While everyone layers up for the cold snap, the human body turns on amazing features to adjust as well. The initial response to whole body cold exposure begins with constriction of the blood vessels across the skin. This conserves heat for the body's vital organs that would otherwise be lost through the skin. Because our fingers and toes are mostly skin, this may leave them deprived of sufficient blood flow and oxygen so localized blood vessels periodically dilate to flush the extremities.
A person may suddenly need to fidget and move around, which stimulates blood flow and heat. Or the body will involuntarily shiver, those rhythmic muscle contractions usually beginning across the torso and spreading from there. While this can warm the body, it requires more oxygen in the muscles, demanding more blood flow, which causes more shivering in a conflicting cycle.
Michael Huckabee is professor and director of the physician assistant program at UNMC. He's worked as a physician assistant for 30 years, primarily in rural Nebraska. He blogs every other week. Read more from Michael.
What's too cold?
Hypothermia occurs when the temperature of the body's core falls below 95 degrees Fahrenheit, about 3.5 degrees below the normal body temperature. In mild cases, the person will shiver and be apathetic, but more pronounced hypothermia results in confused behavior, slurred speech and sleepiness. Then the heart starts to beat more slowly and irregularly which may lead to life-threatening injuries. Resuscitation has been successful even with core temperatures at 56.6 degrees with no perceptible signs of life.
Benefits of fat.
Persons with increased subcutaneous fat are better able to conserve their core body temperature than slim body frames due to the added insulation. Treading lightly here, women are at an advantage as they naturally have a thicker layer of subcutaneous fat. Women generally have a greater body surface area than men, leading to greater heat loss and this need for a little extra protection.
The ideal layers.
Three layers of clothing are recommended. The inner layer next to skin should be lightweight polyester that does not readily hold moisture. Materials that wick sweat to the outer layers for evaporation are best. The middle layer should be fleece or wool for insulation, and the outer layer should be material that repels the wind and rain while allowing evaporation. Heat loss from the head alone can lose up to 50 percent of total resting heat production. Knit caps can substantially reduce the chill. Headbands warm the ears but do not prevent heat loss across the scalp.
Don't get bit.
Frostbite is felt as numbness in exposed skin such as the nose, ears, cheeks or wrists but may also occur in covered fingers and toes because of the vasoconstriction that occurs. When the temperature is around minus 18 degrees Fahrenheit, frostbite can occur in 30 minutes or less to exposed skin. Quick re-warming is typically recommended (such as placing the extremity in lukewarm water), though this is often accompanied by significant pain and your doctor should be contacted. When the frozen part warms up, local inflammation occurs that can lead to microscopic clotting of the blood in the small capillaries of the bloodstream. This can block blood flow and still result in the death of the thawed tissue. Recently, the use of thrombolytic medications (clotbusters) injected into the arteries has been used successfully in small studies to minimize the risk of amputation.