1. Hormones and the Endocrine System
Chapter 45
Hormones and the Endocrine System

2. YOU MUST KNOW
Two ways hormones affect target organs.
The secretion, target, action, and regulation of at least three hormones.
An illustration of both positive and negative feedback in the regulation of homeostasis by hormones.

3. The Body’s Long-Distance Regulators
The endocrine system and the nervous system act individually and together in regulating an animal’s physiology.
The endocrine system of an animal is the sum of all its hormone-secreting cells and tissues.
Endocrine glands are ductless and secrete hormones directly into body fluids.
Hormones are chemical signals that cause a response in target cells.
Positive and negative feedback regulates most endocrine secretion. 3

4. Control Pathways and Feedback Loops
Example
Stimulus
Low blood
glucose
Receptor
protein
Pancreas
secretes
glucagon ( )
Endocrine
cell
Blood
vessel
Liver
Target
effectors
Response
Suckling
Sensory
neuron
Hypothalamus/
posterior pituitary
Neurosecretory
Posterior pituitary
secretes oxytocin
( )
Smooth muscle
in breast
Milk release
Hypothalamic
neurohormone
released in
response to
neural and
hormonal
signals
Hypothalamus
secretes prolactin-
releasing
hormone ( )
Anterior
pituitary
prolactin ( )
Mammary glands
Milk production
(c) Simple neuroendocrine pathway
(b) Simple neurohormone pathway
(a) Simple endocrine pathway
Glycogen breakdown, glucose release into blood
Figure 45.2a–c
There are three types of hormonal control pathways: simple endocrine, simple neurohormone, and simple neuroendocrine.
In each pathway, a receptor/sensor (blue) detects a change in some internal or external variable (the stimulus) and informs the control center (gold).
The control center sends out an efferent signal, either a hormone (red circles) or neurohormone (red squares).
An endocrine cell carries out BOTH the receptor and control center functions.

5. Mechanisms of Hormone Action
Hormones and other chemical signals bind to target cell receptors, initiating pathways that culminate in specific cell responses. There are basically two mechanisms of hormone action:
Cell Surface Receptors: bind the hormone, and a signal transduction pathway is triggered, eliciting a response to the signal.
Ex: The binding of epinephrine to liver cells causes a cascade that leads to the conversion of glycogen to glucose.
Intracellular Receptors: bound by hormones that are lipid-soluble. The receptor then acts as a transcription factor, causing a change in gene expression.
Ex: Testosterone and estrogen enter the nuclei of target cells, bind the DNA, and stimulate transcription of certain genes.
Reception (the target cell’s detection of a signal coming from outside the cell – signal is detected when it binds to a cellular protein on the cell’s surface).
Signal transduction (binding changes receptor protein in some way – initiating transduction which converts the signal to a form that can bring about a cellular response).
Response (transduced signal triggers a specific cellular response).

6. Varying Degrees of Hormonal Effect
The hormone epinephrine has multiple effects in mediating the body’s response to short-term stress
Different receptors
different cell responses
Epinephrine
a receptor
b receptor
Vessel
constricts
dilates
Glycogen
breaks down
and glucose
is released
from cell
(a) Intestinal blood vessel
(b) Skeletal muscle blood vessel
(c) Liver cell
Different intracellular proteins
Glycogen deposits
Note: hormones in the body can affect one tissue, a few tissues, or most of the tissues in the body (as with sex hormones). Or, they may affect other endocrine glands (tropic hormones).
The same hormone may have different effects on target cells that have
Different receptors for the hormone
Different signal transduction pathways
Different proteins for carrying out the response
Figure 45.4a–c

