Introduction to the Endocrine System

Endocrine glands secrete chemical messengers, called hormones ( Box 1.1 ), into the extracellular fluid in a highly regulated manner. Secreted hormones gain access to the circulation, often via fenestrated capillaries, and regulate target organs throughout the body. The endocrine system is composed of the pituitary gland , the thyroid gland , parathyroid glands , and adrenal glands ( Fig. 1.1 ). The endocrine system also includes the ovary and testis , which carry out a gametogenic function that is absolutely dependent on their endogenous endocrine function. In addition to dedicated endocrine glands, endocrine cells reside as a minor component (in terms of mass) in other organs, either as groups of cells (the islets of Langerhans in the pancreas) or as individual cells spread throughout several glands, including the gastrointestinal (GI) tract, kidney, heart, adipose tissue , and liver . In addition, there are several types of hypothalamic neuroendocrine neurons that produce hormones. The placenta serves as a transitory exchange organ, but also functions as an important endocrine structure of pregnancy.