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 and exocrine system, and its responsibilities include metabolism, growth, and sexual development. Most animals with advanced physiology, such as vertebrates and crustaceans, house an endocrine system.

The major glands of the endocrine system are the pituitary, hypothalmus, and pineal located in the brain, the thyroid and parathyroids in the neck, the thymus, adrenals, 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. These glands must control everything from when we fall asleep to when we reach our adult height.

Hormones made in our glands work like messages. Just like in life, the proper message must reach its intended destination to be effective. So certain hormones are designed to only end up at certain cells, called target cells. Once the hormone reaches the right cell, it links to a receptor spot, which informs the cell what to do next. It might start making energy out of sugar or trigger ovulation. But the hormone won't interfere with a non-target cell, and the target cell won't react to any chemical other than its special hormone. Scientists don't completely understand the multiplicitous roles of all hormones, but they do know how good they are at maintaining homeostasis, a healthy balanced state.

The hormones produced by these important 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. The pineal gland makes melatonin, which decides that we sleep when its dark and wake when its light because we are diurnal animals. The pancreas produces insulin, which decides how much sugar to keep circulating in our blood.

We can think of the endocrine system as one that determines the cycles of our body. For instance, we have a reproductive cycle, a sleep cycle, and a nutrition cycle. We don't need to eat at perfectly spaced intervals to have a constant supply of energy. Nor do we have the same fertility rates throughout our lives. Without the endocrine system, we would have no long-term development like bone growth, nor any short-term cycles like hunger.