The endocrine system is your body's chemical messaging network, a collection of glands that release hormones directly into your bloodstream to control how your body works. Unlike messages sent through nerves, which zip around in split seconds, endocrine messages travel through your blood and can affect your whole body over minutes, hours, or even years.

Your endocrine glands include your pituitary gland (which controls growth), your thyroid (which regulates your energy level), your pancreas (which manages blood sugar), and several others. When you're startled, your adrenal glands release adrenaline, an endocrine hormone that makes your heart race and gives you energy to react. When you're growing, your pituitary releases growth hormone. When you hit puberty, endocrine changes transform your body over several years.

Scientists who study this system are called endocrinologists. When your endocrine system works properly, it keeps everything balanced, from your mood to your metabolism to your sleep cycle.