A combination of low oxygen, strong winds and a traffic jam of climbers likely led to the death of a Canadian who reached the top of Mount Everest this weekend but failed to make it back down alive.

Shriya Shah-Klorfine, 33, died over the weekend while descending the south summit of Mount Everest. She collapsed during the descent and is believed to have died from exhaustion.

Jim Elzinga, an accomplished high-altitude climber who has led expeditions up Everest twice, says it's difficult to explain how exhausted one becomes while climbing Everest, whose peak protrudes into the stratosphere to 8,848 metres (29,029 ft) above sea level.

"Especially above 8,000 metres, if you take one step, it will take 10 breaths just to recover. And then you take another step and need another 10 breaths to recover," he explained to CTV's Canada AM Tuesday.

"So it's very, very slow and arduous and it becomes largely a big mental game to keep yourself going upwards."

Dr. Greg Wells, a physiologist and scientist who specializes in health and performance in extreme conditions, says getting enough oxygen to fuel the heart and brains at high altitudes becomes extremely difficult as the atmosphere thins.

"The pressure in the atmosphere decreases (at high altitudes), so there's less drive of oxygen into the body," he says.

That causes energy levels in the body to drop, for muscles to fatigue easily and makes it difficult to concentrate.

Everyone who climbs Everest tries to account for the change in altitude by giving their body time to acclimatize. In some cases, the process works well, and the body learns to deal with the lower oxygen, creating more red blood cells, says Wells.

At other times, the body doesn't cope, leading to the headaches, nausea, and dizziness of altitude sickness.

"When you ascend altitude, your maximum heart rate actually drops, and at some point it reaches your resting heart rate. So you can imagine that simply standing is almost near your maximum," says Wells.

Bottled supplemental oxygen certainly helps, but Elzinga says there are only so many bottles that climbers can carry.

"The challenge with oxygen is that it only lasts a certain amount of time. And so if you're delayed and you run out, that causes people to get into a lot of difficulty," he says.

Such delays are becoming even more common these days, now that there are so many commercial outfits that arrange for tourists to climb Everest.

There is only a short window all year in which climbers can try to summit Everest and that's now, in May. Most of the year, the summit is hit by monsoon-force winds. It is only for a few weeks in May that these winds die down to allow a short seven- to 10-day window for climbers to attempt to summit.

With more than 100 people going up to the summit on any given day during that window, it can become quite crowded at the top, says Elzinga.

"Especially above the last camp on the South Col, from there up to the summit, it's a very narrow ridge," he says. "And there's a particular spot, called the Hillary Step, where if climbers are coming down, you have to wait for them to come down before you get a chance to go up."

Gyanendra Shrestha of Nepal's Mountaineering Department told The Canadian Press that 150 climbers reached Everest's summit on Friday and Saturday, but by Saturday afternoon, a windstorm had set in.

Progress to descend was slow and many had to remain in the dangerous "death zone" above the last camp at the South Col longer than usual, Shrestha said.

The zone is so called because of the low oxygen level, steep icy slope and the fact that dozens of climbers' bodies lay abandoned there.

At least three other climbers died on Saturday along with Shriya Shah-Klorfine: 61-year-old German Dr. Eberhard Schaaf; South Korean mountaineer Song Won-bin; and a 55-year-old Chinese climber named Wang-yi Fa. The last man's Nepalese Sherpa guide is still missing.

Shah-Klorfine hoped to become the first South Asian woman from Canada to ascend the world's highest peak.