The purpose of the endocrine system is to regulate activities within the body. This system maintains homeostasis in the body. Homeostasis is the maintenance of a constant internal environment. The endocrine system consists of organs called glands that produce and secrete chemical messengers called hormones that travel through the blood to target cells.
The endocrine system works in conjunction with the nervous system to exert control over various functions in the body. While the nervous system is responsible for more rapid responses, the endocrine system produces more long-term effects. The glands of the endocrine system produce hormones that affect only specific types of cells. Once a hormone reaches the target cell, it binds to a site on the cell called a receptor. Once bound to a receptor, the hormone initiates activity within the cell.
The hypothalamus and pituitary gland work in conjunction to control the other glands. The pituitary gland, nicknamed the "master gland," receives messages from the hypothalamus and then sends its own messages out to other glands. The endocrine system also includes pure endocrine glands such as the thyroid as well as organs, such as the ovaries. These organs have other functions but also contain endocrine cells that secrete hormones.
The hypothalamus signals the pituitary gland to release hormones that affect other endocrine glands. The release of the target hormones ultimately provides feedback to the hypothalamus, which in turn sends a message to the pituitary gland to stop producing its hormones. This concept, called a feedback loop, controls the actions of the endocrine system.