The Gough island fishermen have been freed after a few icy nights on the island. They were subjected to extreme weather conditions.

But the body is a wonderful thing. It has various defenses to maintain a constant core temperature. This is what happens.

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Why do you shiver when it's cold and sweat when it's hot? These reactions are part of the way your body maintains a steady temperature during weather changes and physical activity.

The biological thermostat that controls this process (thermoregulation) is located in a part of the brain called the hypothalamus. The body's cells and tissues can only function in a limited range of temperature. Problems arise when the body gets too hot or too cold.

Maintaining a stable body temperature as the environment and your activity level change requires a careful and delicate balance of heat production and heat loss. It’s quite remarkable what the body does to maintain the correct temperature: shivering, sweating and other responses, are all orchestrated by the hypothalamus.

Changing the amount of blood flow to the skin is one way the body regulates heat. When nerve endings in the skin detect changes in the air temperature, they communicate this information to the hypothalamus, which will decrease blood flow to the skin if it's cold or increase blood flow if it's hot. Blood from the body's core gives off heat when it reaches the skin surface.

When the air is cold, the body shunts blood away from the skin in order to warm the vital organs in the body's core. (Goose bumps are a trait carried over from a time when humans had thicker body hair that "fluffed up" to help retain heat.) But temperature control is more than skin deep.

Cold Facts

If you go outside without a jacket on a chilly day, your muscles will contract to produce heat. Sometimes, these muscle contractions are so subtle you may not notice them, but if it's very cold, you may start to shiver as your body produces heat to maintain your core body temperature.

If the body's efforts to reduce heat loss and increase heat production are not enough to prevent your core temperature from falling, hypothermia - dangerously low body temperature - can occur.

Hot Stuff

In addition to increasing blood flow to the skin, hot temperatures stimulate the hypothalamus to trigger another cooling mechanism - sweating. As sweat evaporates, the skin surface cools. Exercise, which produces heat, increases both sweating and blood flow to the skin. If the humidity is low, your body can lose a lot of heat by evaporation of sweat. However, when it's very humid, sweat doesn't evaporate very well, which is why high temperatures and high humidity are dangerous.

With age, the body's temperature-regulating mechanisms become less efficient, and chronic diseases and the medications taken to treat them often decrease the effectiveness of these mechanisms even more. Heat exhaustion and heat stroke can occur if the body fails to prevent an increase in core temperature.

A key difference between these two overheating problems is this: Heat exhaustion causes cold, clammy or sweaty skin, whereas heat stroke - a serious problem requiring immediate medical attention - causes hot, dry skin. Heat exhaustion is more common.

Besides a cold sweat, the other signs include fatigue and weakness. Efforts to cool the person will prevent worsening. If heat exhaustion progresses, the person may become confused and lose consciousness, at which point medical attention should be sought.

Heat stroke causes headache, vertigo and fatigue as well as hot, dry skin. Heart rate and breathing speed up, and the person may become disoriented and lose consciousness. Body temperature may climb as high as 41° C, or higher. Heat stroke can be fatal without emergency medical treatment.

Keep Your Cool

Fortunately, it's easy to help your body keep its cool, even during the real hot days of summer (ask the Capetonians about last week!). Drinking plenty of water is critical. When you are dehydrated, as commonly happens during hot weather, your body makes less sweat and sends less blood flowing to the skin, and so is less effective at cooling itself.

Dehydration also "thickens" the blood, increasing the workload on a heart already working overtime to pump blood to the skin. In older people with heart disease, dehydration creates an especially dangerous situation.

Other ways to avoid overheating: Wear loose-fitting, lightweight clothing and stay indoors during the hottest hours of the day. Air conditioning, if available, also helps. In the absence of air conditioning, avoid poorly ventilated areas and use fans to keep air moving so that sweat can evaporate.

Make sure that you stay cool when the heat index -- a measure that takes into account both temperature and humidity -- rises above 90, or if it stays hot for several days and nights without respite. If you can't stay cool on your own, avail yourself of cooling centres and other heat-emergency assistance provided by many communities.

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