Lecture 14 - hormones and human body function :dr. e. l. tsiani. Chemical signals packages in granules or secretory vesicles. Secreted by a cell or group of cells. Activate physiological response at low concentrations (nano molar to pico molar) Hormones are secreted by endocrine glands or cells into the blood. Chemicals released by neurons into blood to act on distant targets. Act on the same cell that secreted them. Secreted by one cell and diffuse to adjacent cells. Cellular mechanism of action: depends on binding to target cell receptors. May have different actions depending on the tissue or stage of development. Controls biochemical and translational and transcriptional events. Transport of ions or molecules across cell membranes. Half life indicates length of activity: a measure of the time needed to decrease the hormone concentration in circulation by 50%. There are degrading enzymes in the circulation, liver and kidney. Amino acid-derived or amine hormones: thyroid hormones, catecholamine"s.