The Endocrine System
Chapter 18
Function of the Endocrine System
• Maintain homeostatic balance of the body
• Endocrine system – the body’s second great
controlling system which influences
metabolic activities of cells by means of
hormones
• The endocrine system is a collection of
“ductless” glands and tissues
• The products of these glands are hormones
Endocrine
glands
Hormones
• Functions
– Regulation:
•
•
•
•
Water balance and body fluid chemistry
Metabolic rate and energy balance
Cardiac and smooth muscle activity
Immune system activity
– Control growth and development
– Reproductive organ function and cycles
– Circadian rythms
Mechanisms of
Intercellular Communication
Table 18–1
Hormones
• Can be divided into 3 groups:
– amino acid derivatives
– peptide hormones
– lipid derivatives
Amino Acid Derivatives
• Small molecules structurally related to
amino acids
• Synthesized from the amino acids tyrosine
and tryptophan
Tyrosine Derivatives
• Thyroid hormones
• Compounds:
– epinephrine (E)
– norepinephrine (NE)
– dopamine, also called catecholamines
Tryptophan Derivative
• Melatonin:
– produced by pineal gland
Peptide Hormones
• Chains of amino acids
• Synthesized as prohormones:
– inactive molecules converted to active
hormones before or after secretion
2 Groups of Peptide Hormones
• Group 1:
– glycoproteins:
• more than 200 amino acids long, with
carbohydrate side chains:
–thyroid-stimulating hormone (TSH)
–luteinizing hormone (LH)
–follicle-stimulating hormone (FSH)
2 Groups of Peptide Hormones
• Group 2:
– all hormones secreted by:
•
•
•
•
•
•
•
hypothalamus
heart
thymus
digestive tract
pancreas
posterior lobe of pituitary gland
anterior lobe of pituitary gland
2 Classes of Lipid Derivatives
• Eicosanoids:
– derived from arachidonic acid
• Steroid hormones:
– derived from cholesterol
Eicosanoids
• Are small molecules with five-carbon ring
at one end
• Are important paracrine factors
• Coordinate cellular activities
• Affect enzymatic processes in extracellular
fluids
Leukotrienes
• Are eicosanoids released by activated white
blood cells, or leukocytes
• Important in coordinating tissue responses
to injury or disease
Prostaglandins
• A second group of eicosanoids produced in
most tissues of body
• Are involved in coordinating local cellular
activities
• Sometimes converted to thromboxanes and
prostacyclins
Steroid Hormones
• Are lipids structurally similar to cholesterol
• Released by:
– reproductive organs (androgens by testes,
estrogens, and progestins by ovaries)
– adrenal glands (corticosteroids)
– kidneys (calcitriol)
Steroid Hormones
• Remain in circulation longer than peptide
hormones
• Are absorbed gradually by liver
• Are converted to soluble form
• Are excreted in bile or urine
Chemical Classifications of
Hormones: Lipid & Water soluble
• Lipid soluble –
– This characteristic allows hormones to pass directly
through the plasma membrane of the target cell
– Lipid soluble hormones include:
• Steroid hormones
• Thyroid hormones
• Nitric oxide (a gas)
• Require a carrier protein to travel in the plasma
Fig. 18.03
Water soluble Hormones
• These are insoluble in lipid but travel freely in
the blood but cannot enter the cell
• They have their activity by binding to receptors
on the plasma membrane and include:
• Amines
–
–
–
–
Catecholamines – epinephrine & norepinephrine
Melatonin
Histamine
Serotonin
More water soluble hormones
• Peptides & Proteins
– Hypothalamic hormones - hypothalamus
– ADH & oxytocin – posterior pituitary
– hGH, TSH, ACTH, FSH, LH, PRL, MSH –
anterior pituitary
– Insulin, glucagon, somatostatin, pancreatic
polypeptide – pancreas
– PTH – parathyroid
– Calcitonin – thyroid
– Gastrin, secretin, CCK, GIP – GI tract
– EPO – Kidneys
– Leptin – adipose tissue
Second messenger
activity of water
soluble hormones
Cyclic AMP
PIP2 – IP3
Steroid
Hormones
Figure 18–4a
Thyroid
Hormones
Figure 18–4b
Endocrine Reflexes
• Functional counterparts of neural reflexes
• In most cases, controlled by negative
feedback mechanisms
Endocrine Reflex Triggers
• Humoral stimuli:
– changes in composition of extracellular fluid
• Hormonal stimuli:
– arrival or removal of specific hormone
• Neural stimuli:
– arrival of neurotransmitters at neuroglandular
junctions
Simple Endocrine Reflex
• Involves only 1 hormone
• Controls hormone secretion by:
–
–
–
–
heart
pancreas
parathyroid gland
digestive tract
Complex Endocrine Reflex
• Involves:
– 1 or more intermediary steps
– 2 or more hormones
Hormone activity
• Down-regulation –
– Excessively high concentrations of hormones
reduces the number of receptors on the target
tissues. This makes the tissue less sensitive to
that hormone
• Up-regulation –
