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Hormones and the Body
Hormones are chemical messengers that coordinate functions in the body in
response to changes in an animal's internal environment or stimuli from the outside
world.
Hormones regulate growth and development.
From fertilization through adulthood, hormones mediate growth and development. Hormones even play a role in
formation of the bond between mother and child.
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Topics Covered in this Module The Chemical Structure of Hormones
Hormones and Homeostasis
When Hormone Regulation Fails
Major Objectives of this Module Classify hormones based on their chemical structure and solubility.
Describe how hormones trigger responses in target cells.
Distinguish between negative and positive feedback loops and give an example of each.
Explain the role of hormones in maintaining homeostasis.
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Most hormones can be classified based on their chemical composition and structure as either amine hormones, peptide
hormones, or steroid hormones. Additionally, hormones can be classified as either water soluble or lipid soluble. The
chemical properties and solubility of a hormone have consequences on how it is secreted, how it is transported through
the blood, and how it signals to its target cell.
The Chemical Structure of Hormones
Hormones are chemical messengers produced in endocrine cells in one part of the body and transported through the
blood to target cells in other parts of the body. Hormones carry out numerous regulatory functions in response to stimuli
from both outside and inside the body. Hormones also regulate reproduction, growth, and the changes that occur during
puberty.
Hormones can be classified based on chemical structure.
Hormones are a diverse group of organic compounds with a variety of chemical structures. The major hormones can be
classified into three categories — amine hormones, peptide hormones, and steroid hormones. Amine hormones are all
derived from single amino acids. Several hormones fall into this category, including thyroxine, dopamine, epinephrine,
and norepinephrine, which are derived from the amino acid tyrosine, and melatonin and serotonin, which are derived
from the amino acid tryptophan.
Peptide hormones contain two or more amino acids joined by peptide bonds or are derivatives of such molecules.
Some peptide hormones are relatively small molecules; for example, the hormones oxytocin and vasopressin are
composed of only nine amino acids. Other peptide hormones are much larger; human growth hormone, for instance, is
191 amino acids in length.
Steroid hormones are derived from cholesterol, a hydrophobic molecule containing four fused rings. Cholesterol is
converted into steroid hormones in several endocrine glands, including the adrenal cortex, the testes (in males), and the
ovaries (in females). Examples of steroid hormones include cortisol, testosterone, and progesterone.
The solubility of a hormone, which is a function of its chemical structure, is important in determining how it is secreted,
how it is transported through the body, and how it communicates its message once it reaches its target cell. All peptide
hormones and most amine hormones are water soluble. Some amine hormones, including the thyroid hormones
thyroxine and triiodothyronine, are lipid soluble. Steroid hormones, like their parent compound cholesterol, are lipid
soluble.
Water­soluble hormones cannot cross the lipid bilayer of plasma membranes and are typically released from endocrine
cells by exocytosis. These hormones diffuse into the bloodstream and travel to target cells, which have cell surface
receptors for the hormones associated with the plasma membrane. Hormone binding induces a conformational change
in the receptor that activates a signal transduction pathway, triggering a response in the cell (Figure 1a).
Most lipid­soluble hormones can freely diffuse through the plasma membranes of both the endocrine cell and the target
cell. However, the lipid­soluble thyroid hormones thyroxine and triiodothyronine must be carried across the plasma
membrane by transport proteins. Lipid­soluble hormones travel through the aqueous environment of the blood
associated with transport proteins. Some receptors for lipid­soluble hormones are on the cell surface, and others are
located inside the cell. To reach intracellular receptors, lipid­soluble hormones diffuse across the plasma membrane.
The intracellular receptor­hormone complex moves into the nucleus, where it binds DNA and activates or inhibits
expression of particular genes (Figure 1b).
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Figure 1: Transmission pathways of water­soluble and lipid­soluble hormones.
Water­soluble hormones (left), which cannot pass through the plasma membrane, bind to a receptor on the outside
of the target cell. Signal transduction transfers the signal from the outside of the cell to the inside. Lipid­soluble
hormones (right), which can diffuse through cell membranes, often bind intracellular receptors that directly mediate
a cellular response.