7. Paracrine Signaling by Local Regulators
In a process called paracrine signaling
Various types of chemical signals elicit responses in nearby target cells
Paracrine signaling involves local regulators – they convey messages between neighboring cells (as opposed to long-distance endocrine signaling by hormones).
These can elicit cell responses more quickly than hormones.
Local regulators have various functions and include
Cytokines and growth factors
Immune responses and cell differentiation
Nitric oxide
Relaxes smooth muscle cells which dilates vessels and improves blood flow
Prostaglandins
Several regulatory functions – including smooth muscle contraction in female uterine helping sperm to reach egg

8. The Major Human Endocrine Glands
Hypothalamus
Pineal gland
Pituitary gland
Thyroid gland
Parathyroid glands
Adrenal glands
Pancreas
Ovary
(female)
Testis
(male)
Figure 45.7
Hypothalamus
Neurosecretory
cells of the
hypothalamus
Axon
Anterior
pituitary
Posterior
HORMONE
ADH
Oxytocin
TARGET
Kidney tubules
Mammary glands,
uterine muscles
The hypothalamus and pituitary integrate many functions of the vertebrate endocrine system:
The HYPOTHALAMUS receives information from nerves throughout the body and from other parts of the brain and then initiates endrocrine signals in response.
The POSTERIOR PITUITARY is an extension of the hypothalamus that stores and secretes two hormones:
Oxytocin causes contraction of the uterine muscles in childbirth and ejection of milk during nursing.
Antidiuretic hormone (ADH) makes the collecting tubules of the kidney more permeable to water, increasing water retention.

9. Relation Between the Hypothalamus and Pituitary Gland
Other hypothalamic cells produce tropic hormones that are secreted into the blood and transported to the anterior pituitary or adenohypophysis
Tropic Effects Only
FSH, follicle-stimulating hormone
LH, luteinizing hormone
TSH, thyroid-stimulating hormone
ACTH, adrenocorticotropic hormone
Nontropic Effects Only
Prolactin
MSH, melanocyte-stimulating hormone
Endorphin
Nontropic and Tropic Effects
Growth hormone
Neurosecretory cells
of the hypothalamus
Portal vessels
Endocrine cells of the
anterior pituitary
Hypothalamic
releasing
hormones
(red dots)
HORMONE
FSH and LH
TSH
ACTH
MSH
TARGET
Testes or
ovaries
Thyroid
Adrenal
cortex
Mammary
glands
Melanocytes
Pain receptors
in the brain
Liver
Bones
Pituitary hormones (blue dots)
Figure 45.8
The ANTERIOR PITUITARY consists of endocrine cells that synthesize and secrete several hormones. Some of these are TROPIC HORMONES, which means they stimulate the activity of other endocrine tissues (FSH, LH, TSH, ACTH).
FSH (Follicle-Stimulating Hormone) stimulates development of the ovarian follicles in females and promotes spermatogenesis in males by acting on cells in the seminiferous tubules.
Luteinizing Hormone (LH) triggers ovulation in females and stimulates the production of testosterone by the interstitial cells of the testes. 9

10. Human Endocrine Glands & Their Hormones
Table 45.1

11. Human Endocrine Glands & Their Hormones
Table 45.1

12. Nonpituitary Hormones
Nonpituitary hormones help regulate metabolism, homeostasis, development, and behavior
Many nonpituitary hormones regulate various functions in the body and include:
Thyroid hormones
Parathyroid Hormone and Calcitonin
Insulin and Glucagon
Adrenal Hormones
Glucocorticoids, such as cortisol
Mineralocorticoids, such as aldosterone
Sex hormones

13. Thyroid Hormones
The thyroid gland consists of two lobes located on the ventral surface of the trachea
Produces two iodine-containing hormones, triiodothyronine (T3) and thyroxine (T4)

14. Negative Feedback Loops Control Thyroid Hormones
The hypothalamus and anterior pituitary control the secretion of thyroid hormones through two negative feedback loops
Hypothalamus
Anterior
pituitary
TSH
Thyroid T3 T4 +
The hypothalamus secretes TSH-releasing hormone (TRH), which stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH).
TSH then stimulates the thyroid gland to synthesize and release the thyroid hormones T3 and T4.
These hormones exert negative feedback on the hypothalamus and anterior pituitary by inhibiting the release of TRH and TSH.
Figure 45.9

15. Hyperthyroidism
The thyroid hormones play crucial roles in stimulating metabolism and influencing development and maturation
Figure 45.10
Hyperthyroidism, excessive secretion of thyroid hormones can cause Graves’ disease in humans
Tissue behind the eyes can become swollen and fibrous, causing the characteristic of bulging eyes.

