Answer: The correct answer for the blank is -
Blood.
Hormones can be described as chemical messengers that are secreted by specific glands and released directly into bloodstream (particularly by endocrine glands), which carry them to particular location in the body.
Example - ACTH (adrenocorticotropic hormone) is released by anterior pituitary gland into the blood, which taked it to adrenal gland and stimulate it to produce steroid hormones.
Thus, blood is the right answer.