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What Happens When It’s Hot Outside

Posted on 20th August, by Aaron Shelley in Articles. Comments Off

What Happens When It’s Hot Outside

It’s official: July 2012 was the hottest month on record for the lower 48 states. Ever. And it doesn’t look like relief is in sight: In fact, this summer is pacing to be one of the warmest on the books since 1950.

We’re guessing no one needed to tell you that — the oppressive afternoon temperatures, pools of sweat and heat-induced grumpiness (wait, is that just us?) likely clued you in.

In serious cases you might have experienced symptoms of heat exhaustion – heavy sweating, dizziness and fainting, muscle cramps, nausea and low blood pressure, to name a few — or the more serious heat stroke – high body temperature, dry skin, nausea, increased heart rate, confusion and even unconsciousness. But have you ever wondered what’s actually happening inside your body to cause that?

Turns out, humans are built with a sophisticated internal thermostat that works to regulate temperature, even in extreme environments. Here’s the basic gist: sitting down in a cool room, just a small bit of blood is traveling to our skin. But as we start to heat up, the heart starts pushing more and more blood to the surface of our bodies, to move heat away from our vital internal organs, and to start the sweating process. “Humans are unique in their ability to have those massive increases in skin blood flow,” W. Larry Kenney, professor of Physiology and Kinesiology at Penn State, tells HuffPost.

Thermoregulation isn’t as well honed in the very young and the elderly, making them more susceptible to heat exhaustion or heat stroke. If the former is suspected, it’s important to stop activity, move somewhere cooler and rehydrate, according to The Mayo Clinic. They recommend calling a doctor if symptoms don’t resolve in an hour, or if you have signs of heat stroke, which is a medical emergency.

Click through the infographic below for a clear, simple explanation of what’s happening to your body during a heat wave — we spoke to Kenney, along with Bret A Nicks, M.D., MHA, FACEP, associate professor of emergency medicine at Wake Forest Baptist Medical Center and a spokesperson for the American College of Emergency Physicians; Fabio Comana, director of Continuing Education for the National Academy of Sports Medicine; and experts from the Centers for Disease Control and Prevention to put it together.

And remember that this is your body at rest (say, sprawled on a lawn chair) — when you exercise, the muscles require significantly more blood, as well as the skin, making you even more susceptible to heat exhaustion and heat stroke (a phenomenon called muscle-pump is also activated to help with circulation).

The Brain

Ever start feeling a little dizzy in the heat? Here’s why: When your body heats up, more blood is diverted to the expanded blood vessels near your skin, meaning the same amount of blood is going to more of your body. And the blood is losing volume as you sweat out liquid. So there’s more volume and less blood, which lowers blood pressure and makes your heart work harder to distribute less blood to more of the body. That can mean you’re not getting quite enough oxygen carried by the blood up to your brain, causing you to feel a little confused or dizzy, or the sensation of tunnel vision. In severe cases, fainting may occur.

The Skin

At rest in a cool environment, just a bit of blood is flowing to the skin (about 300 ml per minute). But as you heat up, the body begins to divert more and more blood to your skin to help the sweating process and carry heat away from your internal organs and back to dilated vessels near your skin, where it can be released into the environment. Once that blood reaches the skin, your sweat glands begin to secrete perspiration. When the air is relatively cool to our body temperature, heat leaves our body pretty easily. But as the air heats up closer to our internal body temperature (and beyond), sweating becomes more important. Perspiration, a mix of water, chemicals, salt and sugar, leaves the sweat glands and hits the skin surface, where the air turns it from a solid to a vapor — and it’s that evaporation that cools you down. Heat rash occurs when the sweat glands become blocked, creating irritation and red bumps. Humidity is a sign that the air is already saturated with moisture, meaning it can no longer absorb the liquid off your skin — in that case, cool compresses, drinks and cold-water misters are key. Profuse sweating is a symptom of heat exhaustion, but eventually it will shut off when the body recognizes that it’s losing too much liquid. Hot, dry skin (sans sweating) is a sign of heat stroke, which is a medical emergency requiring immediate attention.


Proper cell and muscle function requires a balance of electrolytes, or electrically-charged minerals in your blood that include potassium and sodium. But sweating purges electrolytes (as does heat-induced diarrhea). And an imbalance of these minerals can prevent muscles from relaxing and also cause them to fire when they might not need to. The result? A painful heat cramp or muscle spasm. If you experience a heat cramp, move into a cool environment and rehydrate with electrolytes. Plain water won’t do the trick in this case — Powerade can work, as can rehydration salts, which are packed with the necessary sodium and potassium.

The Hands

When our bodies heat up, vessels near the skin respond by expanding, which allows the heart to shuttle more blood closer to the surface of your skin in order to radiate heat and activate the sweating process. The natural pull of gravity tends to bring more of that blood downstream, including to your hands (very similar to the feet). And then gravity works in reverse, as well, making it harder for all that extra blood to travel back up to your heart again, especially when you’re standing. The result can be an edema, which is a swelling caused by excess fluid that should clear up after returning to a cool environment.

The Intestines

As the heart is pumping extra hard to continue pushing less blood through more of the body, it’ll eventually need to pull back on the amount of blood sent to certain organs. One way it compensates is by diverting blood flow away from the gut, including the intestines. Prolonged lack of blood to the intestines can create leaky bowels, resulting in diarrhea — a sign of both heat stroke and heat exhaustion. Diarrhea can also strip the body of electrolytes, which, in turn, affects muscle function.

The Kidneys

While the heart is working overtime to keep pumping less blood through more of the body, something’s got to give. And one way the body compensates is by decreasing blood flow to the gut, including the kidneys. That means less blood volume is traveling through the kidneys, which in the short term can shut down urine production and in the long term — in very severe cases — lead to kidney failure. Even if your kidneys do continue producing urine, it will likely turn a very dark yellow, signaling dehydration from the loss of liquid through sweat.

The Feet

Heat can affect us all the way from our brains down to our feet: as we get warmer, the vessels near the skin expand, which allows the heart to transport more blood to the surface of your skin in order to radiate heat and enable the sweating process. The natural pull of gravity tends to bring more of that blood downstream, especially to your feet (and hands). And then gravity works in reverse, as well, making it harder for all that extra blood to travel back up to your heart again, especially when you’re standing. The result can be an edema, which is a swelling caused by excess fluid. That explains why your feet and ankles might puff up in the heat — keeping your legs propped and avoiding sudden standing can help to avoid the problem.


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