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chemical messenger that, when secreted, works only within the cell that produces it


intercellular protein that binds calcium

circadian rhythm

fluctuations that repeat multiple times due to a set change in time, such as the change from light to dark

down regulation

cellular reduction in the number of hormone receptors to a molecule, which lessens the cell's reaction to the molecule


the bulk transport of large molecules across the cell membrane


intermediate molecule that, when activated during the binding of a receptor, turns on a secondary messenger system


amount of time it takes for half of a hormone concentration to be removed from the blood

hormonal stimulus

release of one hormone in response to another produced by a different organ


chemical secreted by an endocrine gland that targets a particular cell to produce a response

humoral stimulus

release of a hormone due to a change in the ion and nutrient composition of the blood


able to tolerate being close to or in water

hypophyseal portal system

connection of capillaries that exists between the hypothalamus and the anterior pituitary gland that serves to deliver anterior pituitary hormones


able to tolerate being close to or in lipids

negative feedback

system used by the body to counteract a change

neural stimulus

signal sent by nerves to release hormones


chemical messenger that affects the cells in its immediate area


space found between two nerve cells, across which neurotransmitters travel

target cell

cell that is impacted by a particular hormone