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The Hypothalamus Integrates Nervous System Input and Endocrine System Output
The nervous system senses external stimuli such as the scent of a predator and internal stimuli such as an increase in
blood pressure. The endocrine system mediates a response to these stimuli. Thus, these two systems must be closely
coordinated. In vertebrates, the hypothalamus, an endocrine gland located in the brain (Figure 1b), is the primary site at
which sensory input is converted to endocrine output. The hypothalamus receives sensory input from neurons and
sends this information to the pituitary gland, which extends from the bottom of the hypothalamus. The pituitary has
anterior and posterior lobes. The posterior pituitary is an extension of the hypothalamus that contains axons from
specialized neurons originating in the hypothalamus. The specialized neurons, called neurosecretory cells, secrete
neurohormones that circulate in the blood and act on distant cells. The neurosecretory cells of the posterior pituitary
produce two types of neurohormones: oxytocin and vasopressin (ADH, also known as antidiuretic hormone, or ADP).
Vasopressin regulates water and salt balance, and oxytocin moderates behavior and stimulates mammary glands and
uterine contractions.
The anterior pituitary, which is a separate organ from the posterior pituitary, synthesizes and secretes hormones based
on hormonal signals from the hypothalamus. Hormones secreted from the anterior pituitary include growth hormone
(GH), thyroid­stimulating hormone (TSH), follicle­stimulating hormone (FSH), luteinizing hormone (LH),
adrenocorticotropic hormone (ACTH), and prolactin (PRL). GH stimulates growth and metabolism. TSH stimulates the
thyroid gland, which is involved in maintaining metabolic balance. FSH stimulates egg and sperm production. LH
regulates ovaries and testes. ACTH causes the adrenal cortex to release glucocorticoids in response to long­term
stress. Prolactin stimulates the production and secretion of milk.
The hypothalamus regulates activity of other endocrine glands. For example, in response to stress, the hypothalamus
stimulates secretions of the adrenal gland, which sits above the kidneys. The hypothalamus may activate two different
stress­response pathways in the adrenal gland, depending on whether the stress is acute or long term. In an acute
stress response, which is activated by a scary event such as encountering a tiger in the woods, a nerve signal is sent
from the hypothalamus to the adrenal gland. In response, the middle part of the adrenal gland, called the adrenal
medulla, releases the hormones epinephrine and norepinephrine into the blood. Epinephrine and norepinephrine
stimulate the "fight­or­flight" response, which results in the breakdown of glycogen by the liver and an increase in
breathing and heart rate. Glycogen breakdown frees up glucose, an energy source the body will need for fight or flight;
the increased breathing rate enables greater intake of oxygen, which will be needed for the additional cellular
respiration cells will be performing during fight or flight to generate energy; and the increased heart rate speeds the
delivery of that oxygen to the cells that will be performing cellular respiration.
In long­term stress, which is activated by a stressful event such as loss of a job, the hormone corticotropin­releasing
hormone (CRH) is released from the hypothalamus and travels through blood to anterior pituitary. CRH stimulates the
anterior pituitary to secrete adrenocorticotropin hormone (ACTH), which travels to the adrenal gland through the
bloodstream. In response, the outer part of the adrenal gland, called the adrenal cortex, releases glucocorticoids.
Glucocorticoids promote breakdown of fats and proteins to increase blood glucose levels, and reduce immune function.
Increased levels of glucocorticoids are associated with improved memory and vigilance, which are presumably needed
to get an animal through a stressful situation. However, increased this vigilance takes an emotional toll, and the
increased blood sugar necessary to maintain this vigilance takes a physiological toll. Therefore, long­term elevation of
glucocorticoid levels is detrimental to health.
IN THIS MODULE
The Chemical Structure of Hormones
Hormones and Homeostasis
When Hormone Regulation Fails
Summary
Test Your Knowledge
PRIMARY LITERATURE
Adaptor proteins regulate cell
signaling
Structural basis for regulation of the Crk
signaling protein by a proline switch.
