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How do hormones travel from endocrine glands to their target tissues?

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Hormones are primarily secreted by endocrine glands directly into the bloodstream, which serves as the main transport system to reach distant target tissues. Because endocrine glands possess a rich supply of blood vessels, they can readily release their secretions into the circulation.12

While most hormones travel through the general circulation, some utilize specialized portal circulations—such as the hypothalamic-hypophyseal portal system—to be directed specifically to certain organs, like the anterior pituitary gland.2

In the blood, hormones exist in two states:
* Free, unbound hormones: These are biologically active and capable of entering tissues to bind with specific receptors to initiate a response.12
* Protein-bound hormones: Many hormones circulate attached to specific transport proteins. These proteins act as a reservoir, maintaining a stable concentration of free hormones and protecting against sudden fluctuations in secretion. As free hormones are utilized by tissues, bound hormones are released from their carrier proteins to maintain equilibrium.2