Ask Glenn

Why Do We Breathe and Why Do We Need Lungs To Do It?

Question of the month for October 2007, asked by Hope

We breathe to capture oxygen, which helps us convert food into energy in our cells, and to get rid of carbon dioxide – the waste gas which is formed as this happens. Our lungs pump these gases in and out of our bodies, and help us dissolve them in and out of our blood.

So let me get this straight – we breathe so that our cells can eat food?

Not exactly, but close. It’s more like “we breathe so that our cells can breathe, and eat so that our cells can eat”.

What? They breathe, too? Now I’m really confused...

Okay, let’s go back a bit. If you think about it, cells are like little units of life. The smallest living things, bacteria, are just individual cells swimming about eating stuff. They absorb sugars or other nutrients from the air or liquid around them, and turn this “food” into energy – which they can use to grow, multiply, and squidge around looking for more food.

Got that.

Okay. Now some of these bacteria eventually evolved into bigger creatures like fish, frogs, lizards, monkeys and human beings. Those particular types of bacteria are called aerobic bacteria, which means...

...that they wore tracksuits and did lots of exercise?

Err…no. It means they have to “breathe” (or take in) oxygen in order to turn their food into energy.

Oh yeah, of course. I knew that. I mean, nobody makes tracksuits that small, for starters.

Err…right. Anyway, these bacteria and the animal cells they evolved into all need to take in oxygen for that same reason – to fuel their food-processors and produce energy.

But why do they need oxygen to do that? Couldn’t they do it without oxygen?

Well, they can for a while, at least. But the main power source for aerobic bacteria and cells comes from a chain reaction which uses oxygen – so they can’t survive without it for long. Oxygen and nutrients go into this reaction, then energy and carbon dioxide come out. The energy is stored and moved around in special molecules, while the carbon dioxide has to be removed from the cell, as it forms an acid if too much of it builds up. So in a way, aerobic bacteria “breathe in” oxygen and “breathe out” carbon dioxide. These bacteria evolved into complex animals by clustering together to form cells, tissues, organs, systems, and whole animal bodies. But since each cell still needs nutrients and oxygen for energy, the whole animal has to keep eating and breathing just to supply them.

Okay – that kind of makes sense. But why do bigger animals need lungs to breathe?

Well if you think about it, the bigger an animal gets, the more cells it has, and the further away from the air its insides are. Up to a certain size, animals can absorb oxygen through their skin and let it spread through their tissues. But most things bigger than flatworms need air tubes of some kind to get air to the inner parts of the body (that works for insects and spiders, at least). For larger, more complex or more active animals, you need some kind of gas-exchanging air pump. That’s where lungs come in.

But not all animals have lungs, right? I mean, fish don’t, do they?

Right – they don’t. Fish have gills instead. They swallow water and use their gills to absorb the oxygen dissolved in it. Then the gills dump carbon dioxide back into the water before pushing it out. That’s what’s happening when you see the gills “flapping” on a fish.

So why can’t we just swallow oxygen and burp out the carbon dioxide?

Well, then you’d be swallowing continuously, every few seconds, all day long. Like a big, walking air-fish. Not much fun, and kind of tricky to hold down conversations. Plus it wouldn’t work anyway, since swallowed air goes to your stomach - which is already specialized for digesting food, and too small for exchanging breathing gases.

Why does size matter?

You need a large surface area for the gas exchange to happen quickly enough between the air you take in and your bloodstream, which carries it to cells throughout the body. That’s where lungs come in handy. They’re more than just air sacks – they’re like giant air sponges which soak up and exchange gases very quickly through thousands of tiny bobbles, called alveoli.

You’re telling me I have pasta in my lungs?

No. That’s ravioli.

Oh. I knew that.

Alveoli are the clustered, berry-like bobbles found at the end of the branching air tubes deep in your lungs. They help increase the surface area of your lungs so much that if you flattened them all out into one sheet, it’d cover about 75 square meters – roughly the size of a tennis court. When you breathe in, muscles around and beneath the lungs help them suck air into the alveoli. The alveoli are covered in tiny blood vessels, so that oxygen can move into the bloodstream and be carried around the body. Carbon dioxide gas moves the other way – the bloodstream carries it out of organs and tissues and back to the air inside the alveoli, ready to be pushed out again as you breathe out. And there you have it – you and all your cells have all the oxygen they need, provided you keep breathing air, and avoid damaging all your delicate breathing equipment with dangerous fumes or cigarette smoke.

Got it. Don’t breathe cigarette smoke – or pasta – and your cells can keep doing aerobics.

Yes. Something like that.