Signaling molecules released by a cell may have to travel great distances to reach receptor cells in the body. In this case, they are often carried in the circulatory system. Signaling by means of molecules that travel through the bloodstream is called endocrine signaling (Fig. 9.4a; Chapter 38).

Adrenaline (section 9.1) provides a good example of endocrine signaling. Adrenaline, which is produced in the adrenal glands, is carried by the bloodstream to target cells that are far from the signaling cells. Other examples of endocrine signaling involve the mammalian steroid hormones estradiol (an estrogen) and testosterone (an androgen). These hormones travel from the ovaries and the testes, respectively (although there are other minor sources of these hormones), through the bloodstream, to target cells in various tissues throughout the body. The increased amount of estrogen in girls during puberty causes the development of breast tissue and the beginning of menstrual cycles. The increased amount of testosterone in boys during puberty causes the growth of muscle cells, deepening of the voice, and growth of facial hair (Chapter 42).