Exocrine and endocrine The word hormone is derived from the greek word 'hormao' and means -to excite or stir into action-. Hormones are chemicals secreted into the bloodstream by specialized organs and carried to other parts of the body to perform their task. Organs that secrete and manufacture hormones are known as endocrine glands. Exocrine glands such as tear glands, secrete their products outside the body. Whereas exocrine glands are also called ductal glands, endocrine glands are ductless. Endocrine glands come in a variety of sizes and are located through the whole body. "Hormones are chemical substances which are secreted to the blood by special organs." Hormones are found throughout the animal kingdom and even in plants, but only the vertebrates have specialized organs to produce and to store hormones. In many cases the structure of a hormone is the same over a wide variety of animals, although its function can be different. Some human hormones are not secreted by endocrine glands but come from sources as neurons in the hypothalamus, or cells in the digestive tract. Recently the heart has been found to produce a hormone that helps regulate the blood pressure. Communication Untill the beginning of the 20th century the communication within the body was exclusively attributed to the nervous sytem. However, investigaters discovered that the endocrine sytem is also important for this function. Yet, the role of endocrine glands was anticipated in several ancient civilizations in which they were eaten to modify health or behavior. In the fourth century B.C. Aristoteles described the effects of behavior in birds when removing the testes (castration). Although he did not what mechanism was involved, it was clear to him that the testes were important for the male characteristics. Nowadays we know that the testes produce a certain hormone (testosteron) that cause a lower voice and stronger muscles in male human beings.  Further reading:   Endocrine system   Hormones 1   Hormones 2   Hormones 3 Do you have more information about Hormones in general? Please share it in our database. [ TOP ]   © ThinkQuest Team 26618. All Rights Reserved.