Local signaling molecules typically do not enter the circulatory system, but instead act on targets near the cells that synthesize and release them. The distinction is arbitrary. The first endocrine experiments identified the testes as a source of a circulating factor that affected the appearance and behavior of male chickens. The term hormone was first applied to secretin, a hormone secreted by the intestinal epithelium that stimulates alkaline secretions by the pancreas. Endocrine cells are the cells that secrete hormones.
Target cells are the cells with receptors to which a particular hormone can bind.
Cells without receptors for a particular hormone cannot respond to that hormone. Endocrine cells release hormones in response to signals from hormones, signals from nerve cells, changes in local chemical conditions, and signals from the environment. Protein hormones are synthesized before they are needed and released from storage vesicles via exocytosis. Lipid-soluble hormones such as steroids are not stored, but are released upon synthesis. An example of homeostasis is the maintenance of blood glucose within a range of values compatible with normal cell function.