View | Download
SCIENCE ON THE WEB
Overview of Human Hormones
Browse this NIH resource of human
hormones and their clinical relevance
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Hormones and Homeostasis
The external environment, as well as availability of food and water, is constantly changing. Despite these changes,
physiological conditions, such as metabolic rate, body temperature, and blood solute concentrations, must be
maintained within a narrow range. The endocrine system plays a vital role in maintaining constant conditions within the
body, or homeostasis, through a process called negative feedback. However, in some cases, a rapid change in
internal conditions is necessary. This rapid change is often achieved through positive feedback.
Negative feedback minimizes physiological changes.
In negative feedback mechanisms, a change to a physiological system induces responses that reduce the change so
that the system returns to the setpoint. An example of a negative feedback mechanism is regulation of body temperature
by the hypothalamus. In response to a drop in body temperature, the hypothalamus secretes thyrotropin­releasing
hormone (TRH), which causes the anterior pituitary to release thyroid­stimulating hormone (TSH). TSH travels via the
bloodstream to the thyroid gland. TSH stimulates thyroid cells to secrete two hormones, thyroxine (T4 ) and
triiodothyronine (T3 ). T3 and T4 increase metabolic rate in cells throughout the body and and raise the body
temperature. T3 is the more biologically active of the two hormones. When body temperature increases, the
hypothalamus ceases THR production, and metabolism slows (Figure 2).
Figure 2: Negative feedback regulation of body temperature.
Thyroid hormones stimulate metabolism, which raises body temperature.
© 2014 Nature Education All rights reserved.
Test Yourself
Secretin, a peptide hormone made of 27 amino acids, is produced in the duodenum of the
small intestine. Secretin inhibits gastric acid secretion into the stomach and stimulates
bicarbonate secretion into the duodenum (bicarbonate is a base). Describe how pH
homeostasis might be accomplished by adjusting secretin secretion levels.
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Positive feedback amplifies an initial physiological change.
In positive feedback, a change induces a response that amplifies the change. Positive feedback loops are less
common than negative feedback loops because they are associated with instability — they tend to drive a system further
away from a homeostatic setpoint. Nevertheless, they are essential in some processes in which a large or rapid
response to a stimulus is required. As with negative feedback, positive feedback loops can be mediated by hormones. A
classic example of a positive feedback loop is the effect of the hormone oxytocin on uterine contractions during
childbirth (Figure 3). Pressure of the fetus's head against the cervix acts initiates a neural signal to the hypothalamus in
the brain. In response, the posterior pituitary secretes oxytocin. The oxytocin is carried through the blood to the uterus,
where it stimulates contractions of the uterine muscle. In turn, the stronger contractions intensify and prolong the
pressure of the fetus's head on the cervix, further stimulating the release of oxytocin until the fetus is born.
Figure 3: Positive feedback loop during labor.
The fetus puts pressure on the cervix, which induces the production of oxytocin, which further intensifies uterine
contractions. The cycle continues until the fetus is born.
© 2014 Nature Education All rights reserved.
In addition to its role in childbirth, oxytocin facilitates other aspects of maternal behavior. For example, it stimulates the
release of milk for a nursing child. Increased oxytocin levels present during the postpartum period are believed to be
involved in bonding between mother and child.
IN THIS MODULE
The Chemical Structure of Hormones
Hormones and Homeostasis
When Hormone Regulation Fails
Summary
Test Your Knowledge
PRIMARY LITERATURE
Adaptor proteins regulate cell
signaling
Structural basis for regulation of the Crk
signaling protein by a proline switch.
View | Download
SCIENCE ON THE WEB
Overview of Human Hormones
Browse this NIH resource of human
hormones and their clinical relevance
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138 Hormones and the Body
When Hormone Regulation Fails
Failures of the endocrine system can disrupt homeostasis, resulting in disease. For example, certain diseases are
associated with an imbalance in hormone production by the thyroid gland. One of these diseases is hypothyroidism, a
condition in which insufficient thyroid hormone is produced. The two thyroid hormones, T3 and T4 , both contain iodine.
(T3 , or triiodothyronine, contains three iodine atoms, while T4 contains four iodine atoms.) If an iodine deficiency occurs,
the body cannot make sufficient thyroid hormones, resulting in hypothyroidism. The hypothalamus senses low levels of
circulating thyroid hormone and synthesizes TRH in response. TRH stimulates TSH synthesis in the anterior pituitary.
