Intermediate 
How long can you hold your breath? Im trying it right now. The  rst 30 seconds are easy. Im ready to give up at 45 seconds but I continue and it seems to get easier for a while. But, as the second hand ticks past a minute, my heart is pounding. I let out a tiny breath and this helps. Eventually, I give up, releasing the air in my lungs and taking a huge breath. I manage one minute and 12 seconds. Im quite impressed with myself.
The ability to hold your breath is extremely important in some sports, particularly freediving. In 2006, I was  lming a programme about the anatomy and physiology of the lungs for a BBC series. I was lucky to meet Sam Amps, who was captain of the UK freedive team. At a pool in Bristol, she taught me some simple exercises to help me hold my breath for longer while swimming underwater. By the end of the session, Id managed 90 seconds of breath-holding, enough to let me swim a width. Sam swam three widths easily. She could hold her breath for  ve minutes, while swimming. Five!
I asked how she did it: very slow breathing for several minutes before each dive, then a big, deep breath before diving in. She also said that training helped her resist the urge to breathe for far longer than most people.
Some have suggested that the ability to voluntarily hold your breath is evidence that, at some point during our evolution, we lived in water. Some even say that humans have an ability to lower their heart rate in order to breath-hold for even longer. Other facts  our hairlessness, the distribution of our subcutaneous fat and even that we walk on two legs  have been linked to an aquatic phase of our evolutionary development. But, unfortunately, the aquatic ape hypothesis is not true.
Looking at voluntary breath-holding, we are, in fact, not the only non-aquatic mammals that are able to hold their breath. (But its a dif cult thing to investigate in other mammals because, unlike humans, its dif cult to get them to hold their breath.) And evidence shows that our heart rate doesnt drop during breath-holding. At least, it doesnt if youre breath-holding on land. When youre in cold water, its different: this leads to a slower heart rate in most people. But, once again, this isnt evidence of an aquatic ape ancestry. This reduction in heart rate is just one of the physiological responses that are sometimes called the mammalian diving re ex. But physiological responses that are useful in diving are also  and, perhaps, even more importantly  useful for not drowning.
While our ability to breath-hold may not be special, when we compare ourselves with other animals, its now becoming very useful in one particular area of medicine. Radiotherapy for breast cancer involves pointing radiation, very precisely, at the tumour. This may require several minutes of radiation and, so, its usually done in short periods, between breaths. But, if the patient can keep her chest perfectly still for several minutes, it means that the entire dose can be given, in the right place, in one go. The problem, of course, is that most people, just like me, cannot hold their breath for much longer than a minute. But doctors at University Hospital Birmingham have shown that, if patients are given oxygen-rich air before holding their breath, they can hold it for  ve-and-a-half minutes. 
Surprisingly, the trick seems to be the ability to fool the diaphragm. When you breathe in, youre pulling the muscle of your diaphragm  at so that the volume of your chest increases  and air is pulled into your lungs. When you hold your breath, you keep your diaphragm like that. If you arti cially raise oxygen levels and reduce carbon dioxide levels before a breath-hold, as in the Birmingham radiotherapy experiments, you may be able to delay tiredness in the diaphragm. So, its your diaphragm, the main muscle of breathing, that is in charge when it comes to holding your breath. Eventually, even if youve fooled it for a while, the signals from the diaphragm are just too strong and you have to give up  and take a breath.