Zoology · Ch 9 — Chemical Coordination and Integration
Endocrine Glands and Hormones
Endocrine Glands and Hormones
Endocrine glands are unusual among glands because they have no ducts — no tubes to carry their product to a particular spot. For this reason they are also called ductless glands. The chemical substances they pour out are known as hormones, and because there is no duct, these secretions pass straight into the bloodstream.
What a hormone is
The older, classical way of describing a hormone was fairly narrow: a chemical made by an endocrine gland, released into the blood, and carried to a target organ located some distance away where it produced its effect. Scientific understanding has since broadened this idea, and the modern definition is stated more generally:
- Hormones are non-nutrient chemicals — they are not foods or fuels.
- They act as intercellular messengers, carrying signals from one set of cells to another.
- They are produced and needed only in very small (trace) amounts.
The advantage of this newer definition is that it is wider. It takes in not only the familiar hormones released by the well-organised endocrine glands, but also a range of other signalling molecules that fit the same description.
The idea of a target
Because a hormone travels widely in the blood yet acts only in particular places, the notion of a target tissue (or target organ) is central: a hormone influences only those cells that are equipped to respond to it. This selective action is what allows a trace amount of a chemical, distributed body-wide, to bring about a specific, localised effect.