You have two adrenal glands and they are located on top of your kidneys. Their main function is to counter stress by producing a number of key hormones that send chemical messages to your body.
Cortisol is a very important anti-inflammatory hormone produced by the adrenal glands. Its production can be activated by any stress, good or bad. It is commonly referred to as the "stress" hormone. It is an anti-inflammatory hormone and will release glucose from your liver and muscles into your blood stream to provide an instant fuel source for a hyper-critical event. So this effect can be a good thing when things out on the street go bad and gives you that adrenal rush. It goes along with increased blood pressure and pupil dilation, and all of your body's other responses to a "fight-or-flight" event.
Now if this occurs a number of times a shift, for a number of years, along with all the other more minor stressors you have in your life, you now have all the right conditions for stress to turn into a chronic life-altering mode for the officer.
This is where cortisol comes about in a very bad way for you physiologically. Being in a constant state of yellow or orange awareness has an unwanted, long-term downside. The chronic release of cortisol results in a severe fat-storing effect on an officer's body as a way to deal with the excess glucose put into the blood stream. It becomes visceral fat, which equals weight gain. There is more "bad stuff" that can result from elevated stress and cortisol levels as well.
Your cortisol levels also influence and regulate or modulate many of the changes that occur in your body in response to stress, including blood pressure, blood sugar (glucose) levels, immune responses, anti-inflammatory actions, and metabolizing of fat, protein, and carbohydrates. After years in stressful police work, these levels may very well be in an unhealthy range, becoming risk factors for a variety of diseases. Which is why reducing and controlling your stress is highly important to you as a law enforcement officer.