NCERT Book Class 11 Biology Chapter 19 Chemical Coordination and Integration

CHEMICAL COORDINATION AND INTEGRATION 239

C HAPTER 19
C HEMICAL C OORDINATION
AND INTEGRATION

19.1 Endocrine You have already learnt that the neural system pr ovides a
Glands and point-to-point rapid coordination among organs. The neural
Hormones coordination is fast but short-lived. As the nerve fibres do not innervate
all cells of the body and the cellular functions need to be continuously
19.2 Human
regulated; a special kind of coordination and integration has to be
Endocrine
provided. This function is carried out by hormones. The neural system
System
and the endocrine system jointly coordinate and regulate the
19.3 Hormones of physiological functions in the body.
Heart, Kidney
and
19.1 E NDOCRINE GLANDS AND HORMONES
Gastrointestinal
Tract
Endocrine glands lack ducts and are hence, called ductless glands. Their
19.4 Mechanism of secretions are called hormones. The classical definition of hormone as a
Hormone Action 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
possess very simple endocrine systems with few hormones whereas a large
number of chemicals act as hormones and provide coordination in the
vertebrates. The human endocrine system is described here.