You are endothermic, or warm-blooded; your body's processes, such as metabolism and heart rate, are regulated internally. Exothermic organisms must seek an external heat source to warm themselves. Because you are endothermic, your body's internal processes continue to function over a much greater temperature range than those of cold-blooded animals, such as reptiles and insects. This ability to regulate your body temperature and other internal functions keeps your heart rate steady, circulating needed blood throughout your body.
Effects of Cold Temperature on Heart Rate
Although you can function normally across a broad range of external temperatures, extreme cold will begin to affect your heart rate. While you are normally able to maintain a body temperature near 98.6 F, prolonged exposure to extreme cold temperatures causes your body temperature to plummet. As your temperature drops below 95 F, your normal body processes slow. MayoClinic.com notes that your heart and nervous system begin to fail during this time, as you begin to suffer from hypothermia. Even before hypothermia sets in, exposure to cold temperatures will begin to slow your heart rate and metabolism.
Benefits of a Slowed Heart Rate Due to Hypothermia
Although extreme low temperatures can be deadly, there is also an advantage to being affected by the hypothermia. Because your metabolism and heart rate slow, your body uses oxygen at a much lower rate than it would at higher temperatures. Your lower rate of oxygen consumption allows your body to persist longer under compromised conditions and, in essence, prolongs dying. Rescuers will have a longer window of opportunity to find, recover and revive you if you drown in very cold water.
Increases in Heart Rate Due to Heat
Extreme heat has the effect of increasing your heart rate. You experience this whether you are exposed to high ambient temperatures or you have a high fever. In either case, your aorta and your medulla react to the higher temperature, influencing your heart rate. As your heart beats faster, blood flows more quickly and you use more oxygen. In high but bearable temperatures, you adjust to the ambient heat around you, and your heart rate gradually returns to normal.
Weight Loss From Heat-Induced High Heart Rate
You may have used a hot tub or sauna to encourage weight loss or boost your metabolism, but the boost you experienced was likely short-lived. As your body adjusts to heat and returns your heart rate to normal, you quickly lose the advantages of the high ambient heat. While some additional weight loss may occur due to sweating, hot tubs, saunas and other sources of high ambient heat have only a temporary effect on your heart rate and energy consumption.
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