ABSTRACT
A hormone is a chemical messenger signaling from one cell (or group of cells) to another in order to carry information to the target cells. Endocrine hormone molecules are secreted (released) directly into the bloodstream, whereas exocrine hormones are secreted directly into a duct, followed by either ow into the bloodstream or ow from cell to cell by diffusion in a process known as paracrine signaling (Figure 27.1). Well-known hormone classes are amine-derived hormones (e.g., catecholamines and thyroxine), lipid-derived hormones (e.g., steroid hormones like testosterone and cortisol), and peptide hormones (e.g., vasopressin, growth hormone, and insulin). Peptide hormones are peptides that are secreted into the bloodstream and have endocrine functions in living animals.
