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How do hormones reach distant target organs?

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Diagram showing the locations of major human endocrine glands in male and female bodies, including pineal gland, hypothalamus, pituitary gland, parathyroid glands, thyroid gland, thymus, adrenal glands, pancreas, kidneys, testes, and ovaries. — Endocrine system The glands of the human endocrine system.

Most hormones reach distant target organs by being secreted into the general circulation, where they travel through the blood to exert their effects on specific tissues.1

To facilitate this transport, many hormones circulate in the blood in two forms: as free, unbound molecules or bound to specific transport proteins. These proteins, produced by the liver, act as a reservoir that maintains a stable concentration of biologically active free hormone, protects against sudden secretory surges, and ensures even distribution to large organs.1

In addition to general circulation, some hormones are transported via specialized portal systems:
* Hypothalamic-hypophyseal portal circulation: Directs neurohormones from the hypothalamus to the anterior pituitary gland.1
* Hepatic portal circulation: Transports hormones from the pancreas, such as insulin and glucagon, directly to the liver before they enter the general bloodstream.1

Once a hormone reaches its destination, it binds to specific receptors on or within target cells to initiate a physiological response.2