NEET Biology Chapter 22

Chemical Coordination and Integration

Endocrine glands lack ducts and are hence, called ductless glands. Their secretions are called hormones. The classical definition of hormone as a chemical produced by endocrine glands and released into the blood and transported to a distantly located target organ has current scientific definition as follows : Hormones are non nutrient chemicals which act as intercellular messengers and are produced in trace amounts. The new definition covers a number of new molecules in addition to the hormones secreted by the organised endocrine glands. Invertebrates posses very simple endocrine systems with few hormones whereas a large number of chemicals act as hormones provide coordination in the vertebrates. The human endocrine system is described here.

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