The endocrine system consists of a series of glands that signal each other by secreting chemicals called hormones into the blood. These hormones travel to target tissues to regulate various critical functions, which include metabolism, reproduction, growth and development. Exposure to endocrine active chemicals (EACs) can affect reproduction and development in both humans and wildlife by inducing adverse hormonal changes in the tightly regulated endocrine pathways. Chemicals capable of acting as EACs are ubiquitous with environmental sources that include household detergents, pesticides, plastics, pharmaceutical estrogens, industrial chemicals, and byproducts of incineration and fuel combustion.
The endocrine system regulates various processes vital for life. It is a dynamic control system of the body which coordinates responses of a target tissue to internal and external signals. The primary function of the endocrine system is to maintain a balanced physiological system that can sustain a dynamic equilibrium by avoiding large fluctuations in hormone levels and their responses. By integrating specialized responses of individual organs, the endocrine system allows the desired cellular environment to be maintained. The physiological processes regulated by the endocrine system include short- and long-term metabolism, reproduction, growth, functions of the gut, kidneys, and cardiovascular system, and stress responses. 
The endocrine system is organized to stabilize the cellular environment by a seesaw-type mechanism. The seesaw-type mechanism operates by feedback signals between the target cells and the regulating cells, with the result that secretion of a target cell-stimulating hormone is altered by the products of the target cells. The balance of the two sides of the seesaw is determined by a programmed set point, which determines the level at which each side of the seesaw will respond to signals from the other side. The preprogrammed set point represents the optimal physiological condition. The endocrine system is organs and tissues integrate their information using crosstalk, where signaling from hormones are shared between different biological pathways, and responses to hormones can activate multiple responses in the organism.
Glands of the endocrine system signal each other by secreting hormones into the blood, which travel to target tissues to regulate critical functions. The brain initiates the process by sending neural signals. The hypothalamus responds to these neural signals and secrets hormones into the blood and transported to the pituitary. In response, the pituitary secretes different hormones that travel to specific downstream endocrine glands, such as the thyroid, adrenal gland, and gonads. In response, these glands release hormones that travel to target tissues to regulate specific physiological processes. Hormones produced by the thyroid regulate general metabolic rate, growth and possibly embryonic development. Hormones secreted by the adrenal gland are involved in stress responses, osmoregulation and carbohydrate metabolism. The hormones released by the gonads control reproduction. To maintain control of these physiological processes, some hormones secreted by the downstream glands travel back to the hypothalamus and pituitary to form a closed-loop negative feedback control system.

Endocrine system related literatures
1. Basic & clinical endocrinology[M]. Stamford, CT: Appleton & Lange, 1997.
2. Capen C C, Karbe E, Deschl U, et al. Endocrine system[M]//International Classification of Rodent Tumors. The Mouse. Springer Berlin Heidelberg, 2001: 269-322.