16. Parathyroid Hormone and Calcitonin:
The maintainence of blood calcium level is one example of how homeostasis is maintained by negative feedback.
Calcitonin
Thyroid gland
releases
calcitonin.
Stimulates
Ca2+ deposition
in bones
Reduces
Ca2+ uptake
in kidneys
STIMULUS:
Rising blood
Ca2+ level
Blood Ca2+
level declines
to set point
Homeostasis:
Blood Ca2+ level
(about 10 mg/100 mL)
level rises
Falling blood
Ca2+ release
from bones
Parathyroid
gland
Increases
in intestines
Active
vitamin D
Stimulates Ca2+
uptake in kidneys
PTH
Figure 45.11
Two antagonistic hormones, parathyroid hormone (PTH) and calcitonin play the major role in calcium (Ca2+) homeostasis in mammals
Calcitonin, secreted by the thyroid gland
Stimulates Ca2+ deposition in the bones and secretion by the kidneys, thus lowering blood Ca2+ levels
PTH, secreted by the parathyroid glands
Has the opposite effects on the bones and kidneys, and therefore raises Ca2+ levels
Also has an indirect effect, stimulating the kidneys to activate vitamin D, which promotes intestinal uptake of Ca2+ from food

17. Maintenance of Glucose Homeostasis
Beta cells of
pancreas are stimulated
to release insulin
into the blood.
Insulin
Liver takes
up glucose
and stores it
as glycogen.
Body cells
take up more
glucose.
Blood glucose level
declines to set point;
stimulus for insulin
release diminishes.
STIMULUS:
Rising blood glucose
level (for instance, after
eating a carbohydrate-
rich meal)
Homeostasis:
(about 90 mg/100 mL)
rises to set point;
stimulus for glucagon
Dropping blood glucose
skipping a meal)
Alpha cells of pancreas
are stimulated to release
glucagon into the blood.
Liver breaks
down glycogen
and releases
glucose into
blood.
Glucagon
Figure 45.12
Two types of cells in the pancreas secrete insulin and glucagon, antagonistic hormones that help maintain glucose homeostasis.
Insulin reduces blood glucose levels by
Promoting the cellular uptake of glucose
Slowing glycogen breakdown in the liver
Promoting fat storage
Glucagon increases blood glucose levels by
Stimulating the conversion of glycogen to glucose in the liver
Stimulating the breakdown of fat and protein into glucose

18. Diabetes Mellitus
Diabetes mellitus, perhaps the best-known endocrine disorder
Is caused by a deficiency of insulin or a decreased response to insulin in target tissues
Is marked by elevated blood glucose levels
Type I diabetes mellitus (insulin-dependent diabetes)
Is an autoimmune disorder in which the immune system destroys the beta cells of the pancreas
Type II diabetes mellitus (non-insulin-dependent diabetes)
Is characterized either by a deficiency of insulin or, more commonly, by reduced responsiveness of target cells due to some change in insulin receptors

19. Adrenal Hormones: Response to Stress
The adrenal glands are adjacent to the kidneys and are actually made up of two glands: the adrenal medulla and the adrenal cortex
The adrenal medulla secretes epinephrine and norepinephrine
Hormones which are members of a class of compounds called catecholamines
These hormones
Are secreted in response to stress-activated impulses from the nervous system
Mediate various fight-or-flight responses

