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A hormone is a chemical secreted by an endocrine gland that targets a particular cell to produce a response and is transported via blood to other locations in the body, where it affects a particular organ. The means by which hormones are derived is one classification system, which includes three categories: amino acids, peptides, and lipids. Nerves or special chemical signals cause cells to turn on genes responsible for the production and secretion of hormones. Once produced, hormones are released from the cell by way of exocytosis. Each hormone acts on a particular organ that has the specific receptors needed for the hormone to operate. Hormones can move freely within the bloodstream, or hormones can be bound to a carrier protein. The presence of a hormone in the blood can last for varying amounts of time. Hormones are regulated by the amplification of the signal for hormone production, by adding or removing receptors on target cells, through interactions with other hormones, or by removing the hormone altogether. Paracrine signals act locally in the same tissue. Autocrine messenger signals act on the same cell that secreted them.

At A Glance

  • Hormones are chemicals produced by glands and are secreted into the blood to have an effect on another organ.
  • Hormones are classified as either hydrophilic or lipophilic and can be amino acid derived, peptide derived, or lipid derived.
  • Cells receive neural or chemical stimulation to turn on genes responsible for production and secretion of hormones, which happens via exocytosis.
  • Each hormone acts on target organs and may bind to a receptor on the target cell membrane, which sets off a second messenger system within the cytoplasm, or may diffuse into the cell and bind to a receptor.
  • Hormones can circulate freely in the bloodstream or bind to carrier proteins to activate or retard a response as the body requires.
  • The effect of hormones can be regulated by negative feedback, by adding or removing receptors in target cells, by circadian rhythms, and through a process of hormone removal.
  • Paracrine signals act locally in the same tissue, while autocrine messengers bind to receptors on the cell that produced them.