I want to say frequencies would be the answer. I could be wrong though. Hope this may have helped!
Id say this is more of a biology question or even a psychology question , well its not a question at all but the endocrine system is a collection of glands that secrete hormones direct to the blood system (circulatory system) to be sent to the desired / target organ , an example of a gland could be the pituitary gland (located towards rear of brain)
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