Body Heat Infographic: 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).