20. Stress Hormones from the Adrenal Cortex
Hormones from the adrenal cortex also function in the body’s response to stress and fall into three classes of steroid hormones:
Glucocorticoids, such as cortisol
Influence glucose metabolism and the immune system
Mineralocorticoids, such as aldosterone
Affect salt and water balance
Sex hormones
Are produced in small amounts

21. Stress and the Adrenal Gland
Nerve
signals
Spinal cord
(cross section)
Hypothalamus
Releasing
hormone
Nerve
cell
Anterior pituitary
Blood vessel
Adrenal medulla
secretes epinephrine
and norepinephrine.
Nerve cell
Adrenal cortex
secretes
mineralocorticoids
and glucocorticoids.
ACTH
Adrenal
gland
Kidney
Stressful stimuli cause the hypothalamus to activate the adrenal medulla via nerve impulses and the adrenal cortex via hormonal signals.
The adrenal medulla mediates short-term responses to stress by secreting the catecholamine hormones epinephrine and norepinephrine.
The adrenal cortex controls more prolonged responses by secreting steroid hormones.
(a) Short-term stress response
(b) Long-term stress response
Effects of epinephrine and norepinephrine:
Effects of
mineralocorticoids:
Effects of
glucocorticoids:
1. Glycogen broken down to glucose; increased blood glucose
1. Retention of sodium ions and water by kidneys
1. Proteins and fats broken down and converted to glucose, leading to increased blood glucose
2. Increased blood pressure
3. Increased breathing rate
4. Increased metabolic rate
2. Increased blood volume and blood pressure
5. Change in blood flow patterns, leading to
increased alertness and decreased digestive
and kidney activity
2. Immune system may
be suppressed
Figure 45.13a,b

22. Gonadal Sex Hormones
The gonads (testes and ovaries) produce most of the body’s sex hormones: androgens, estrogens, and progestins
The testes primarily synthesize androgens, the main one being testosterone - which stimulate the development and maintenance of the male reproductive system
Testosterone causes an increase in muscle and bone mass and is often taken as a supplement to cause muscle growth, which carries many health risks
Estrogens, the most important of which is estradiol, are responsible for the maintenance of the female reproductive system and the development of female secondary sex characteristics
In mammals, progestins, which include progesterone are primarily involved in preparing and maintaining the uterus

23. Melatonin and Biorhythms
The pineal gland, located within the brain secretes melatonin
Release of melatonin
Is controlled by light/dark cycles
The primary functions of melatonin
Appear to be related to biological rhythms associated with reproduction

24. Invertebrate Regulatory Systems
In insects, molting and development are controlled by three main hormones:
Brain
Neurosecretory cells
Corpus cardiacum
Corpus allatum
EARLY LARVA
LATER LARVA
PUPA
ADULT
Prothoracic gland
Ecdysone
Brain hormone (BH)
Juvenile hormone (JH)
Low JH
Neurosecretory cells in the brain produce
brain hormone (BH), which is stored in
the corpora cardiaca (singular, corpus
cardiacum) until release. 1
BH signals its main target
organ, the prothoracic
gland, to produce the
hormone ecdysone. 2
Ecdysone secretion
from the prothoracic
gland is episodic, with
each release stimulating
a molt. 3
Juvenile hormone (JH), secreted by the corpora allata,
determines the result of the molt. At relatively high concen-
trations of JH, ecdysone-stimulated molting produces
another larval stage. JH suppresses metamorphosis.
But when levels of JH fall below a certain concentration, a
pupa forms at the next ecdysone-induced molt. The adult
insect emerges from the pupa. 4
Invertebrate regulatory systems also involve endocrine and nervous system interactions. Diverse hormones regulate different aspects of homeostasis in invertebrates
Brain hormone is produced by neurosecretory cells and stimulates the release of ecdysone from the prothoracic glands
Ecdysone promotes molting and the development of adult characteristics
Juvenile hormone promotes the retention of larval characteristics
Figure 45.15