It used to be that only mad dogs and Englishmen went out in the noonday sun. These days you might see anybody who owns a pair of Reeboks pounding around the track, down the street or through the park, working hard to live forever, paying no mind to the current heat wave.

So he looks a little pink, like a lover engulfed by a blush. It probably won't kill him -- not if he's in shape, accustomed to the climate and has been pumping fluids into that most efficient and adaptable of machines we call the human body.

If he's not in shape, has recently lived in Alaska or has refrained from water and such, he could be in trouble.

What precisely is happening to our determined jogger, this macho jock undeterred by Fahrenheit levels approaching 100?

This kind of heat has already killed well over 100 people in the Deep South and Western states, and it has finally found its way here. Yesterday it reached 98 degrees by 4 p.m., with a heat index of 102.

The jogger's blood is doing its best to rescue him from his brain's recklessness, in deciding he should exercise in weather like this. Blood is flowing out from the core of his body to the surface of his skin so the heat being generated within his body by his exertion can be dissipated into the air.

Body's response

The red color -- the blush -- is evidence of the dilation of the capillaries on his skin; they are filling up with this large volume of blood.

Now, according to physicians and physiologists this is good. It is the body defending itself. And the body has an array of specific physiological defenses against heat besides this, things like heat-shock proteins which are being produced in the cells to protect them from heat stress.

As with all defenses, this one, the migration of the blood, has its limitations. In this case, the limitation relates to the supply. The blood engaged in getting rid of the heat at the surface of the body has to come from somewhere, right?

So where does it come from?

From vital organs like the kidneys, the brain, the heart. These need fresh blood, too, and don't like being deprived. Take the heart, for instance. With so much blood out on the periphery of the body, it has less to pump. So what happens?

Artin Shoukas, a physiologist and biomedical engineer at Johns Hopkins Medical School, explains the sequence: "Your blood pressure decreases. You start to feel faint. And, usually because you are standing, you don't have enough blood going to the brain. So you faint."

If the blood flow to the heart drops enough, the heart can fail. You can die.

This doesn't happen all that often. "The human organism, says Dr. Robert Fitzgerald, a respiratory physiologist at Hopkins, "really knows how to manage a precipitous fall in blood pressure."

But athletes can run extra risk.

With the International Lacrosse World Championship going on this week at Hopkins' Homeland campus, "I thought that these guys better watch themselves out there," said Fitzgerald. "When you exercise you give off a lot of heat. They're trained at reasonable ambient temperatures, but these days temperatures are not reasonable. What you got is internally generated heat and ambient heat."

Which is to say, it is increasingly difficult to lose heat in an atmosphere that is also overheated. And it is increasingly difficult to evaporate sweat -- another of the body's cooling mechanisms -- in an atmosphere saturated with humidity.

There are other troubles when it comes to exertions made in high temperatures.

"When you start exercising, the blood flow is diverted to working muscles in the legs and limbs, and your heart has to start #F pumping more blood to those muscles," Shoukas says. "They need oxygen and nutrients."

Trouble is, your blood flow is already decreased because of that flow of blood away from the heart to the surface of the skin.

"In these circumstances you often can't get enough blood to the muscles, so the muscles start to fatigue," he says. "You can cramp up."

Worse, you can put too great a strain on your heart.

Sweating up a storm

Sweating is the most efficient way of cooling the body, short of diving into a mountain lake or going into an air-conditioned bar. Sweat cools by carrying heat away through evaporation. Human beings are well-equipped for this: Each one of us has 2 million to 4 million sweat glands. Taken together, they weigh about 3 ounces. Women have more than men.

At the lacrosse tournament on University Parkway, there's plenty of sweat out on the field. And, of course, there are plenty of fluids for the players to drink to replace body fluids. For the fans there are drinks, and something else: Misting tents have been set up on the adjacent baseball field.

Step in, walk through a fine spray and you come out exceedingly damp. It's a great way to cool down fast, if you don't mind soggy clothes.

This technique has also been put to use at the Baltimore Zoo. Roger Birkel, zoo director, they've begun misting the snow leopards -- creatures who are really not too heat tolerant. They're also trying to make the polar bear comfortable with ice blocks in his pool and frozen fish, or "fishcicles."

Actually, Birkel says, "the polar bear is probably the most comfortable animal out here. The mammals I worry about in this kind of heat are our keepers and maintenance people."

Since many of the zoo's inmates come from Africa and other places with warm climates, they can withstand the occasional blast of Baltimore heat with little difficulty. Which is not to say that animals don't feel the stress.

The animal that handles the heat best, says Birkel, is the dromedary, the one-humped camel.

"They store water very well in their fat layers and have the ability to turn off evaporation, to keep the water inside," says Birkel. "They can actually allow their body temperature to rise by as much as six to seven degrees centigrade [Celsius, or 21 degrees Farenheit]. Most animals can't do that. If your body temperature did that, you'd go into a heatstroke."

Obviously, days like these are not at all bothersome for the camel. Nor for certain others. "We have a dama gazelle. They are found at the edge of the Sahara," says Birkel. "They are light-colored, almost white. That's to reflect the sun. I've noticed that even on these hot days they're right out in the sun and don't seem to be bothered at all."

Like people, animals alter their behaviors to accommodate changes in the weather. People cast off articles of clothing, seek shade, move slowly. Gazelles and other plains herd animals face away from the sun to reduce the amount of heat they are exposed to. When they want to absorb heat, they put their bodies perpendicular to the sun.

Imagine a giant herd of impala out in the Great Rift Valley: "On three, everybody turn. ..."

Animal 'sweating'

A lot of animals don't sweat, but cool themselves by other means, like panting. Dogs do this, but everybody knows that. But so do jackals, wolves and foxes. And all cats, from alley cats to Bengal tigers. And it gets pretty hot in Bengal.

Among the more ingenious animals when it comes to dealing with the heat is the American prairie dog. Their natural environment provides very little shade, and plenty of heat. So to escape it they dig burrows, 10 feet deep or more. These are cool inside, owing to the animal's cleverly contrived air-conditioning system, the way they construct their labyrinth.

Prairie dogs always have two entrances to their homes. One is cone-shaped, like a volcano. The other is a flat hole. The effect of the cone-shaped opening is to push the air blowing across it upward; this draws air out of the tunnel. The effect of the flat hole is to draw air down into it. The stream of air cools as it passes through the deep recesses of the burrow.

How did they learn how to do that, you might ask?

Evolution, says Birkell. "It's part of natural selection."

Pretty smart for a bunch of rodents.

Pub Date: 7/23/98