Chapter 45
Hormones And Endocrine System
Key concepts:
• Hormones and other signaling molecules bind to target
receptors, triggering specific response pathways.
• Negative feedback and antagonistic hormone pairs are
common features of the endocrine system.
• The endocrine and nervous systems act individually and
together in regulating animal physiology.
• Endocrine glands respond to diverse stimuli in
regulating metabolism, homeostasis, development and
behavior.
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HORMONES AND OTHER SIGNALING MOLECULES BIND
TO TARGET RECEPTORS, TRIGGERING SPECIFIC
RESPONSE PATHWAYS (PP. 975-981).
Types of secreted signaling molecules:
 Hormones are secreted into extracellular fluids by
endocrine cells or ductless glands and reach target cells via
blood.
 Local regulators act on neighboring cells in paracrine
signaling and on the secreting cell itself in autocrine
signaling.
 Neurotransmitters also act locally, but some secrete
neurohormones that act throughout the body.
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HORMONES AND OTHER SIGNALING MOLECULES
BIND TO TARGET RECEPTORS, TRIGGERING
SPECIFIC RESPONSE PATHWAYS.
Chemical classes of hormones:
 Hormones can be polypeptides, amines or steroids and can
be water soluble or lipid soluble.
Hormone receptor location:
 Protein hormones bind to receptors embedded in the
plasma membrane, steroid hormones and thyroid
hormones enter target cells and bind to receptors in the
cytoplasm or nucleus.
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HORMONES AND OTHER SIGNALING MOLECULES
BIND TO TARGET RECEPTORS, TRIGGERING
SPECIFIC RESPONSE PATHWAYS.
Cellular response pathways:
 Binding of water soluble hormones to cell surface receptors
triggers intracellular signal transduction leading to specific
responses in the cytoplasm but lipid soluble hormones and
its receptors act in the nucleus to regulate transcription
specific genes.
Multiple effects of the same hormone:
 The same hormone may have different effects on the target
cells.
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SIGNALING BY LOCAL REGULATORS
Several types of chemical compounds function as local
regulators:
 Neurotransmitters: are local regulators in the synapses of the
nervous system.

Growth factors: are peptides and proteins that stimulate cell
proliferation.

Prostaglandins (PGs): are modified fatty acids, released from most
types of cells into the interstitial fluid, affecting nearby cells.

Nitric oxide (NO): it is a gas with many types of cells can produce and
it has multiple function. Highly reactive and potentially toxic, NO
usually trigger changes in a target cell within a few seconds of contact
and then breaks down.
When NO secreted by neurons, function as neurotransmitter; secreted
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by white blood cells, it kills certain bacteria and cancer cells.

Negative feedback and antagonistic hormone pairs are
common features of the endocrine system (pp.981-984).
Simple endocrine pathway
Negative feedback is a loop in which the response reduces the
initial stimulus
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NEGATIVE FEEDBACK AND ANTAGONISTIC HORMONE
PAIRS ARE COMMON FEATURES OF THE ENDOCRINE
SYSTEM.
To see how such control systems operate, we’ll consider the
regulation of blood glucose levels.
 Two antagonistic hormones, insulin and glucagon regulate
the concentration of glucose in the blood.
 Insulin from beta cells of pancreas reduce blood glucose
level by promoting cellular uptake of glucose, glycogen
formation in the liver, protein synthesis and fat storage.
 Glucagon from alpha cells increase blood glucose level by
stimulating conversion of fat and glycogen to glucose in the
liver and breakdown of the fat and protein to glucose.
 Diabetes mellitus which is marked by elevated blood
glucose levels result from inadequate production of insulin
(type 1) or loss of responsiveness of target cells to insulin10
(type 2)
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THE ENDOCRINE AND NERVOUS SYSTEMS ACT
INDIVIDUALLY AND TOGETHER IN REGULATING ANIMAL
PHYSIOLOGY (PP. 984-990)
Coordination of endocrine and nervous systems in
vertebrates:
 The hypothalamus, contain sets of neurosecretory cells.
Some produce direct acting hormones that are stored and
released from the posterior pituitary.
 Other hypothalamic cells produce hormones that are
transported by portal blood to the anterior pituitary.

