7 Endocrine System Fun Facts

There Are More Glands Than You Think

Textbooks cite variable numbers of endocrine glands, largely because many cells groups can secrete hormones. The primary endocrine system glands are:

• Hypothalamus
• Pituitary gland
• Pineal gland
• Thyroid gland
• Parathyroid glands
• Adrenal gland
• Pancreas
• Ovary (in females)
• Testis (in males)

However, other groups of cells may secrete hormones, including the placenta (estrogen and progesterone) and stomach (ghrelin). Older sources may cite the thymus as a member of the endocrine system, but it is excluded from modern texts because it doesn't actually secrete any hormones.

Endocrinology Has Been Practiced Over 2,000 Years

The medical and scientific study of the endocrine system is called endocrinology. Even though ancient healers had no way to understand the function of endocrine glands, Chinese healers in 200 B.C. used the compound saponin from seeds and the mineral gypsum to extract pituitary and sex hormones from human urine to make medicine. Endocrinology wasn't recognized as a science in its modern form until the nineteenth century.

Hormones Weren't Discovered Until the 20th Century

While Chinese healers extracted and used hormones for centuries, the chemical nature of those hormones remained elusive. In the 1800s, scientists knew that some form of chemical messaging occurred between organs. Finally, in 1902, English physiologists Ernest Starling and William Bayliss coined the word "hormones" to describe pancreatic secretions.

A Gland Can Have Both Endocrine and Exocrine Functions

Endocrine glands are clusters of cells, rather than entire organs. The pancreas is an organ that contains both endocrine and exocrine tissue. Insulin and glucagon are two endocrine hormones released by the pancreas. Pancreatic juice, secreted by a duct into the small intestine, is an exocrine product.

The Endocrine System Responds to Stress

Physical and emotional stress cause the endocrine system to produce more hormones. For example, more adrenaline and growth hormone are released, to aid in physical exertion and speed the metabolism. However, the system is designed to improve short term survival. Prolonged stress causes endocrine disorders, including obesity and the autoimmune thyroid disorder Graves' disease.

Other Animals Have Endocrine Systems

Humans and other vertebrates (e.g., cats, dogs, frogs, fish, birds, lizards) all have a hypothalamus-pituitary axis that serves as the basis for the endocrine system. Other vertebrates also have a thyroid, although it may serve a slightly different function. For example, in frogs, the thyroid regulates the transformation from a tadpole into an adult. All vertebrates have an adrenal gland, too.

Endocrine signaling isn't limited to vertebrates. All animals with a nervous system have an endocrine system.

Plants Produce Hormones Without an Endocrine System

Plants don't have an endocrine or exocrine system, but they still produce hormones to control growth, fruit ripening, repair, and metabolism. Some hormones diffuse to local tissue, like exocrine hormones. Others are transported through plant vascular tissue, much like endocrine hormones.

Endocrine System Key Takeaways

• The endocrine system is a chemical messaging network.
• Endocrine glands secrete hormones, which are carried by the circulatory system throughout the body.
• The primary endocrine glands are the pituitary, hypothalamus, pineal gland, thyroid, parathyroid, adrenal, pancreas, ovary, and testis.
• Hormones maintain homeostasis in the body. Improper function is associated with diseases, including osteoporosis, obesity, diabetes mellitus, and thyroid disease.

Sources

• Hartenstein V (September 2006). "The neuroendocrine system of invertebrates: a developmental and evolutionary perspective". The Journal of Endocrinology. 190 (3): 555–70. doi:10.1677/joe.1.06964.
• Marieb, Elaine (2014). Anatomy & physiology. Glenview, IL: Pearson Education, Inc. ISBN 978-0321861580.
• Temple, Robert G (1986) The Genius of China: 3000 Years of Science, Discovery, and Invention. Simon and Schuster. ISBN-13: 978-0671620288
• Vander, Arthur (2008). Vander's Human Physiology: the mechanisms of body function. Boston: McGraw-Hill Higher Education. pp. 345–347. ISBN 007304962X.