from old greek: ορμàν (horman) - to drive
Hormones are signalling and messaging molecules that ensure regulation of different body functions. They are secreted by hormone producing cells into surrounding tissue (paracrine delivery) or in blood vessels (classical endocrine delivery).
Action of hormones originates from their binding to receptors that are located either in the cell membrane or in cytoplasm. After interaction with their target molecule they trigger in the cell a signalling cascade comprising different intracellular transmitters, e.g. second messengers.
Secretion of hormones is controlled by - often complex - regulation mechanisms on molecular level that are embedded in elaborate feedback control systems and frequently organised in antagonistic redundancy mechanisms. Temporally limited action of hormones is determined by their degradation, e.g. in target tissues, and measured as half life.
Examples for endocrine glands are:
Although, many other tissues are apt to secrete hormones that act as tissue hormones.
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