I have written about body excretions before, one on mucous was published a month or so back.  

The subject once again came to my attention when I read about a new book by Sarah Everts, titled “The Joy of Sweat: The Strange Science of Perspiration.”  Her book is divided into three parts:  the science of sweat, sweat and society and the war on sweat.  It contains all there is to know about diaphoresis — the process of perspiration. Parts of her book are included in this article.

There are two forms of perspiration: insensible and sensible. When the body releases sweat via the skin pores the body cools down as the water evaporates from the skin. ... This is called sensible perspiration. This is to distinguish it from the other type of perspiration (i.e. insensible perspiration) in which the body is not able to sense it is perspiring.  Word origin: French perspiration, from perspirer, from Latin perspirare (to breathe through), from per (through) and spirare (to breathe).

According to Evert’s book, “sweating isn’t just splashy and conspicuous; it’s also so very human. Most other animals do not regulate their body temperature by sweating. In fact, some evolutionary biologists argue that our ability to perspire has helped humans to dominate the natural world.”

Sweat Glands

There are three main types of sweat glands: eccrine, apocrine and apoeccrine.  Eccrine sweat glands are the most numerous, distributed across nearly the entire body surface area, and responsible for the highest volume of sweat excretion.  Humans have 2 to 4 million eccrine sweat glands and are found on both glabrous (palms, soles) and non-glabrous (hairy) skin.

Gland density is not uniform across the body surface area. The highest gland densities are on the palms and soles (~250–550 glands/cm2)  and respond to emotional, as well as thermal stimuli.

The density of eccrine glands on non-glabrous skin, such as the face, trunk and limbs are ~2–5-fold lower than that of glabrous skin [16], but distributed over a much larger surface area and are primarily responsible for regulating heat.  

By contrast, apocrine and apoeccrine glands play a lesser role in overall sweat production as they are limited to specific regions of the body.

Body Heat

Your body  produces as much heat as a 60-watt light bulb. That’s if you are a smallish person. If you are big and burly, you will be as hot as a 100-watt light bulb. Our bodies can’t help but exude heat even in a state of utter relaxation because our cells are workaholics, diligently dealing with the never-ending to-do list involved in keeping us alive: breaking down nutrients, shuttling oxygen around, building hormones, copying DNA, fighting pathogens. Taking care of these tasks requires billions of chemical reactions. And many of these reactions produce heat. That internal heat makes us warm.

Physiology

Sweat as a heat-loss strategy is based on the simple fact that hot surfaces (such as your skin) can evaporate fluids (for example, sweat). As anyone who has tried to simmer down a sauce knows, water evaporation requires heat. In the same way, your body’s heat evaporates sweat off your skin. Because heat is consumed for sweat evaporation, there’s a net cooling effect on your skin.

It’s evolution being crafty with physics and biochemistry: There’s heat on your skin and water in your body, so why not dispatch the water to the skin in order to help the body cool down?

“Sweat is a great way to dump body heat,” says Yana Kamberov, a geneticist at the University of Pennsylvania who is studying the evolution of sweat. “No animal is as good as we are at releasing water on skin and using it to cool down.”

But as ingenious as it is, humans are nearly unique in their reliance on sweat to regulate temperature. Most other species use other ways to cool down — some of them unusual and even bizarre. Elephants use their enormous ears to dissipate heat away from their bodies, dogs pant to cool down, and vultures poop on themselves.

All of these techniques work to shed excess heat. But none of them work as well as what humans have evolved. As our ancestors evolved from furry primates into relatively naked, upright creatures, sweating to cool down became one of our species’ unique powers.

When the weather turned cold, we wore the pelts of other animals to keep warm. But when the weather got hot, sweating was the most efficient way evolution found for keeping us cool. When our predecessors began evolving body-wide sweat glands some 35 million years ago, perspiration was as peculiar as it was precious.

Final Thoughts

Many evolutionary biologists include sweating in the portfolio of idiosyncrasies that have helped our species dominate the natural world.

“Everybody is very aware of the fact that humans have big brains and that we have language and that we make tools. People should also know and be excited about sweat,” says Daniel Lieberman, an evolutionary biologist at Harvard University. “Sweat may seem like an unpleasant problem on the subway but it helped make us human. You can’t be physically active the way humans are without sweating. Without sweating, we wouldn’t have been able to become hunter-gatherers.”

Max Sherman is a medical writer and pharmacist retired from the medical device industry.  His new book “Science Snippets” is available from Amazon and other book sellers. It contains a number of previously published columns.  He can be reached by email at  maxsherman339@gmail.com.