BIOLOGY FOR AP® COURSES
Chapter 28 ENDOCRINE SYSTEM
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28.1 THE ENDOCRINE SYSTEM
In this section, you will explore the following questions:
• What are the different types of hormones?
• What is the role of hormones in maintaining homeostasis?

HORMONES
• Hormone
• Regulatory chemical that is secreted into extracellular fluid
and carried by the blood
• Can act at a distance from source
• Multicellular animals need hormones to help homeostasis
• Hormone will travel to cell that has receptor for that hormone,
attachment to the receptor will induce cellular response
• Watch this video:
• Endocrine system
• Organs and tissues that produce hormones

LIPID-DERIVED HORMONES
The structures shown here represent (a) cholesterol, plus the steroid
hormones (b) testosterone and (c) estradiol.

AMINO ACID-DERIVED HORMONES
(a) The hormone epinephrine, which triggers the fight-or-flight response,
is derived from the amino acid tyrosine. (b) The hormone melatonin,
which regulates circadian rhythms, is derived from the amino acid
tryptophan.

PEPTIDE HORMONES
The structures of peptide hormones (a) oxytocin, (b) growth hormone,
and (c) follicle stimulating hormone are shown. These peptide hormones
are much larger than those derived from cholesterol or amino acids.

COMPARISON OF NERVOUS & ENDOCRINE SYSTEMS
• Neuroendocrine System
Nervous system controls through nerve impulses
conducted by axons.
Responses occur within milliseconds.
Relatively local, specific effects.
Stops when stimulus stops, adapts quickly.

COMPARISON OF NERVOUS & ENDOCRINE SYSTEMS
Endocrine system controls through hormones
transported in the blood.
May have widespread general effects.
Responses occur after seconds to days & are more
prolonged, adapts slowly

28.2 HOW HORMONES WORK
In this section, you will explore the following questions:
• How do hormones work?
• What is the role of different types of hormone receptors?

HOW HORMONES WORK
Hormones are very specific
• Need a specific receptor in order to work
Receptors change with cellular activity
• Receptors may be found on many different cells or limited to a
small number of specialized cells
• Number of receptors can change
• Up-regulation
• Down-regulation
• Hormone receptors may be found within the cell or on the
plasma membranes

INTRACELLULAR HORMONE RECEPTORS
Lipid-derived hormones bind to transport proteins
• Once they reach the target cell, they are released from their
carrier protein and pass through the plasma membrane and
bind to a target within the cell.
• Lipophilic – cross membrane and bind to intra-cellular receptor
• The hormone/receptor complex regulates transcription by
increasing or decreasing the synthesis of mRNA.

INTRACELLULAR HORMONE RECEPTORS
An intracellular nuclear receptor (NR) is located in the cytoplasm bound to a
heat shock protein (HSP). Upon hormone binding, the receptor dissociates from
the heat shock protein and translocates to the nucleus. In the nucleus, the
hormone-receptor complex binds to a DNA sequence called a hormone
response element (HRE), which triggers gene transcription and translation. The
corresponding protein product can then mediate changes in cell function.

PLASMA MEMBRANE HORMONE RECEPTORS
• Amino acid derived hormones and peptides cannot pass
thorough the plasma membrane
• Hydrophilic – cannot cross the membrane, have to bind to
receptor on the membrane
• Bind to receptors on the outer surface of the plasma
membrane and initiates signaling pathway (called the
2nd messenger system)

PLASMA MEMBRANE HORMONE RECEPTORS
The amino acid-derived hormones epinephrine and norepinephrine bind to
betaadrenergic receptors on the plasma membrane of cells. Hormone
binding to receptor activates a G-protein, which in turn activates adenylyl
cyclase, converting ATP to cAMP. cAMP is a second messenger that
mediates a cell-specific response. An enzyme called phosphodiesterase
breaks down cAMP, terminating the signal.

PLASMA MEMBRANE HORMONE RECEPTORS
Do not have to know or memorize this for this class, but it’s important
to understand how intricate these systems can be and how important
they are in maintaining homeostasis.
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28.3 REGULATION OF BODY PROCESSES
In this section, you will explore the following questions:
• How do hormones regulate the excretory system?
• What roles do hormones play in the reproductive system?
• How do hormones regulate metabolism?
• What is the role of hormones in different diseases?

HORMONAL REGULATION OF BODY PROCESSES
Excretory System
• Maintains water balance
• Antidiuretic hormone (ADH)
• Regulates amounts of water excreted by kidneys therefore
regulating blood pressure
• Released when water levels of bloodstream is lower
(function of osmo-receptors)
• Underproduction causes diabetes insipidus
• Different from diabetes mellitus (type 1 and type II)

EXCRETORY SYSTEM
• Aldosterone (steroid hormone)
• Produced by adrenal cortex
• Main regulator of water and electrolytes such as sodium
and potassium in the body
• Promotes reabsorption of water
• Released due to the signaling of the renin angiotensin
system, whereas ADH is released with the function of osmo-
receptors

REPRODUCTIVE SYSTEM
• Follicle Stimulating Hormone (FSH)
• Males: maturation of sperm cells
• Females: development of egg cells
• Luteinizing Hormone (LH)
• Males: production of testosterone
• Females: surge causes ovulation
• Oxytocin
• Stimulates uterine contraction during childbirth

METABOLISM
• Insulin
• Produced by beta cells of pancreas
• Lowers blood sugar by increasing cellular uptake and
utilization by cells
• Deficiency results in diabetes mellitus
• Glucagon
• Antagonist to insulin
• Produced by alpha cells of pancreas
• Raises blood sugar by targeting liver to break down glycogen

METABOLISM
• Insulin and glucagon
regulate blood glucose
levels.
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In response to glucagon,
the liver breaks down
glycogen and releases
glucose into the blood.

