The endocrine system is a collection of glands and organs that produce and regulate hormones in the bloodstream to control many functions of the body. This system overlaps with the nervous system and exocrine system, and its responsibilities include metabolism, growth and sexual development. Most animals that have advanced physiology, such as vertebrates and crustaceans, have an endocrine system.
The major glands of the endocrine system are the pituitary gland, hypothalmus and pineal gland in the brain; the thyroid and parathyroids in the neck; the thymus, adrenal glands and pancreas in the abdomen; and the gonads — either ovaries or testes — in the lower abdomen. To a lesser degree, organs such as the heart, lungs and stomach are involved in hormone management. The glands of a person's endocrine system control many things, such as when he or she falls asleep and when the person reaches his or her adult height.
Hormones that are made in the body's glands work like messages. Just like with other types of communication, the proper message must reach its intended destination to be effective. For this reason, certain hormones are designed to end up only at certain cells, called target cells.
After the hormone reaches the correct cell, it links to a receptor spot, which informs the cell what to do next. For example, it might be told to start making energy out of sugar or to trigger ovulation. The hormone won't interfere with a non-target cell, however, and the target cell won't react to any chemical other than its special hormone. As of early 2012, scientists did not completely understand the roles of all hormones, but they did know that hormones are important for maintaining homeostasis — a healthy balanced state within the body.
The hormones produced by these glands are far too numerous and complicated to list. For example, the pituitary gland is often called the master gland because it controls the functioning of other members of the endocrine system. Pineal glands makes melatonin, which regulates people's sleeping cycles. The pancreas produces insulin, which controls how much sugar is kept circulating in the bloodstream.
The endocrine system can be thought of as the system that determines the cycles of the body, such as reproductive cycles, sleep cycles and nutrition cycles. People don't need to eat at perfectly spaced intervals to have a constant supply of energy, nor do they have the same fertility levels throughout their lives. The endocrine system regulates these cycles to ensure that the body has the appropriate amount of energy and the proper fertility levels. It also is responsible for long-term development such as bone growth and short-term cycle stages such as hunger.