| Hormones are a class of bioactive chemicals made by living things. Although most of these chemicals are either proteins , peptides, or steroids; a certain type of chemical structure is not what defines a hormone. Hormones are defined by the way that they act - and that's to activate (or otherwise signal) a cell at a distance.
Hormones have specific effects that occur only when it binds to a receptor on (or in) the cell that responds to it.
- Ordinarily- hormones cause their effects within the body of the organism that produced them.
- The hormone might be produced by one type of cell within an organ, and exert its effects on another cell in that same organ, another organ, or even on cells in that are widely scattered throughout the body.
- That's thought to be their purpose: hormones provide a way that cells can be signalled and co-ordinated to act within a multicellular organism.
- Those effects are often extremely powerful, in fact hormones are substances that have can an enormous effect on living tissues - even in very small amounts.
- It can take as little as one molecule of a hormone to stimulate a cell.
- Unlike some substances, (like say - battery-acid), hormones do not exert their effects willy-nilly. Hormones are only able to directly effect those tissues that 'invite them' to enter.
- Describing a receptor site as "an invitation" is a bit fanciful, admittedly - but it's accurate. Unless there is a molecular gateway for entry, and unless it properly "opens" for a given hormone - there will be no effect.
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- Since there is often similarity in the shapes of the hormones and receptors between different species of mammals, it's sometimes possible inject, say, a human hormone into a rabbit and still get an effect.
- That's the basis of the old fashioned 'rabbit test', by the way, it had to checking for hormones in a pregnant lady using a lab rabbit. Now the hormones are checked for in a lab kit that does not require response by a living thing.
- This cross-effectiveness is what allows animal hormones to be used as therapy for people.
- The first hormone treatments were begun about a hundred years ago, and then, were extracts of whole glands (like thyroid or adrenal glands) of animals (like cows or pigs) taken fresh from slaughter-houses.
- Even though these treatments have become refined and safer over the decades, using human hormones avoids a number of problems, and is now technically possible for many kinds of hormones.
Hormones are made by secretory cells that release them directly into circulating fluids, usually the bloodstream. The first hormones that were discovered in humans (and other animals) are made by endocrine glands. Hormones are the effectors of the endocrine system.
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