REGULATION OF METABOLISM BY THYROID
HORMONES
• Basal metabolism rate (amount of calories required by
body at rest)
• determined by thyroxine (T4) and triiodothyronine (T3).
• Released from thyroid gland in response to stimulation
by TSH
• Hypothyroidism = low metabolic rate
• Weight gain, sensitivity to cold, lethargy
• Hyperthyroidism = high metabolic rate
• Weight loss, irritability, increased heart rate

HORMONAL CONTROL OF BLOOD CALCIUM
• Calcium regulated by:
• PTH (produced by parathyroid glands)
• Released in response to low blood calcium levels
• Antagonist is Calcitonin (produced by thyroid gland)
• Antagonist hormone has opposite effects on body

HORMONAL CONTROL OF GROWTH
• Growth and cell replication regulated by:
• GH (Growth Hormone)
• Stimulates growth, cell reproduction, and cell regeneration
• Increases rate of protein synthesis
• Is glucose sparing
• Regulated by growth hormone releasing and inhibiting
hormones from hypothalamus

HORMONAL CONTROL OF STRESS
• Short Term Stress
• Epinephrine for “fight or flight” from adrenal gland
• Long Term Stress
• Cortisol from adrenal gland
• Anti-inflammatory

28.4 REGULATION OF HORMONE PRODUCTION
In this section, you will explore the following questions:
• How is hormone production regulated?
• What are examples of different stimuli that control hormone levels in
the body?

HORMONE PRODUCTION
• Controlled by a negative feedback mechanism
• 3 types of stimuli regulate hormone production
1. Humoral stimuli – response to changes in bodily fluids
2. Hormonal stimuli – release of a hormone in response to
other hormones
3. Neural stimuli – direct stimulus by the nervous system on
the endocrine glands

28.5 ENDOCRINE GLANDS
In this section, you will explore the following questions:
• What are the roles of different glands in the endocrine system?
• How do different glands work together to maintain homeostasis?

HYPOTHALAMUS
• Integrates the endocrine and nervous systems.
• Synthesizes and secretes regulatory hormones that control
endocrine cells in the anterior pituitary.
Caption: 1807 Posterior Pituitary Complex (c) OpenStax, Public domain

THE PITUITARY GLAND
• Also known as the hypophysis
• Hangs by a stalk from the hypothalamus
• Consists of two parts
• Anterior pituitary (adenohypophysis)
• Appears glandular
• Not part of brain
• Hypothalamus releases neurohormones into anterior pituitary,
causing the anterior pituitary to release tropic hormones (these
hormones then work on other endocrine glands)
• Posterior pituitary (neurohypophysis)
• Appears fibrous because it contains axons from Hypothalamus
• Part of brain
• Releases ADH and oxytocin

THE PITUITARY GLAND

THE ANTERIOR PITUITARY
• Produces at least 7 essential hormones
• Peptide Hormones
• Adrenocorticotropic hormone (ACTH) – stimulates adrenal to
make cortisol
• Melanin-stimulating hormone (MSH) – stimulates dispersion of
pigment
• Protein Hormones
• Growth hormone (hGH)
• Prolactin (PL) – stimulate mammary glands
• Glycoprotein Hormones
• Thyroid-stimulating hormone (TSH) – acts on thyroid
• Luteinizing hormone (LH) – acts on ovaries, testes
• Follicle-stimulating hormone (FSH) – development of ovarian
follicles, development of sperm

THE POSTERIOR PITUITARY
• 2 hormones are produced in the hypothalamus, and
stored in special cells in the posterior pituitary.
• Antidiuretic hormone (ADH)
• Oxytocin

ADRENAL GLANDS
The location of the adrenal glands on top of the kidneys is shown.
(credit: modification of work by NCI)
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PANCREAS
• Pancreatic Cells – Islets of Langerhans
• 2 types of cells within islets
1. Alpha cells – produce glucogon
2. Beta cells – produce insulin

PINEAL GLAND
• Found within the brain
• Produces melatonin
• Affected by photoperiod (more is produced at
night) – circadian rhythm
• Effective antioxidant

GONADS
• Male testes and female ovaries
• Produce steroid hormones
• Testes produce:
• Androgens
• Testosterone
• Ovaries produce:
• Estradiol
• Progesterone

ENDOCRINE SYSTEM SUMMARY

ORGANS WITH SECONDARY ENDOCRINE FUNCTIONS
Organs like:
• Heart
• Kidneys
• Intestines
• Thymus
• Gonads
• Adipose tissue