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The Danger of Dehydration in Cold W-ather
From http://health-faq.com

On a hot summer day, when the sun is blazing and the sky is completely cloudless, the beads of sweat on your brow prompt you to ensure that you are consuming plenty of water. This is especially the case when you are doing hard physical work or exercise outdoors - because your body loses fluid much faster. The fluid loss from your body, which stimulates thirst when the weather is warm, does not trigger the same reaction when the temperature drops. This is not just because you feel cold, it's because cold temperatures affect thirst sensations.
Getting the correct balance of fluid in your body depends on thirst stimulation. When you feel thirsty, you consume fluids voluntarily - and your kidneys will conserve or excrete the fluid as necessary. This process is facilitated by fluid controlling hormones, like AVP (plasma argentine vasopressin), and a couple of things can stimulate these hormones. When your body loses fluid, the level of sodium in your blood rises, and your blood volume reduces overall as well. Both of these reactions stimulate your hypothalamus to discharge AVP, which makes your kidneys produce urine at a slower rate. This replenishes the fluid in your body. Simultaneously, your hypothalamus sends a signal to the cortex of your brain, to generate a thirst sensation - encouraging the intake of water required to restore healthy salt levels.
When the weather is cold, the thirst response of your body declines by as much as forty percent, even if you are suffering from dehydration. This occurs because your blood vessels tighten when you are cold, to stop blood from traveling easily to the body's periphery. This allows your body to preserve warmth, by attracting extra blood to its center.
However, due to this, your body is "tricked" into believing that it is correctly hydrated. Your brain will not detect the reduction in blood volume, so the AVP hormone is not discharged at the normal rate - in spite of increased blood sodium levels. Consequently, your body will not preserve water and you will not feel particularly thirsty. This is why athletes tend to drink less water, when training in cold weather. Also, their kidneys do not receive the hormone signals to preserve water, so urine production rises. This condition is known as cold induced urine diuresis.
There are a few other factors that can cause dehydration during the winter too. In cold weather, you lose more fluid via the respiratory process. For instance, when your breath is visible, that is really water vapor being lost from your body. If you exercise vigorously in extremely cold temperatures, you will lose lots of vapor when you breathe.
Insulated underwear, thick coats and other warm clothing items can help your body preserve heat. However, the extra weight also means that your body has to work ten to forty percent harder. As a result, it generates more sweat - which leads to a loss of fluid. In cold air, sweat evaporates much quicker. Often, this makes people wrongly believe that they are not sweating in cold conditions. This can cause a reduced thirst sensation.
Revealingly, animals such as dogs and rats also exhibit reduced thirst sensations in colder weather. Typically, they drink back the lost fluid though. They also tend to experience increases in central blood volume, because of cold induced vasoconstriction.
It is important to drink plenty of water, particularly when doing physical activities outdoors in cold weather. You can check by examining your urine to see if you are correctly hydrated - it should have a clear color. In addition, you might be dehydrated if your lips and mouth become dry, if your skin is flushed or if your saliva thickens. Severe symptoms include muscle cramps, abdomen and chest pains, sickness, dizziness and confusion, dimmed vision and a raised pulse. You need to seek medical assistance immediately if you experience any of the aforementioned dehydration symptoms, but more importantly, stay hydrated to avoid them entirely.

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