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WHY DOES HORMONES ONLY ACT IN TARGET TISSUES?
Hormones are not directed, and have no chemical path finders letting them reach the appropriate target tissues - they are blind messengers. Hormones are released and carried by the bloodstream, the blood takes the hormone to different parts of the body. The target tissues have recognition molecules which detect the presence of the hormone and are programmed to respond to the presence of the hormone - thus the hormone itself is a passive entity and does not act in anyway. The target cells and tissues react to the hormones in the blood and induce whatever changes are necessary. The actual mechanism for the detecting hormones is via a receptor molecule on the target tissue or cell. Most receptors are essentially large protein molecules, there is a factor known as receptor specificity and the receptor molecule is on a continual alert to react to a specific type of chemical substance in the blood. Hormone receptors react only to the specific hormone they interact with and ignore all other hormones in the blood. The passage of a specific hormone in the blood near a target tissue will be recognized immediately by the receptor molecule on the target tissue. The receptor can be said to attract the specific hormone and captures it - this process means the message has been passed and the tissue reacts. Most receptors are very efficient and sensitive to the availability of their specific hormone in the blood, this is important as most hormones are produced very minute quantities. For the full expression of the hormonal activity, the single most important factor may well be this receptor sensitivity.


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