The Endocrine System

Hormones

• From Greek hormon, meaning to “excite or set into motion”
• Function as part of a negative feedback mechanisms (the action of the hormones shuts down the synthesis and release of that hormone)
• Produced by a specialized gland or organized group of cells; sometimes stored then secreted directly into bloodstream.
• Exerts its effect elsewhere than where it was produced, so must be stable for transport.
• Only required in very small amounts, they “trigger” a response that amplifies their signal.
• Acts upon specific target cells (e.g. calcitonin acts only on bone and kidneys to reduce blood Ca²⁺ levels) because they are recognized by a receptor on the target cell.
• Enzymatically cleared from the body once they have activated a cell/tissue
• Some are released as prohormones that are modified to an active form in the target cell.