Endocrine glands are composed of aggregations of epithelial or epithelioid cells which secrete chemically specific and biologically highly active substances, known as hormones, directly or indirectly into the blood stream. In consequence they are often known as glands of internal secretion. Blood then conveys the glandular secretion to the target organs whose cells react by a change in activity. Since the primary receptors (site of action) of an hormone within a target organ may be the cell membrane or a constituent of nucleus or cytoplasm or both, a complex series of biochemical changes results, involving in some cases genetic transcription and translation, and in others, effects on enzyme activity in specific metabolic pathways.
KeywordsIodine Respiration Retina Noradrenaline Progesterone
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