The endocrine system is the system in which a message is transmitted from endocrine tissues; for example the pancreas and insulin, by circulating body fluids mainly the bloodstream. The specificity of the signal depends the receptors on target tissues and cells. The signal is released in the form of a chemical messenger known as hormones. Each hormone can have different effects in similar tissues depending on the receptor present; an example being epinephrine and how in arterioles it can cause vasodilation and/or vasoconstriction depending on the specific receptors present. Endocrine cells within endocrine glands release hormones conveyed particularly by the bloodstream and can act on distant cells: examples of endocrine glands being the anterior pituatry, the thyroid gland and the adrenal gland.