This is what happens when you are dehydrated

We’re often told to drink eight glasses of water a day – and much more if we are exercising. But just how much do we actually need? And what are the dangers of being dehydrated?

In this clip from the BBC’s Bang Goes The Theory, Dallas Campbell undergoes a gruelling workout without taking a sip to drink. By the time he’s finished, he has lost enough fluid for his blood to become significantly more viscous – causing his heart to pump extra hard to make up for it. But that’s the least of his worries compared with the striking effects to his mental reaction times – consequences you may want to consider when you drive home from the gym.