When the brain detects stress in the environment, the stress–response system goes into action. This begins with the hypothalamus–pituitary–adrenal (HPA) axis. Those are some very long words, so scientists just call it the HPA axis. When the brain detects stress, it first sends a message to a part of the brain called the hypothalamus. The job of the hypothalamus is to wake up the pituitary gland. Although the pituitary is only about the size of a small pea, it has a mighty job. The pituitary releases hormones, which are the messengers in the stress–response system. These hormones travel out of the brain to the adrenal glands. The adrenal glands sit on top of the kidneys. The adrenal glands release cortisol into the body.
Cortisol is known as the stress hormone. Cortisol is a messenger that sets other organs in the body into action. It is like the superpower of the stress response system. Cortisol helps the brain to think clearly, sends energy to important muscles, and increases heart rate and breathing.

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