The glands are the organs that are responsible to produce and secrete substances that affect the body's metabolism. Glands are classified into external and exocrine secretion, which are substances that release the exterior, such as sweat and sebaceous glands, the ductless or endocrine, which produce substances called hormones, which are released into the blood vessels and regulate the physiological activities the body, and finally mixed glands, formed by a party other exocrine and endocrine, as the pancreas and liver. The pancreas produces insulin, a hormone that converts blood glucose and glycogen. The latter, being degraded in the muscles, produces the energy that allows the body's movements.
The hormones have a role to influence various organs, and can excite, inhibit or regulate their activity. Each hormone is secreted by specialized cells.