Tissues and cells capable of secreting/responding to hormones. A chemical substance produced in one part of the body that is secreted into the bloodstream and then circulates throughout the body; but a lot of variability. Can affect one or a number of organs (i. e. gnrh vs. t3) Effective in small quantities (i. e. high affinity) Some hormones can act in different ways (i. e. paracrine signalling; autocrine signalling: nervous vs. endocrine system. Two ways that our body exerts control; work together. Endocrine chemical messenger; nervous ap. Endocrine amplitude (concentration) of hormone; nervous frequency of ap. Endocrine receptors; nervous wiring : endocrine system. Many major glands (i. e. hypothalamus, pituitary [master control system], adrenals, thyroids, gonads, pancreas, etc. ) Many other cells that aren"t in discrete glands that also produce hormones. Gut cells secrete many hormones (i. e. gastrin, secretin, cck) Fat cells produce leptin: regulation of hormones. Positive feed forward: classes of hormones.