Every day thousands of chemical reactions happen inside your body’s cells. All of these reactions are collectively called metabolism. These reactions do not occur randomly: they are strictly coordinated and organized by substances in the body called hormones.
The endocrine system is made of glands that are responsible for producing and releasing hormones into your blood. There are two types of glands: endocrine and exocrine.
Endocrine glands are fully a part of the endocrine system, making and releasing hormones (for example, insulin and estrogen) into the bloodstream. Exocrine glands do not release hormones. Instead, they release substances through ducts to the exterior of the body (such as sweat and saliva).
Some organs in other systems have a secondary endocrine function besides their primary function. For example, besides pumping blood, the heart in the cardiovascular system produces hormones and releases them when necessary. The same happens with the kidneys, liver and gonads, among other organs 1,8,9.