Glands are organs that produce and release substances to regulate certain activities in the body. Glands are classified into two primary groups: exocrine glands and endocrine glands. Exocrine glands are duct bearing glands that secrete their substances onto an epithelial surface through ducts. Endocrine glands secrete substances called hormones directly into the blood, from where these are transported to different parts of the body.

Hormones are otherwise called chemical messengers. These are organic chemical substance synthesised by a specific organ or local group of cells and secreted into the blood or tissue fluid, which carries it to other places of the body where its actions are exerted. Bayliss and Starling defined hormone as a chemical agent released from one group of cells and is conducted easily through the transporting body fluid to affect different cell groups. Hormones are low molecular weight compounds that bring about physiological changes after binding to specific receptors located in the target organ. Some examples of hormones are thyroxine, oxytocin, ADH, oestrogen, testosterone, etc.