When heat overcomes the body


When it comes to hot weather, the human body acts remarkably like a home's central air conditioning - complete with a thermostat and cooling mechanism.

But extreme conditions can overwhelm that system - resulting in heat exhaustion, heat stroke and death.

"You get hot and the mechanisms you use to regulate that heat go haywire," said Dr. Joshua Sharfstein, Baltimore's health commissioner, who declared a Code Red heat alert yesterday.

To avoid heat-related illness, experts recommend drinking plenty of fluids, finding a cool place indoors and avoiding overexertion. Here's why that's important:

The body's target core temperature is within a degree or so of 98.6 degrees Fahrenheit. Blood temperature is continually monitored by sensors in the spinal cord and the hypothalamus, a part of the brain. When the core blood temperature rises as a result of, say, playing touch football, those sensors tell the body's cooling systems to rev up.

Unlike dogs and cats, which have to pant to cool down, humans have two internal mechanisms that transfer heat from internal organs to the external environment. The first is evaporation, in the form of sweating. The second is convection, through blood flow near the skin.

Both mechanisms have limits - particularly when high heat and humidity conspire to undermine the body's cooling ability.

As sweat evaporates, it cools the skin and the blood underneath - much as an air conditioner cools air by passing it over pipes chilled by the evaporation of a refrigerant.

Heavy sweating can sap the body's fluid reserves, leaving no reserve for cooling. It also depletes electrolytes such as potassium and salt, both crucial for metabolism. High humidity compounds the problem.

"When humidity rises, evaporation doesn't work as well," said Robert Koos, a physiology professor at the University of Maryland School of Medicine.

Increasing blood flow near the skin is another way of cooling the body's core. Blood vessels near the skin can widen, and the heart can pump faster to increase blood flow. That releases heat by convection through the skin.

However, heat exchange becomes inefficient when outside air is warmer than the body. On summer days, heat and humidity can become so intense that the body's internal cooling systems are unable to cope.

"To drive that blood flow to the skin, cardiovascular output can almost double," said Koos. "In an elderly person whose ability to increase cardiac output is diminished, it can be devastating."

Sharfstein notes that while children and the elderly are particularly at risk, anyone can overheat when temperatures soar. "You have NFL lineman dying of heat stroke," he said.

Part of the problem is that people are misled by thirst and drink too little on hot days.

"Drink more than you feel like you need to - the thirst mechanism is inadequate," Sharfstein said. "What you want to avoid is the vicious spiral."

That spiral begins when the body gets so hot that its cooling system starts to break down.

Heat exhaustion is the first stage of that breakdown. The loss of fluids and electrolytes through sweating and an increase in core temperature begin to strain the body's metabolism.

Victims of heat exhaustion often suffer dizziness, nausea and headaches. They tend to feel weak, and their skin feels clammy. As the heart pumps faster to carry blood to the skin, people with weakened cardiovascular systems can suffer heart attacks.

But even in healthy young people, heat exhaustion can lead to heat stroke if nothing is done to counter the heating.

Heat stroke occurs when the nervous system gets so hot that the hypothalamus and other heat sensors go on the fritz - the thermostat breaks and the cooling system spins out of control.

Heat stroke victims often stop sweating and lose the ability to control blood flow to vessels near the skin. That causes core temperature to rise even higher. Heat stroke victims often have body temperatures over 104 degrees.

Brain damage, organ failure and death can occur when body temperature reaches 107 or 108 degrees, said Dr. Kenneth Butler, an emergency room physician at the University of Maryland Medical Center.

Butler said sudden heat waves give little time for people to acclimate to higher temperatures - a metabolic process that usually takes about two weeks.

When patients come into the emergency room with symptoms of heat exhaustion or heat stroke, he said, doctors drape them with a wet sheet and turn on fans to cool them down.

It is crucial to look for symptoms in others, Butler said, because overheating diminishes the victim's ability to realize that he or she is in trouble and respond appropriately.


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