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A person is likely to die of Hypothermia, if body core temperature is below 33 degree Celsius. n this stage the body has effectively stopped trying to keep itself warm and some final steps are taken to avoid death. The heart rate and breathing slow so that they are hard to detect at all. Only or two breaths per minute may be taken. The skin is very pale and icy cold to the touch. The limbs are stiff, pupils dilate and are unreactive to light. To all intents and purposes the person appears to be dead, metabolism has slowed so far that they are almost in a state of suspended animation.
Every year there are tales of people surviving cold ordeals that had apparently killed them, in fact even as far as being declared clinically dead.
Children in particular survive the effects of hypothermia particularly successfully. The reason for this somewhat ironically is the increased surface area to body volume ratio that causes the child to cool down more rapidly and means that the body enters the more stable 3rd stage of profound hypothermia sooner.
Children's organs are easier to bring back to life as they are less likely to be affected by any kind of stress or disease that may may affect older organs. Even so, successful stories of survival are rare, it's just that we get to hear about them as they make the news, once ice starts to form in the body, it kills all of the cells that it touches.
Any "miracle" or "magic" is more to do with the person being found in time, recovery is just a matter of physics and biology.
Vigorous exercise causes the urine to become more concentrated?
During exercise body loses the water by the way of sweating. So, less water in the blood plasma so kidney does not filter much water from the blood, this makes urine more concentrated.
In very cold conditions we may shiver?
A final response to decreasing temperature is the increase of heat production. Muscular contraction is an inefficient process and causes much heat to be produced, so when we start to feel cold, we may begin to shiver, movements that are useless in themselves, but that generate heat as a by-product and so help to warm us up. Shivering can increase the production of heat five-fold.
Temperature-sensitive nerve endings in the nervous system monitor the skin and core. When warnings of cold start coming through, the brain takes the brakes off particular motor signals and muscles start to twitch, releasing heat as a by-product.
When we get too cold, muscles contract rapidly, we shiver.
The average temperature of the human body is 37 degree Celsius. If the environment is cold, then the temperature receptors in skin send this information to the processing centre in the brain called hypothalamus. The processing centre also has temperature receptors to detect the change in temperature of the blood. The processing centre automatically triggers changes to the effectors to ensure our body maintain temperature at 37 degree Celsius. These effectors are sweat glands and muscles. If we are too cold, the processing centre sends nerve impulse to the skin to decrease the heat loss from the body's surface. This makes the hair on the skin trap more warmth if they are standing up, and tiny muscles in the skin can quickly pull the hairs upright to reduce heat loss.
When we are too cold the blood vessels supplying warm blood to the skin become narrow or constrict called vasoconstriction. This reduces the flow of warm blood near the skin's surface and reduces the heat loss. Muscles attached to our skeleton can also receive signal from hypothalamus when we feel too cold. They respond by shivering. The rapid contraction of muscles during shivering results in heat being produced during respiration. This heat then warms up surrounding tissues.