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Davis, California

Saturday, March 2, 2024

The science behind sweating

Why do we sweat? Where in the long path of human history did we evolve this embarrassing characteristic? How does it work?

Well, the truth is that sweating is what gave us the ability to achieve greatness. Most other mammals don’t sweat, causing them to run internal body temperatures much higher than our own. They can pant, but they can’t hide. Humans, on the other hand, are unique in that we can direct heat off of our bodies and (more importantly) away from our big brains.

On top of giving our brains an easier time, we can use our perspiration powers for endurance activities that others cannot. The four-legged prey we used to run after with spears had to eventually slow down to pant. We didn’t. We could keep pace with them, outrun them and then eat them.

Almost every aspect of science plays an important role in keeping us cool. Let’s start with how we know when to sweat.

As we become active, our body temperature rises. Then, when the hypothalamus of the brain notices a temperature above the normal threshold, it activates a process called negative feedback.

“In negative feedback loops, a change causes the body to respond by doing what it can to reverse that change,” said Sahana Rajan, a second-year neurobiology, physiology and behavior major.

So as temperature goes up, the human body fights back by dilating blood vessels to give off heat and by sweating. When body temperature returns to normal, the brain inactivates these processes.

Once the body has been alerted that it needs to cool off, its sweat glands become active. There are 4 million of these glands all over the body — everywhere but the lips. The eccrine glands on the foreheads, palms and soles of our feet are connected to emotional signals as well as temperature changes. This is why we get sweaty palms before giving a speech.

How does sweating actually cool us off? Water released from our pores evaporates, cooling our skin and releasing heat. This requires understanding a bit of molecular chemistry. The molecules of water (H­2O) are in constant motion. If the liquid water increases in temperature, the average kinetic energy — energy from the movement of the different water molecules — also increases. If it reaches a certain temperature, the fastest moving molecules will move so quickly they can jump out of the pool, taking their energy with them.

“The hotter water becomes a gas, leaving behind the cooler water still in liquid form. That is why when water evaporates, it leaves behind cooler water,” said Maria Zarefakis, a chemistry graduate student.

Furthermore, we’ve all noticed that a nice breeze can cool us down even quicker.

“[This] has to do with heat transfer and the fact that the rate of evaporation of sweat increases as the wind blows … Sweat is evaporating and carrying heat away from our skin … the convective heat transfer coefficient is increased as air is ‘blown’ over the skin which also results in heat being carried away faster,” said Josef Efron, a fourth-year mechanical and aerospace engineering double major.

This technical explanation is essentially another way of saying that as the wind blows, in addition to heat from our bodies being directly carried away by the air (aka convective heat transfer), water evaporates more quickly.

Along with water, we release salt and nitrogenous wastes, like urea. While it may seem efficient to dump these unwanted materials out with our sweat, it actually reduces the efficiency of evaporative cooling. This is due to a concept in chemistry called colligative properties.

“Colligative properties only apply to solutions. They are a property of solutions that depend on the number of dissolved particles in the solution. What the solute is doesn’t matter,” Zarefakis said.

Essentially, when we add things to water, water behavior changes.

For example, colligative properties cause a boiling point elevation. The temperature required to change molecules from a liquid phase to a gas phase increases. All that salt in our sweat just gets in the way of the water molecules that are speeding up and trying to turn to gas. In the end, however, we are still able to cool off enough to keep our bodies happy, while also getting the added benefit of getting rid of all those extra waste products.


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