Why does my face turn red in the heat or when I feel embarrassed? asks a reader.

Hot. Sweaty. Red? If the face is a barometer of our emotions, it's also a thermometer of our physical discomfort, at least in summer. Being red-faced on a 90-degree day is like being drenched in sweat after running full tilt: a sign your body is helping you cool off.

By pumping out water to the skin surface, where it can evaporate into thin air, sweating cools us. But it takes more than sweat to keep our body temperature in a safe range on a hot day.

So what's with the red face (and arms, and torso?) The autonomic nervous system (ANS) sends signals to dilate blood vessels just under the skin. As the vessels expand, blood rushes from the body's overheating core toward the surface, where some of its heat helpfully escapes into the air.

Heat makes its exit by radiating through the skin (radiation), being carried away by moving air (convection), and by direct contact with air cooler than body temperature (conduction.) As we chill out, the air surrounding us gets just a tiny bit warmer.

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Just as your face turns red as the body tries to get rid of heat, blushing when we're emotional is involuntary, like sneezing in pollen season.

Imagine squirting mustard across your shirt in a crowded school cafeteria, or tripping over the doorjamb as you enter the room. Even being singled out for praise in a group can make us uncomfortable, embarrassed and, yep, red-faced. Which only heightens the we're-all-looking-at-YOU misery.

Human beings are champion blushers. Our brains are trained from an early age to consider social and moral issues, and to pay close attention to the people around us. And then there's our bare facial skin, fur- and feather-free, ideal for displaying every fleeting emotion.

Like heat-prompted reddening, blushing starts when blood vessels near the skin's surface suddenly expand. Blushing is controlled by a part of the ANS called the sympathetic nervous system, which prepares us for "fight or flight" in dangerous or stressful situations. Champion blushers, especially the fair-skinned, may find the rosy glow spreading to their neck, chest and arms.

While most human beings start out as at least occasional blushers, we blush less often as we age. Over time, the response of our sympathetic nervous system to embarrassment may lessen, perhaps in part because we get more comfortable in our own (non-blushing) skins. We realize that eventually, everyone spills food, says the wrong word, or gets a piece of toilet paper stuck to their shoe.