Two kinds of hormones secreted into the blood:
 Releasing hormones make the anterior pituitary secrete its
hormones.
 Inhibiting hormones make the anterior pituitary stop
secreting hormone.
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POSTERIOR PITUITARY HORMONE
The pituitary gland has two lobes: anterior and
posterior lobes:
The posterior pituitary or neurohypophysis, that
stores and releases two hormones: oxytocin and
antidiuretic hormone, ADH.
 They function in simple neurohormone pathways.
Oxytocin induces uterine contractions during
childbirth and milk ejection
 ADH enhances water re -absorption in the kidneys.

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Simple neurohormone pathway
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Some
hormones
called tropic
hormones, have
other endocrine
glands as their
targets in a
hormone
cascade
pathways.
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A hormone cascade pathway
ANTERIOR PITUITARY HORMONES
The anterior pituitary produces:
Tropic hormones:
1. Thyroid stimulating hormone (TSH), regulate the thyroid gland
secretions
2. Follicle stimulating hormone (FSH) and (LH), they also called
gonadotropins because they stimulate the activities of the male and
female gonads.
3. Adrenocorticotropic hormones (ACTH), control the production and
secretion of steroid hormones by the adrenal cortex.
Nontropic hormones:
1.
Prolactin (PRL) stimulate mammary glands growth and milk
synthesis
2. Melanocyte – stimulating hormone (MSH), it stimulate skin
pigmentation.
3. Endorphins, the brain’s natural opiates, inhibit perception of pain.
Growth hormone (GH):
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has both tropic and nontropic actions. It promotes growth directly and
stimulate the production of growth factors.
ENDOCRINE GLANDS RESPOND TO DIVERSE
STIMULI IN REGULATING METABOLISM,
HOMEOSTASIS, DEVELOPMENT AND
BEHAVIOR (PP. 990-994).

Thyroid hormones:
 The thyroid gland consists of two lobes located on the
ventral surface of the trachea.
 The thyroid gland produces iodine containing
hormones triiodothyronine (T3) and thyroxine (T4)
that stimulate metabolism and influence development
and maturation.
 The thyroid gland also secrete calcitonin which lowers
calcium levels in the blood.
 The hypothalamus and anterior pituitary control the
secretion of thyroid hormones.
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PARATHYROID HORMONE AND VITAMIN D:
CONTROL OF BLOOD CALCIUM

The four parathyroid glands embedded in the
surface of the thyroid gland
 The parathyroid glands raise plasma calcium
levels by secreting parathyroid hormone (PTH)
 PTH stimulate the kidneys to activate vitamin D,
which in turn stimulates the intestines to
increase uptake of calcium from food.
 In bone, PTH induces specialized cells called
osteoclasts to decompose the mineralized matrix
of bone and realse Ca+2 into the blood.
 Calcitonin
and (PTH) maintain calcium
homeostasis by acting on kidneys and bone.
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ADRENAL HORMONES: RESPONSE TO
STRESS
Neurosecretory cells in the adrenal medulla release
epinephrine and norepinephrine in response to stress.
 These hormones mediate various fight or flight
responses.
 The adrenal cortex releases three functional classes of
steroid hormones:
 Corticosteroids (Including sex hormones),
 Glucocorticoids, such as cortisol, influence glucose
metabolism and the immune system;
 Mineralocorticoids, primarily aldosterone, affect salt and
water balance.

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MELATONIN AND BIORHYTHMS
The pineal gland is a small mass of tissue near the center
of the brain
 The pineal gland secretes melatonin which is secreted at
the night, which influences skin pigmentation, biological
rhythms and reproduction in various vertebrates.

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