TSH stimulates the growth of thyroid tissue, which results in a swelling in the neck called a goiter. In extreme cases, a
goiter can grow to the size of a grapefruit or larger. Hypothyroidism can also result in fatigue, reduced heart rate, and
weight gain. Iodine deficiency is particularly dangerous in children because thyroid hormone is essential to normal
brain development; hypothyroidism can result in a type of mental retardation historically known as "cretinism." The
fortification of table salt with iodine in the modern era has dramatically reduced the incidence of hypothyroidism due to
iodine deficiency, especially in the developing world.
Paradoxically, hyperthyroidism, or an overactive thyroid gland, can also result in an enlarged thyroid. An autoimmune
condition known as Graves' disease can lead to hyperthyroidism. In Graves' disease, the body inappropriately produces
antibodies against the receptor for TSH on thyroid cells. The binding of the antibodies to the receptor mimics the binding
of TSH to the receptor, and the thyroid cells secrete T3 and T4 into the blood as a result. The increase in circulating
thyroid hormone reduces production of TRH by the hypothalamus and TSH by the anterior pituitary. However, even
without TSH, the autoimmune antibodies continue to stimulate the production of thyroid hormone and growth of the
thyroid gland as a whole — again resulting in the formation of a goiter. In addition to the goiter, patients with Graves'
disease present with exophthalmos (bulging eyes), fatigue, and weight loss with increased appetite. Treatment of
Graves' disease includes drugs that interfere with the enzymes that add iodine to thyroid hormones to make them
functional. Another treatment involves radioactive iodine, which accumulates in the thyroid and kills some of the thyroid
tissue, lowering the amount of thyroid hormone produced to a more normal level.
Test Yourself
Both hypothyroidism and hyperthyroidism can result in a goiter. If a patient came to you with
a goiter, how could you use a simple blood test to determine which condition she has?
Submit
IN THIS MODULE
The Chemical Structure of Hormones
Hormones and Homeostasis
When Hormone Regulation Fails
Summary
Test Your Knowledge
PRIMARY LITERATURE
Adaptor proteins regulate cell
signaling
Structural basis for regulation of the Crk
signaling protein by a proline switch.
View | Download
SCIENCE ON THE WEB
Overview of Human Hormones
Browse this NIH resource of human
hormones and their clinical relevance
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Summary
OBJECTIVE
Classify hormones based on their chemical structure and solubility.
Most hormones can be classified based on their chemical composition and structure as either amine hormones, peptide
hormones, or steroid hormones. Additionally, hormones can be classified as either water soluble or lipid soluble. The
chemical properties and solubility of a hormone have consequences on how it is secreted, how it is transported through
the blood, and how it signals to its target cell.
OBJECTIVE
Describe how hormones trigger responses in target cells.
Hormones are produced by endocrine cells and travel through the blood to reach their target cells. Water­soluble
hormones are frequently released from endocrine cells by exocytosis. Once in the blood, they diffuse freely to their target
cells. At the target cell, they bind to cell surface receptors to transmit their signal. Most lipid­soluble hormones can diffuse
through the plasma membranes of both the endocrine cell and the target cell. They require specialized transport
proteins to move through the aqueous environment of the blood. At the target cell, they may transmit their signal by
interacting with intracellular receptors or by binding to receptors on the cell surface.
OBJECTIVE
Distinguish between negative and positive feedback loops and give an example of each.
Negative feedback loops produce responses that mitigate the effect of a physiological change. An example of a negative
feedback loop is regulation of body temperature by adjustment of thyroid hormone levels. Thyroid hormones stimulate
metabolism and increase body heat. Thyroid hormone production increases when body temperature is low and
decreases when body temperature is high. Positive feedback loops produce responses that intensify the effect of a
physiological change. A positive feedback loop occurs during childbirth. Pressure of the fetus's head on the cervix
induces the secretion of the hormone oxytocin, which stimulates uterine contraction, resulting in more pressure on the
cervix; the feedback loop continues until the fetus is born.
OBJECTIVE
Explain the role of hormones in maintaining homeostasis.
