Naked Body: Out of breath

What happens in your lungs to get oxygen into the body and carbon dioxide out? Stefan Marciniak's helpful answer means we don't have to hold our breath...

Most living things need to exchange gases like oxygen and carbon dioxide with their environment.  Humans do this by breathing.  At rest, an adult takes about 12 breaths of about 500 ml every minute – in that way we move about 6 litres of air into and out of our body per minute. 

We need to move air because we constantly consume oxygen to keep the body alive, and we produce carbon dioxide as a waste product.  If we don’t get enough oxygen, we can’t burn sugar and fat, which is our fuel, and we suffocate – first the brain stops working and we pass out because it needs so much power, but then other body systems start to fail.

If our blood vessels were just full of water, it wouldn’t be a very good system. Water can only carry about 3 ml of dissolved oxygen in every litre. So it’s lucky we have our blood. Our blood has specialised red blood cells packed with a protein called haemoglobin. This protein binds to oxygen, increasing the amount of oxygen that can be dissolved in blood to about 200 ml per litre.  Incidentally, haemoglobin gives blood its colour – changing from a dark red to a vivid bright red when it’s carrying oxygen.

To get oxygen into our blood, the lungs are made of millions of tiny air sacs called alveoli, each surrounded by blood vessels. Each time we breathe in, the air in these sacs is refreshed and more oxygen diffuses into the surrounding blood.  The heart pumps this oxygenated blood to our tissues where the lower levels of oxygen and higher levels of carbon dioxide cause the haemoglobin to release oxygen, which then diffuses into the cell where it’s needed.

Carbon dioxide, on the other hand, is constantly produced as cells burn fuel, but unlike oxygen, it’s a soluble gas so can be carried easily in the blood.  The problem is, however, that it’s acidic.  The more carbon dioxide dissolved in a fluid, the more acidic it becomes.  The chemical reactions that make our bodies work depend upon the acidity being kept within very specific limits.  If we become too acidic (say if carbon dioxide builds up because we stop breathing) our bodies begin to malfunction.  On the other hand, if we blow off too much carbon dioxide, our blood isn’t acidic enough causing different problems, such as nerves to misfire.  This is why hyperventilation (breathing too rapidly) causes us to feel pins-and-needles in our hands and around the mouth.  This can happen when we’re anxious, and only makes us feel worse.  Most of the gas we breathe is nitrogen, which makes up 80% of the air.  Nitrogen is inert, but at high pressures it can dissolve into the blood.  This happens during SCUBA diving and is why divers have to surface very gradually.  As they ascend slowly at the end of a dive, the nitrogen dissolved in their blood can be breathed out harmlessly.  But if they come up too quickly, bubbles form in tissues including joints (like bubbles forming when you open a bottle of fizzy-drink).  This causes severe pain that makes the diver double over – you may have heard of this as the “the bends”.

But if all that talk of the “the bends” is making you a little nervous, relax and take a deep breath, your lungs know what they’re doing.