It's 10 p.m. Time to walk the dog. You open the front door and notice that the streetlight is out. In the darkness, you hear a rustling in the bushes. Your heart races. Your stomach contracts. You feel like you have enough energy to run the three miles to the police station in 60 seconds.

Stressed? You bet. But what precisely occurs in your body when you are under stress?

In general, when your brain senses a threat, it sends a message throughout your body via the hypothalamic-pituitary-adrenal axis. This system triggers the "fight-or-flight response," prompting several physical changes that prepare you to survive a potential threat:

• Your heart rate, blood pressure and respiration rise to ensure an adequate oxygen supply.
• Blood flow increases to the muscles and brain, where it is most needed, while blood flow decreases to the "rest and digest" areas of the body (such as your gastrointestinal tract) because these areas are less important in moments of crisis.
• Sweating and muscle tension increase to help adjust body temperature and prepare for movement.
• Blood sugar increases to boost energy resources.