You can hold your breath, but soon you must breathe. why is this?

You can hold your breath for sometime, but you must breath to take in oxygen and give out the carbon dioxide formed in the body. The buil up of poisonous carbon dioxide can lead to failure of lungs and heart if it is not breathed out. Oxygen is needed by our system to mix with the blood and help in continuation of all the body functions. In case a person is unable to breath in oxygen and breath out carbon dioxide for sometime, then the person will surely die because of choking. People die under water because they can breath out the carbon dioxide but cannot breath in oxygen and this makes the person 's lungs fail.