7.1 - The endocrine system
Locate the main endocrine glands and distinguish a hormone’s source, blood transport and target organ. Explain why a circulating hormone causes a response only in particular cells.
7.1 - Hormones: source, route and target
An endocrine gland is an organ that produces and releases hormones. Hormones do not travel along neurones or through a duct: an endocrine gland releases them directly into the blood.
Hormone
A chemical messenger produced by an endocrine gland and carried in the blood to a target organ, where it causes a response.
The route has three distinct parts:
- Source: cells in an endocrine gland produce a hormone and release it into the bloodstream.
- Transport: circulating blood carries the hormone around the body.
- Target: cells in the target organ have receptors that the hormone can bind to, so those cells respond.
Blood may carry the hormone past many organs, but an organ without the relevant receptor does not make the same response. The source gland, the transport medium and the target organ are therefore not interchangeable terms.
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