Hormones play a significant role in maintaining homeostasis in the human body. Internal stimuli, such as changes in
blood glucose or osmolarity, and external stimuli, such as changes in environmental temperature, trigger the release of
hormones that allow the individual to produce a response. Disruptions of hormonal regulation can result in disease. For
example, hypothyroidism and hyperthyroidism result from failures in the production or regulation of thyroid hormone
secretion.
Key Terms
amine hormone
A hormone that is chemically derived from an amino acid; examples include epinephrine (derived from tyrosine)
and serotonin (derived from tryptophan).
homeostasis
Maintenance of a set value for a given physiological parameter.
hormone
One of many types of substances produced in a small amount by specialized glands or cells at one site and
transported via a circulatory system or other body fluid to target cells at another site in an organism.
hypothalamus
An endocrine gland in the brain that integrates neural and endocrine functions.
negative feedback
A form of regulation in which a change in some physiological parameter triggers a response that mitigates the
change; an important mechanism for maintaining homeostasis.
neurohormone
A signaling molecule released from neurons directly into body fluids; mediator of neuroendocrine signaling.
neurosecretory cell
A neuron that secretes neurohormones.
peptide hormone
A hormone that is composed of multiple amino acids linked by peptide bonds; examples include oxytocin and
insulin.
pituitary gland
A gland at the base of the hypothalamus; consists of a posterior lobe that secretes neurohormones and an anterior
lobe that is an endocrine gland.
positive feedback
A form of regulation in which a change in some physiological parameter triggers a response that amplifies,
prolongs, or strengthens the change.
receptor
A protein to which a signaling molecule binds.
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steroid hormone
A hormone that is chemically derived from cholesterol; examples include testosterone, progesterone, and cortisol.
target cell
Any cell that has receptors for a specific hormone.
IN THIS MODULE
The Chemical Structure of Hormones
Hormones and Homeostasis
When Hormone Regulation Fails
Summary
Test Your Knowledge
PRIMARY LITERATURE
Adaptor proteins regulate cell
signaling
Structural basis for regulation of the Crk
signaling protein by a proline switch.
View | Download
SCIENCE ON THE WEB
Overview of Human Hormones
Browse this NIH resource of human
hormones and their clinical relevance
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IN THIS MODULE
The Chemical Structure of Hormones
Test Your Knowledge
Hormones and Homeostasis
When Hormone Regulation Fails
1. Based on their chemical structure, most hormones can be classified into which of the following three groups?
Summary
Test Your Knowledge
amine, peptide, and steroid
amine, peptide, and polypeptide
hydrophilic, hydrophobic, and lipophilic
hydrophilic, lipophilic, and steroid
steroid, hydrocarbon, and cyclic
PRIMARY LITERATURE
Adaptor proteins regulate cell
signaling
Structural basis for regulation of the Crk
signaling protein by a proline switch.
View | Download
2. Which of the following statements is true about a hormone's solubility?
Steroid hormones are soluble in water.
Peptide hormones cannot diffuse across the plasma membrane.
All amine hormones are water soluble.
None of the statements made in the other choices are correct.
All of the statements made in the other choices are correct.
SCIENCE ON THE WEB
Overview of Human Hormones
Browse this NIH resource of human
hormones and their clinical relevance
3. What must a target cell have in order for a hormone to initiate a response?
a cell membrane
a second messenger
a receptor
a nucleus
None of the answers are correct.
4. Which of the following statements about homeostasis is true?
A negative feedback loop increases the strength of contractions during childbirth.
Positive feedback loops produce responses that mitigate the effect of a physiological change.
Positive feedback loops produce responses that intensify the effect of a physiological change.
Negative feedback loops produce responses that intensify the effect of a physiological change.
Body temperature is regulated by a positive feedback loop.
5. What hormone does the anterior pituitary produce during the stress response?
epinephrine
norepinephrine
thyroid­stimulating hormone
ACTH
glucagon
6. What stimulus initiates the positive feedback loop that takes place during childbirth?
the release of oxytocin from the adrenal medulla
Neural signals from the uterus to the hypothalamus
the release of oxytocin from the posterior pituitary
pressure of the baby's head against the mother's cervix
secretion of milk in the breast tissue
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