gradient (same thing)
  • A passive process, requiring no energy input

And to make diffusion happen from the blood to the lung air spaces fast, the lungs need to tick all these boxes:

  • The concentration gradient needs to be kept steep:
    the blood needs to be low in oxygen and the air spaces high in oxygen.
  • The barrier between blood and air needs to be thin.
  • There needs to be a low diffusion distance everywhere in the lungs:
    every air space in the lungs has to be close to a blood vessel.
  • The lungs have to have a large surface area:
    the more lining in the lungs, the more places there are where blood can pick up oxygen.

So the lungs needs to have a folded-up shape, with lots of surface area and lots of tiny blood vessels behind it. Now let’s see how they tick those boxes.

The structure of the lungs

The lungs have lots of air spaces, called alveoli. (Singular alveolus: it’s borrowed from Latin.) With a folded up structure, they maximise the surface area of lining in the lungs.

Alveolus_diagram

The alveolus wall is called epithelium: single, flattened-out cells. Behind them, there’s lots of tiny blood vessels, called capillaries. These, too, have walls only one cell thick. Capillary walls are called their endothelium. That means nowhere in the lungs is far from a capillary: diffusion distance is low.

Maintaining the concentration gradient

To keep the concentration steep, the lungs need to keep two things at extreme levels: there needs to be high-oxygen air in the alveoli, and low-oxygen blood in the air spaces.

That’s done by breathing and the heartbeat:

  1. breathing, or ventilation: moves old air out and new air in
  2. blood being pumped round the body: takes blood away once it’s picked up oxygen and replaces it with some more

The other thing breathing needs to make it work is for the lungs to be squishy. How breathing works is a separate post, but basically the lungs need to be squashed to push all the old air out and stretched to make space for the new air.

Other parts of the lungs you need to know about

This post is focusing on how the lungs work, but you need to know about a few other bits of the lungs:

  • the bronchi (singular bronchus) and bronchioles, big and then smaller tubes going into the lungs
  • the trachea or windpipe

Acknowledgement

The diagram of the alveolus was created by Mariana Ruiz Villarreal, alias Lady of Hats. (Twitter, blog.) It’s released into the public domain. All pictures used with thanks.

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