23 3.5. The Endocrine System
The endocrine system consists of a network of glands that produce chemical substances known as hormones (Figure 3.34). Like neurotransmitters, hormones are chemical messengers that must bind to a receptor in order to have an effect. However, unlike neurotransmitters, which are released in close proximity to the receptors they affect, hormones are secreted into the bloodstream and travel widely throughout the body. Thus, whereas neurotransmitters’ effects are localized, the effects of hormones are widespread. Also, hormones are slower to take effect, and tend to be longer lasting.
Hormones are involved in regulating all sorts of bodily functions, and they are controlled by the hypothalamus (in the central nervous system) through its action on the pituitary gland (in the endocrine system). Imbalances in hormones are related to a number of disorders, including diabetes, obesity, infertility, thyroid disease, insomnia, and even acne! This section explores some of the major glands that make up the endocrine system and the hormones secreted by these glands (Table 3.2).
Table 3.2. Major Endocrine Glands and Associated Hormone Functions
|
Endocrine Gland |
Associated Hormones |
Function |
|---|---|---|
|
Pituitary |
Growth hormone, releasing and inhibiting hormones (such as thyroid stimulating hormone) |
Regulate growth, regulate hormone release |
|
Thyroid |
Thyroxine, triiodothyronine |
Regulate metabolism and appetite |
|
Pineal |
Melatonin |
Regulate some biological rhythms such as sleep cycles |
|
Adrenal |
Epinephrine, norepinephrine |
Stress response, increase metabolic activities |
|
Pancreas |
Insulin, glucagon |
Regulate blood sugar levels |
|
Ovaries |
Estrogen, progesterone |
Mediate sexual motivation and behavior, reproduction |
|
Testes |
Androgens, such as testosterone |
Mediate sexual motivation and behavior, reproduction |
Major Glands
The pituitary gland is located at the base of the brain, and receives messages from the hypothalamus. The pituitary is often referred to as the “master gland” because its messenger hormones control all the other glands in the endocrine system. In addition to messenger hormones, the pituitary also secretes growth hormone, endorphins for pain relief, and a number of key hormones that regulate fluid levels in the body.
The thyroid gland is located in the neck and releases hormones that regulate growth, metabolism, and appetite. In hyperthyroidism, or Grave’s disease, the thyroid secretes too much of the hormone thyroxine, causing feelings of agitation, bulging eyes, and weight loss. In hypothyroidism, reduced hormone levels cause people with this condition to experience tiredness, weight gain, and they often complain of feeling cold. Fortunately, thyroid disorders are often treatable with medications that help reestablish a balance in the thyroid hormones.
The adrenal glands sit on top of our kidneys and secrete hormones involved in the stress response, such as epinephrine (adrenaline) and norepinephrine (noradrenaline). The pancreas is an internal organ that secretes two hormones, insulin and glucagon, which regulate blood sugar levels. These pancreatic hormones are essential for maintaining stable levels of blood sugar throughout the day by lowering blood glucose levels (insulin) when levels are too high or raising them when they are too low (glucagon). People who suffer from diabetes do not produce enough insulin; therefore, they must take medications that stimulate or replace insulin production, and they must closely control the amount of sugars and carbohydrates they consume.
The gonads secrete sexual hormones, which are important in reproduction, and mediate both sexual motivation and behavior. The female gonads are the ovaries; the male gonads are the testes. Ovaries secrete estrogens and progesterone, and the testes secrete androgens, such as testosterone.
Dig Deeper
Athletes and Anabolic Steroids
Although it is against Federal laws and many professional athletic associations (The National Football League, for example) have banned their use, anabolic steroid drugs continue to be used by amateur and professional athletes. Anabolic steroid drugs mimic the effects of the body’s own steroid hormones, like testosterone and its derivatives. These drugs have the potential to provide a competitive edge by increasing muscle mass, strength, and endurance, although not all users experience these results. Moreover, use of performance-enhancing drugs (PEDs) does not come without risks. Anabolic steroid use has been linked with a wide variety of potentially negative outcomes, ranging in severity from largely cosmetic (acne) to life threatening (heart attack). Furthermore, use of these substances can result in profound changes in mood and can increase aggressive behavior (National Institute on Drug Abuse, 2001).
Baseball player Alex Rodriguez (A-Rod) spent the latter part of his playing career at the center of a media storm regarding his use of illegal PEDs. Rodriguez excelled while using the drugs; his success played a large role in negotiating a contract that made him the highest paid player in professional baseball. A subsequent scandal and suspension tarnished his reputation and, according to a statement he made once retired, cost him over $40 million (Gaines, 2013). Even lower-profile athletes, particularly in cycling (e.g., US Lance Armstrong who had several Tour de France trophies revoked) and Olympic sports, have been revealed as steroid users. What are your thoughts on athletes and doping? Why or why not should the use of PEDs be banned? What advice would you give an athlete who was considering using PEDs?
