Module 2 The Endocrine System

  Reflect on the Big Picture

At the beginning of this module, you were introduced to Emily. She has all of the same endocrine glands as you do. She even secretes all the hormones that you do. However, due to growth of tiny tumors in her endocrine glands, she may secrete too much of one kind of hormone and not enough of another. Her body is under constant stress, and her negative feedback mechanisms do not restore homeostasis. Instead, positive feedback results and she becomes ill due to the over secretion of hormones putting constant stress on her body.

Going Beyond

Melatonin is a hormone that is produced by the tiny pineal gland located within the brain, and it is involved in your sleep and awake cycle. Many people experience tiredness, inability to concentrate, and irritability when they are not exposed to periods of sunshine. Read “Light up Your Life!” on p. 443 of your textbook and address the following two questions:

1. Explain why people living in northern countries such as Canada, parts of Russia, and the Scandinavian countries may be more susceptible to Season Affective Disorder (SAD), as compared to people who live in more southern regions.
2. How do innovative technologies such as the “Litebook” contribute to society

Lesson Summary

In this lesson, you investigated the following focusing question:

• How is the endocrine system organized, and how do its parts communicate with each other and with various parts of the body?

To answer this question, the major glands of the endocrine system including the hypothalamus/pituitary complex, the thyroid gland, the parathyroid glands, the adrenal glands, and the islet cells of the pancreas were introduced and studied. These glands secrete hormones which have unique effects on the body by interacting with target cells. Some of these hormones work opposite each other and are called antagonistic hormones. Other hormones, called tropic hormones, target other endocrine gland, causing them to consequently secrete more hormones. Levels of hormonal secretions are regulated by negative feedback mechanisms. Negative feedback tends to stabilize a system because the response compensates for the change in the internal environment. This leads to the reestablishment of homeostasis. Reinforcing the change in the internal environment leads to instability and an imbalance or deviation from homeostasis called positive feedback. It is rarely beneficial in the body.

Glossary

antagonistic hormones: two hormones that produce opposite effects

endocrine gland: a cell, tissue, or organ that produces secretions that are released directly into the bloodstream; ductless gland e.g. thyroid gland
exocrine gland: a cell, tissue, or organ that produces secretions that are released through ducts or channels; e.g. a sweat gland

gonadotropic hormones: hormones that affect the reproductive organs; also called the gonads

homeostasis: the maintenance of a constant internal environment or one within narrow limits despite possible fluctuations in the external environment

hormone: a chemical messenger that is produced by specialized cells, circulated in the bloodstream, and coordinates the various parts of the body by interacting with target cells

lipid soluble hormones: hormones that are chemically identified as lipids or steroids, such as testosterone, estrogen, progesterone, or cortisol
negative feedback: a mechanism where deviation from the optimum state causes a return to the optimum state; acts to eliminate any deviation from optimal conditions and leads to stability
positive feedback: a mechanism where movement away from the optimum state causes further deviation from the optimum state; usually leads to instability and is tolerated by the body only for a short time
set point (set value): ideal or optimum conditions

tropic hormone: a hormone that has another endocrine gland as its target cell

water soluble hormones: hormones that are chemically identified as being either amino acids or proteins, such as epinephrine, human growth hormone, thyroxine, insulin, and glucagon