When the temperatures rise or fall to extremes, most people run for cover. But if you love the outdoors and have to stay active, scorching heat or freezing cold may not deter you from venturing out into the elements. In fact, some of your favorite activities may take place in less-than-ideal climate conditions. Your heart plays a vital role in helping your body cope when the mercury's in the end zone.

Heart Rate and Core Temperature

Core temperature is the internal temperature in a living organism. Optimal core temperature for the human body is around 37 degrees Celsius, or about 98.6 degrees Fahrenheit. Human core temperature is monitored and regulated by sensors in the hypothalamus of the brain. When core temperature falls or rises, reactions are set in motion to restore homeostasis, or balance. The heart plays a role in restoring homeostasis by regulating the circulating blood volume to heat or cool the internal environment.

Heart Rate and Hot Temperatures

Prolonged exposure to extreme heat can cause heat-related illness which usually progresses in three stages. In the earliest stage, muscle cramps accompany profuse sweating as the body attempts to cool itself. The next stage, heat exhaustion, is marked by a weak, rapid pulse as blood is shunted to the skin's surface, seeking cooler temperatures to carry back to overheated internal systems. In the most extreme stage, heat stroke, a racing heart and strong pulse attempt to circulate fluid-depleted blood to the vital organs and brain in a fight for survival. Heat stroke is a life-threatening 911 emergency.

Heart Rate and Cold Temperatures

A drop in favorable core temperature also invokes a systemic response. Initially, shivering and hairs standing on end attempt to generate and hold in heat. Prolonged exposure to colder temperatures can lead to hypothermia, the opposite of heat stroke, particularly if you are sweaty, wet or submerged in water. According to the University of Minnesota's Sea Grant information page on hypothermia, when core temperature drops below 37 degrees, heart rate initially increases to generate heat 3. As core temperature continues to drop, the heart rate slows to conserve energy and preserve vital organs.

Causes and Risk Factors for Evironmental Illness

Extremes in temperature, whether hot or cold, put some populations at greater risk for environment-related health problems. The very old, the very young, people with mental illness, those with certain medical conditions or those taking some medications may be more vulnerable to extremes in climate. Inappropriate clothing, dehydration, poor nutrition and sub-standard physical fitness can all be contributing factors. Poor heart health can impede the body's efforts to restore core homeostasis.

Preparing for Environmental Extremes

Taking time to prepare for extreme temperatures can dramatically reduce your risk of hyper- or hypothermia. Staying hydrated by drinking water before, during and after activities helps keep your core temperature stable. Beverages that contain alcohol and caffeine do not hydrate, and can have a negative impact on core temperature. Dressing in layers that you can remove or add on as temperatures rise and drop helps dissipate heat or keep it in as needed. If you have a heart condition, or are taking medications like diuretics which dehydrate you, activities in extreme environmental conditions are not advisable. If you suspect you have an environmentally induced illness, take shelter as soon as possible and seek medical intervention if symptoms warrant.