Learning objectives
By the end of this section, you will be able to:
- Identify the major glands of the endocrine system
- Identify the hormones secreted by each gland
- Describe each hormone’s role in regulating bodily functions
The endocrine system consists of a series of glands that produce chemical substances known as hormones (CNX_Psych_03_05_Endocrine). Like neurotransmitters, hormones are chemical messengers that must bind to a receptor in order to send their signal. However, unlike neurotransmitters, which are released in close proximity to cells with their receptors, hormones are secreted into the bloodstream and travel throughout the body, affecting any cells that contain receptors for them. 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.
View original figure (external; image not reproduced in this text edition)
Original figure description: A diagram of the human body illustrates the locations of the thymus, several parts within the brain (pineal gland, thalamus, hypothalamus, pituitary gland), several parts within the thyroid (cartilage of the larynx, thyroid gland, parathyroid glands, trachea), the adrenal glands, pancreas, uterus (female), ovaries (female), and testes (male).
Hormones are involved in regulating all sorts of bodily functions, and they are ultimately controlled through interactions between the hypothalamus (in the central nervous system) and the pituitary gland (in the endocrine system). Imbalances in hormones are related to a number of disorders. This section explores some of the major glands that make up the endocrine system and the hormones secreted by these glands (Table_03_05_01).
Major Glands
The pituitary gland descends from the hypothalamus at the base of the brain, and acts in close association with it. The pituitary is often referred to as the “master gland” because its messenger hormones control all the other glands in the endocrine system, although it mostly carries out instructions from the hypothalamus. 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.
Located in the neck, the thyroid gland releases hormones that regulate growth, metabolism, and appetite. In hyperthyroidism, the thyroid secretes too much of the hormone thyroxine, causing agitation, bulging eyes, and weight loss. One cause of hyperthyroidism is Graves' disease, an autoimmune disease in which one’s own body attacks itself. In hypothyroidism, reduced hormone levels cause sufferers to experience tiredness, and they often complain of feeling cold. Fortunately, thyroid disorders are often treatable with medications that help reestablish a balance in the hormones secreted by the thyroid.
The adrenal glands sit atop 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 hormones that regulate blood sugar levels: insulin and glucagon. These pancreatic hormones are essential for maintaining stable levels of blood sugar throughout the day by lowering blood glucose levels (insulin) or raising them (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.
| 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 |
Summary
The glands of the endocrine system secrete hormones to regulate normal body functions. The hypothalamus serves as the interface between the nervous system and the endocrine system, and it controls the secretions of the pituitary. The pituitary serves as the master gland, controlling the secretions of all other glands. The thyroid secretes thyroxine, which is important for basic metabolic processes and growth; the adrenal glands secrete hormones involved in the stress response; the pancreas secretes hormones that regulate blood sugar levels; and the ovaries and testes produce sex hormones that regulate sexual motivation and behavior.
Review Questions
The two major hormones secreted from the pancreas are:
- estrogen and progesterone
- norepinephrine and epinephrine
- thyroxine and oxytocin
- glucagon and insulin
D
The ________ secretes messenger hormones that direct the function of the rest of the endocrine glands.
- ovary
- thyroid
- pituitary
- pancreas
C
The ________ gland secretes epinephrine.
- adrenal
- thyroid
- pituitary
- master
A
The ________ secretes hormones that regulate the body’s fluid levels.
- adrenal
- pituitary
- testes
- thyroid
B
Critical Thinking Questions
Hormone secretion is often regulated through a negative feedback mechanism, which means that once a hormone is secreted it will cause the hypothalamus and pituitary to shut down the production of signals necessary to secrete the hormone in the first place. Most oral contraceptives are made of small doses of estrogen and/or progesterone. Why would this be an effective means of contraception?
The introduction of relatively low, yet constant, levels of gonadal hormones places the hypothalamus and pituitary under inhibition via negative feedback mechanisms. This prevents the alterations in both estrogen and progesterone concentrations that are necessary for successful ovulation and implantation.
Chemical messengers are used in both the nervous system and the endocrine system. What properties do these two systems share? What properties are different? Which one would be faster? Which one would result in long-lasting changes?
Both systems involve chemical messengers that must interact with receptors in order to have an effect. The relative proximity of the release site and target tissue varies dramatically between the two systems. In neurotransmission, reuptake and enzymatic breakdown immediately clear the synapse. Metabolism of hormones must occur in the liver. Therefore, while neurotransmission is much more rapid in signaling information, hormonal signaling can persist for quite some time as the concentrations of the hormone in the bloodstream vary gradually over time.
Personal Application Questions
Given the negative health consequences associated with the use of anabolic steroids, what kinds of considerations might be involved in a person’s decision to use them?
Glossary
- adrenal gland
- sits atop our kidneys and secretes hormones involved in the stress response
- diabetes
- disease related to insufficient insulin production
- endocrine system
- series of glands that produce chemical substances known as hormones
- gonad
- secretes sexual hormones, which are important for successful reproduction, and mediate both sexual motivation and behavior
- hormone
- chemical messenger released by endocrine glands
- pancreas
- secretes hormones that regulate blood sugar
- pituitary gland
- secretes a number of key hormones, which regulate fluid levels in the body, and a number of messenger hormones, which direct the activity of other glands in the endocrine system
- thyroid
- secretes hormones that regulate growth, metabolism, and appetite