15.1.1—Endocrine system features

The endocrine system is a set of glands whose secretions are hormones. An endocrine gland releases a hormone into the blood, allowing it to travel to distant cells and coordinate a response.

• A hormone acts as a chemical signal carried in the bloodstream.
• Only target cells with a complementary receptor respond directly. Receptor location depends on the hormone and target cell.
• This receptor match makes signalling selective even though the hormone circulates widely.

Blood-borne endocrine signalling is suited to coordination that does not require an instant response and can persist beyond the initial release. The same system can influence distant target tissues, while receptor distribution determines which cells respond.

Endocrine signalling is not the same route as a nerve impulse: the message is a hormone carried in blood rather than an electrical impulse travelling along a neurone. The detailed nervous pathway, synapse, and muscle-response mechanisms are covered separately.