Slowing down: The body and the big chill
Plenty of us have felt the icy blast of the winter weather over the past week.
But the cold conditions nearly cost retired gardener Brian Courtney his life, after he collapsed in a Salisbury street and developed hypothermia over several hours before an ambulance was called.
Doctors say the 77-year-old was lucky to survive after his body temperature dropped to just 26C, compared with a normal temperature of around 37C.
So what effect does the cold weather have on our bodies?
Frostbite is an obvious danger, which occurs when the body's extremities - fingers, toes, cheeks, noses and ears, get so cold their temperature drops below freezing.
Caught early enough it can be reversed, but if not it can result in tissue loss.
But hypothermia is one of the cold's most dramatic manifestations in the human body.
It occurs when the body's core temperature drops and, in the most severe cases, leads to uncontrolled shivering, a loss of control over hands, feet and limbs and sometimes blue, puffy skin.
But the brain slows down too, meaning thinking becomes sluggish, speech is difficult and the victim can become irrational.
Dr Kevin Fong of University College London is an expert in space medicine, where it tends to be either extremely cold or extremely hot, but rarely in-between.
He says humans operate in a narrow temperature range.
"We're useless more than two or three degrees above or below 37C - we cook above and shut down below."
Dr Fong says that when the body's temperature drops to between 32C and 35C, the heart starts to slow down.
If the temperature falls still further, things start to get very worrying indeed.
At a cellular level, the spread of the electrical impulses that drive the heart can be disrupted, increasing the risk of heart attack.
Mechanisms for coping
If the core temperature drops to the high 20s, like it apparently did for Mr Courtney, the heart may just stop altogether.
The body has all sorts of elaborate mechanism to combat heat loss.
Shivering, for example, is an attempt by the body to create warmth through tiny muscle movements round vital organs.
It is triggered by a part of the brain that responds to even small fluctuations in the core temperature.
But Dr Fong says in humans our most basic defence is behaviour.
"The way to avoid freezing to death in a ski resort is to put on a coat and go inside.
"We are very attuned to doing the right thing when we get cold."
Earlier this year Dr Fong took part in a Horizon documentary for BBC TV, looking at the technique of extreme cooling to bring people back from the dead.
"I had to sit in a pool filled with water at 12C.
"The hardest thing was sitting there doing nothing about being cold.
"Every fibre of my being was crying out to do something that would warm me up, like swimming."
And the cold can be a double edged sword.
While extreme cold can stop the heart, it also preserves the brain.
So in extremely cold conditions someone may look dead, but could still be very much alive.
Dr Fong quotes the old medical aphorism: "You're not dead until you're warm and dead."