What happens when you hold your breath for a long time?

Anaerobic respiration is happening. This is so you get a different biochemical pathway that your body uses when there isn't oxygen as an energy source available. You produce lots of lactate, lots of CO2 and there's lots of hydrogen protons in the blood. That makes it acidic. Everybody has buffers in their blood to counteract the effects of this anaerobic respiration: buffers like bicarbonate which is alkaline so it means that you buffer the pH of your blood to keep it neutral. So these are chemicals that soak up acid and essentially stop the acid being just acid. You can put them into your bloodstream and the blood won't become acidic because the acid has bound onto something. That's the buffer.

You can find in some people that can hold their breath for a particularly long period of time that they'll excrete more of these buffers because they'll be producing higher levels of them [lactates] in the blood. In people who can hold their breath for a very long period of time they'll be very practised at it so they'll be very relaxed. This is true if you've ever had a scuba diving lesson you might have found you were very nervous. So when you practise with these things, the same as scuba diving and holding your breath. You can slow down your respiration and relax a lot more so you can hold your breath for longer.