ENDOCRINE SYSTEM AND HORMONES
Endocrine glands lack ducts and are hence called ductless
glands. Their secretions are called hormones.
The classical definition of hormone is a chemical produced by endocrine glands and released into the blood and trans- ported to a distantly located target organ
messengers and are produced in trace amounts.
few hormones while in vertebrates, a large number of e Hormones are non-nutrient chemical which act as intercellulare Invertebrates possess very simple endocrine systems with
. The first hormone was discovered by William M. Bayliss chemicals act as hormones and provides coordination.
and Ernest H. Starling in 1903. It was secretin coming from the duodenal mucous membrane
digestion, excretion, reproduction, etc.
This is called synergism, e.g., FSH and LH0 2
. Hormones regulate metabolism, growth, secretion,
. When some hormones work together to control a process,
When two hormones work against each other to control a process, this is called antagonism, e.g, insulin and glucagon calcitonin and parathormone.
The tissue on which the hormone acts is known as 'target tissue'.
A target cell responds to a hormone because it bears receptors for the hormone.
The secretion of hormones is regulated by feed back mechanisms.
Synthesis and release of some hormones is regulated by
nerves and the hormones may also influence nerve activities.
