What if you could not sweat?

You run around on a sunny day. You get hot.

Then tiny drops of sweat come out of your skin. What if no sweat came out at all?

After a game of tag on a summer day, your face feels hot, your cheeks go pink, and your forehead is wet. That wet stuff is sweat, and it's doing an important job.

So what if your body couldn't make sweat at all? Would you just feel hot, or would something else have to change? We can't switch off anyone's sweat, but we can follow the clues: how sweat cools you, how dogs manage with very little sweat, and other ways to stay cool.

When the body warms, the sympathetic nervous system, which keeps watch on body temperature, stimulates the sweat glands. The cooling comes not from the sweat itself but from its evaporation, which carries heat away from the skin.

This thought experiment removes sweating and asks how else the body could shed heat. It follows three threads: what happens to heat when sweat is gone, how dogs cool themselves with little sweat, and the everyday cooling strategies people use. Measured facts are separated from reasoned ideas.

You have lots of tiny sweat glands in your skin.

There are many on your hands, your feet and your forehead. They make sweat when you get hot.

Your skin is dotted with tiny sweat glands, coiled tubes that open onto the skin's surface. The ones that cool you are found all over your body, and there are especially many on the palms of your hands, the soles of your feet and your forehead.

You sweat a little all the time, about half a liter a day, even when you don't notice it. When you get really hot, from sunshine or exercise, an active person can sweat 0.7 to 1.5 liters an hour, according to the OpenStax anatomy textbook.

Eccrine sweat glands are coiled glands deep in the dermis whose ducts open at skin pores. They occur over the whole body but are especially abundant on the palms, soles and forehead, and they are a primary component of human thermoregulation (OpenStax). Their sweat is mostly water, with some salt, antibodies, traces of metabolic waste and dermcidin, an antimicrobial peptide.

Even without visible sweating, about 500 mL of insensible perspiration is secreted daily. In heat or vigorous activity, the sympathetic nervous system drives output as high as 0.7 to 1.5 L per hour in an active person.

Now pretend you can't sweat. You run and play. You get hot.

But there is no sweat to cool you. You might need other ways to cool down!

Without sweat, the body would lose its main way to cool down in the heat. It would still have one other trick: the tiny blood vessels in your skin widen, so warm blood can give off heat through the skin. That's why your face goes pink when you exercise.

But on a hot day, with no sweat drying on your skin, heat might build up much faster. You might have to slow down, rest in the shade or find other ways to cool off, a bit like dogs do. Those are our ideas about a pretend body.

OpenStax describes two skin-level responses to heat: sweating, and dilation of dermal arterioles so blood carries excess heat to the skin, causing the flush seen in exercise. Remove sweating and only the second remains, along with behavior.

It is reasonable to infer that a non-sweating person would shed heat far more slowly during exercise or in hot weather, and would depend on reducing heat gain: rest, shade, light clothing and cooled surroundings. How much activity they could sustain is speculative here; the point is that evaporation of sweat does work that blood flow alone may not match.

Sweat is mostly water. When it dries off your skin, it takes heat away.

That is why a breeze feels cool when you are sweaty!

Sweat cools you when it evaporates, changing from liquid water on your skin into water vapor in the air. To evaporate, the water needs energy, and it takes that energy from your skin as heat. So as the sweat dries, your skin cools down.

The OpenStax physics textbook explains that very humid air slows evaporation, so body temperature can rise and sweat stays sitting on your brow. And the American Kennel Club notes that fur limits how well sweat can evaporate, which is one reason dogs rely on other ways to cool.

Evaporation requires energy. OpenStax's physics text gives the figure: at body temperature, evaporating perspiration takes 2,428 kJ per kilogram, about 10 percent more than the latent heat of vaporization at 100 °C, and that heat comes from the skin. High humidity inhibits evaporation, so body temperature may rise, leaving unevaporated sweat on the brow.

That is why evaporative cooling depends on exposed, wet skin and air that can take up vapor. The American Kennel Club notes that because most dogs are covered in fur, which limits evaporation, their sweat glands are most useful where there is little or no fur, on the paw pads.

Dogs can't sweat much. So they pant with their tongues out!

Water dries off their tongues and cools them. People can rest in the shade and drink water.

Dogs do sweat a little, through their paw pads, which is why you might see damp paw prints on a hot day. But sweating plays only a small part in keeping dogs cool, the American Kennel Club says. Mostly, dogs pant: moisture evaporates from their tongue, nose and lungs and carries heat away.

People use clever tricks too. The National Weather Service suggests slowing down in the hottest part of the day, wearing loose, light-colored clothes, staying out of strong sun, and drinking plenty of water.

The American Kennel Club explains that dogs have merocrine sweat glands in their paw pads, but that sweating plays only a minor role; dogs rely mainly on panting, blood-vessel expansion and the insulation of their coat. When they pant, moisture evaporates from the tongue, nasal passages and lining of the lungs: evaporative cooling, moved from skin to airways.

For people, the National Weather Service's heat guidance focuses on reducing heat gain: rescheduling strenuous activity to the coolest time of day, lightweight light-colored clothing to reflect sunlight, limiting direct sun, drinking plenty of water and using air-conditioned spaces.