To reach Everest's summit Edmund Hillary and Tenzing Norgay had to climb sheer rock, while battling treacherous ice and a deadly lack of oxygen on the most dangerous part of the mountain. Seventy-two years ago, they shared their victory with the BBC.

"I think my first reaction was definitely one of relief," New Zealander Edmund Hillary told the BBC on 3 July 1953, as he described how he and Nepalese sherpa Tenzing Norgay felt when they stood on the highest point on Earth. "Relief that we had found the summit for one thing and relief that we were there." Tenzing too, having survived the precarious icy terrain and the biting cold, said through his translator, the expedition's team leader Colonel John Hunt, that his first feeling on reaching the top was "immense relief", followed by joy. This was because in order to stand on Everest's summit the two men had managed to scale a seemingly insurmountable sheer 40ft vertical rock face in the mountain's most treacherous region – the infamous "death zone".

The mountain, which towers 8,849m (29,032ft) above sea level, straddling the border of Nepal and Tibet, goes by many names. The British named it after surveyor George Everest in 1856, but it has long been known locally as Sagarmatha in Nepal and is called Chomolungma, meaning goddess mother of the world, in Tibet.

The death zone was a term given to a particular section of Everest by Edouard Wyss-Dunant, a doctor who led the Swiss attempt to scale it in 1952. Tenzing had been a member of this expedition, too. The moniker refers to the altitude that climbers reach on the mountain – 8,000m (26,000ft) above sea level – where the low-oxygen atmosphere starts to have disastrous effects on their physiology and their cells start to die. The majority of the climbers who have died on Everest have met their end in the death zone.

Humans have simply not evolved to survive in the incredibly cold temperatures, brutal winds and lack of oxygen that exists there. The thinness of the atmosphere means that mountaineers suffer hypoxia, where their vital organs do not get enough oxygen and bodies begin to break down. As their brains and lungs get starved of oxygen, their heart rate spikes, increasing their risk of a heart attack. The shortage of oxygen to the brain causes it to swell, triggering headaches, nausea and quickly impairing a climber's judgment and ability to make decisions, especially when they are under stress. As their brains swell, mountaineers have been known to experience delirium, talking to people who aren't there, burrowing in the snow or even shedding their clothing.

Tenzing and Hillary – along with the others on the expedition – had planned for this slowly acclimating themselves to the harsh conditions in the Himalayas by establishing a series of camps at increasing altitudes, gradually making their way up the mountain through April and May, 1953. This allowed their bodies time to expand their lung capacity and produce more haemoglobin – the protein in red blood cells that helps carry oxygen from the lungs to the other parts of the body – to compensate for the decreasing oxygen as they moved towards Everest's peak. But this acclimatisation was also not without risk for the team as too much haemoglobin thickens the blood. This makes circulation more difficult, which increases the likelihood of a stroke and accumulation of fluid in the lungs.

However, it is virtually impossible to acclimatise your body at any altitude above 6,000m (19,700ft) and the vertical rock face they needed to scale that sat 8,790m (28,839ft) above sea level. So, the climbers had brought with them specially designed oxygen apparatus, which would help combat the effects of the altitude's atmosphere. But they were under no illusion about........

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