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Pamplona, Spain— the face where danger is sought and survival instincts are put to the test. Every year during the festival of San Fermin, people come from all over the world to run with the bulls. It's the ultimate adrenaline rush.

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Since the bull run began, hundreds of people have been horribly gored and many have lost their lives. Thousands invade Pamplona to experience the sheer thrill of facing death. This summer, firefighter Andy is one of them.

At home I had a lot of friends telling me I'm crazy. But it's something I've always wanted to do, ever since I was a teenager, and finally put some money together and here it is. Hopefully it's the first of many trips to Pamplona.

Andy's agreed to take part in our experiment to see how the body responds at moments of acute danger, like being chased by six angry bulls.

OK, we got a heart rate monitor here. It's going to go around my torso.

He'll be wired up to a whole variety of medical sensors. First, a heart rate monitor. This will relay information to a special watch he'll wear on his wrist.

Look like it's fluctuating between 104 and 110. That's a little high. That's a little high. I can feel my heart beating— beating in my chest right now. It's a blood pressure cuff. It's going to go on my left wrist. I'm going to get a blood pressure reading right now, see how excited I am about an hour and 15 minutes before the run. Got 159 over 112. So it's a little high already.

Finally, I'm going to put a swab in my mouth. It's going to measure the cortisol level. It's a stress hormone.

The body chemical cortisol is closely associated with adrenaline. Adrenaline may save Andy's life, enabling him to run faster than he's ever done before. Now that Andy is rigged and ready, all he has to do is to wait.

I feel excited. It's an hour before the run, and we've watched them erect the barriers. I'm ready to go.

The possibility of death by a bull is— it's there. It's real. It's, uh, it's in the back of my mind.

The bulls are released.

Andy waits for them to come near. In the tradition of the bull run, the challenge is to get as close to a charging bull as you dare without getting gored.

The bulls are just inches away, and Andy's body is flooded with a massive burst of adrenaline. His cortisol levels more than double. From a normal resting rate of 72 beats per minute, Andy's heart rate rockets, tripling to an amazing 230 beats per minute.

Definitely in fear. You're definitely in fear. You can feel the stomp of the feet and the hooves and everything. It's— wow. It's incredible.

The fear instincts that we rely on to save our lives are immensely powerful, so it's very difficult to turn them off at will.