The endocrine system is comprised of a series of varied glands located throughout the body that secrete a variety of chemical hormones that regulate different systems in the human body. The levels of the different hormones in the bloodstream are regulated by different means including negative feedback loops. A negative feedback loop is a system whereby the production of a substance is controlled by the level of that substance or a related substrate in the bloodstream. One example is insulin. Insulin is a hormone manufactured in the pancreas that helps control glucose levels in the bloodstream. When glucose levels increase in the bloodstream this activates the production of insulin. When the insulin lowers the glucose levels to normal, the production of insulin is stopped. So in this way, insulin levels in the bloodstream control insulin production through the glucose substrate.