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Biology of stress

When a stressor is perceived and interpreted, the stress response system starts up a cascade of biological events. As a result, we release the stress hormones adrenaline and cortisol.

Where does cortisol come from and where does it go?

In scientific lingo, the stress response system is called the hypothalamic-pituitary-adrenal axis (HPA axis). After perceiving a stressor, a small brain area called the hypothalamus sends a chemical message to the pituitary gland. From here a new chemical message is sent out of the brain through our blood, to the producers of stress hormones called the adrenal glands that sit on top of the kidneys. This message says “secrete cortisol”.


Think of it this way, if your boss gives you negative feedback about something you have done, then you will surely stop doing it. The same applies here; negative feedback from cortisol and its receptors stops the stress response system dead in its tracks.

In order to do their job, stress hormones have to bind (become attached to) what are known as receptors. Think of it this way, stress hormones carry a message that tells cells in the body what they need to do. But, something has to receive that message before the orders contained in the message can be carried out. The message receivers in this case are the stress hormones receptors.

When cortisol attaches to its receptors it can then interact with other cells to get their job done. Since cortisol is needed for many biological functions it has receptors almost everywhere in the body, including the brain.

i.e.: If you need a fast supply of energy to fuel your muscles and run, then priority will be given to the receptors in your leg muscles.

Interpreting a stressor gets the stress system started, but what stops it? How do the adrenal glands know when it is time to stop making and sending out cortisol?

Mother Nature is very efficient and minimizes the amount of work done by any one biological system. As we said before, cortisol attaches to its receptors to work. But these receptors are smart little buggers and can sense when enough is enough.

So, when enough stress hormones have been released and the job is done, cortisol travels backwards up the relay message system and the simple act of binding to its receptors in the system, shuts it down. This is known as negative feedback.

What time is it?

Have you ever wondered why you have lots of energy when you wake up (after you have cleared the cobwebs of course) and why you sometimes feel a little slow in the afternoon? Well, here again, cortisol plays a role.

Cortisol is not only released in times of stress but is also important to many normal or basal bodily functions. To accomplish this we need a steady stream of cortisol in our blood. In fact, cortisol is secreted at a very predictable and pretty much universal rhythm in all of us. This is called a circadian rhythm.

The circadian rhythm of cortisol is as followed: there is the cortisol peak early in the morning, then cortisol levels decline slowly over the course of the day to their maximum low, and eventually rise again in the early night time hours slowly preparing for the morning peak again.

To summarize, there are two kinds of stress hormone levels:

  • Resting (basal) cortisol levels. These are the normal everyday levels essential for normal functioning.
  • Reactive cortisol levels. These are increases in cortisol in response to stressors.