• Hormones are the messenger molecules of the endocrine system.
  • Endocrine hormones travel throughout the body in the blood.
  • Each hormone affects only certain cells, called target cells.
  • A target cell is affected by a particular hormone because it has receptor proteins that are specific to that hormone.
  • A hormone travels through the bloodstream until it finds a target cell with a matching receptor it can bind to.
  • When the hormone binds to a receptor, it causes a change within the cell.
  • Exactly how this works depends on whether the hormone is a steroid hormone or a non-steroid hormone.
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