Hormone

Hormone, chemical that transfers information and instructions between cells in animals and plants. Often described as the body’s chemical messengers, hormones regulate growth and development, control the function of various tissues, support reproductive functions, and regulate metabolism (the process used to break down food to create energy). Unlike information sent by the nervous system, which is transmitted via electronic impulses that travel quickly and have an almost immediate and short-term effect, hormones act more slowly, and their effects typically are maintained over a longer period of time.
Hormones were first identified in 1902 by British physiologists William Bayliss and Ernest Starling. These researchers showed that a substance taken from the lining of the intestine could be injected into a dog to stimulate the pancreas to secrete fluid. They called the substance secretin and coined the term hormone from the Greek word hormo, which means “to set in motion.” Today more than 100 hormones have been identified.
Hormones are made by specialized glands or tissues that manufacture and secrete these chemicals as the body needs them. The majority of hormones are produced by the glands of the endocrine system, such as the pituitary, thyroid, adrenal glands, and the ovaries or testes. These endocrine glands produce and secrete hormones directly into the bloodstream. However, not all hormones are produced by endocrine glands. The mucous membranes of the small intestine secrete hormones that stimulate secretion of digestive juices from the pancreas. Other hormones are produced in the placenta, an organ formed during pregnancy, to regulate some aspects of fetal development.
Hormones are classified into two basic types based on their chemical makeup. The majority of hormones are peptides, or amino acid derivatives that include the hormones produced by the anterior pituitary, thyroid, parathyroid, placenta, and pancreas. Peptide hormones are typically produced as larger proteins. When they are called into action, these peptides are broken down into biologically active hormones and secreted into the blood to be circulated throughout the body. The second type of hormones is steroid hormones, which include those hormones secreted by the adrenal glands and ovaries or testes. Steroid hormones are synthesized from cholesterol (a fatty substance produced by the body) and modified by a series of chemical reactions to form a hormone ready for immediate action