When You're the One Canceling a Flight. In defense of Spirit, most airlines operate on razor thin margins and refund dollars do add up. Even airlines that allow refunds-generally only under very special circumstances-may make you jump through hoops and even pay a fee.
Medical Definition of receptor. 1  1: a cell or group of cells that receives stimuli : sense organ. 2  2: a chemical group or molecule (as a protein) on the cell surface or in the cell interior that has an affinity for a specific chemical group, molecule, or virus.
Hormones that are circulated in the blood come in contact with a number of cells. However, they only influence certain target cells. Target cells have specific receptors for a specific hormone. Target cell receptors can be located on the surface of the cell membrane or inside of the cell. When a hormone binds to a receptor, it causes changes within the cell that influence cellular function. This type of hormone signaling is described as endocrine signaling because the hormones influence target cells over a distance.