– Low concentrations cause cells to produce more
receptors resulting in increased sensitivity of
the tissue
The Hormones and their glands
Hypothalamus
Figure 18–5
the Pituitary
Anterior Lobe
• Also called adenohypophysis:
1. pars distalis
2. pars intermedia
3. pars tuberalis
Anterior Lobe
Figure 18–6
Hypothalamic
Regulatory
Hormones
• Rate of
secretion is
controlled by
negative
feedback
Figure 18–8a
Hypothala
mic
Regulator
y
Hormones
Figure 18–8b
Thyroid-Stimulating
Hormone (TSH)
• Also called thyrotropin
• Triggers release of thyroid hormones
Adrenocorticotropic
Hormone (ACTH)
• Also called corticotropin
• Stimulates release of steroid
hormones by adrenal cortex
• Targets cells that produce
glucocorticoids
Gonadotropins
• Regulate activities of gonads (testes,
ovaries)
• Follicle-stimulating hormone
• Luteinizing hormone
Follicle-Stimulating
Hormone (FSH)
• Also called follitropin
• Stimulates follicle development and estrogen
secretion in females
• Stimulates sustentacular cells in males:
– promotes physical maturation of sperm
• Production inhibited by inhibin:
– peptide hormone released by testes and ovaries
Luteinizing Hormone (LH)
• Also called lutropin
• Causes ovulation and progestin production
in females
• Causes androgen production in males
FSH and LH Production
• Stimulated by gonadotropin-releasing
hormone (GnRH) from hypothalamus:
– GnRH production inhibited by estrogens,
progestins, and androgens
Prolactin (PRL)
• Also called mammotropin
• Stimulates development of mammary
glands and milk production
• Production inhibited by prolactin-inhibiting
hormone (PIH)
Prolactin (PRL)
• Stimulates PIH release
• Inhibits secretion of prolactin-releasing
factors (PRF)
Prolactin (PRL)
Figure 18–8b
Growth Hormone (GH)
• Also called somatotropin
• Stimulates cell growth and replication
• Production regulated by:
– growth hormone–releasing hormone
(GH–RH)
– growth hormone–inhibiting hormone
(GH–IH)
Melanocyte-Stimulating
Hormone (MSH)
• Also called melanotropin
• Stimulates melanocytes to produce melanin
• Inhibited by dopamine
Melanocyte-Stimulating
Hormone (MSH)
• Secreted by pars intermedia during:
– fetal development
– early childhood
– pregnancy
– certain diseases
Posterior Lobe
• Also called neurohypophysis
• Contains unmyelinated axons of
hypothalamic neurons
• Supraoptic and paraventricular nuclei
manufacture:
– antidiuretic hormone (ADH)
– oxytocin (OT)
Antidiuretic Hormone
• Decreases amount of water lost at kidneys
• Elevates blood pressure
• Release inhibited by alcohol
Oxytocin
• Stimulates contractile cells in mammary
glands
• Stimulates smooth muscles in uterus
• Secretion and milk ejection are part of
neuroendocrine reflex
Summary: The Hormones
of the Pituitary Gland
Figure 18–9
Summary: The Hormones of the Pituitary Gland
Table 18–2
Fig. 18.10
The
Thyroid
Thyroid Gland
Figure 18–10a, b
Thyroid
Follicles
Figure 18–11a, b
Regulation of
thyroid
hormone by
TSH
Thyroglobulin
Figure 18–10c
Thyroid Gland
Table 18–3
Parathyroid Glands
Figure 18–12
4 Effects of PTH
1. It stimulates osteoclasts:
–
–
accelerates mineral turnover
releases Ca2+ from bone
2. It inhibits osteoblasts:
–
reduces rate of calcium deposition in bone
3. It enhances reabsorption of Ca2+ at kidneys,
reducing urinary loss
4. It stimulates formation and secretion of
calcitriol at kidneys
Calcitriol
• From cholecalciferol (made in the skin)
• Effects complement or enhance PTH
• Enhances Ca2+, PO43— absorption by
digestive tract
Calcium
homeostasis
Calcitriol
• Stimulates calcium
and phosphate ion
absorption along
digestive tract
Figure 18–17a
The Adrenals
Hormones of the Adrenal Cortex
• Mineralcorticoids from the zona glomerulosa
– Aldosterone, it’s primary function is regulation of Na and
K ion concentrations.
– It works with the “renin-angiotensin” mechanism in
maintaining water balance and blood volume. This called
the RAA (renin – angiotensin – aldosterone) pathway
– It also has a major role in pH regulation by promoting
excretion of H+.
The Renin–Angiotensin System
Figure 18–17b
Adrenal Cortex Hormones, continued
• Glucocorticoids from the zona fasciculata
– Cortisol (hydrocortisone) 95%
– Corticosterone
– Cortisone
• Effects:
–
–
–
–
–
–
Protein catabolism
Gluconeogenesis
Lipolysis
Resistance to stress
Anti-inflammatory response
Immune suppression
Regulation of
glucocorticoids
Adrenal cortex hormones iii
• Sterocorticoids (Androgens) from the zona
reticularis
• Mostly DHEA (dehydroepiandrosterone)
– Unimportant in males after puberty
– Promote libido in females and converted to estrogen
– Following menopause this is the source of estrogens for
women
Adrenal Cortex
Table 18–5
Adrenal Medulla
•
•
•
•
Catecholamines – epinephrine and norepinephrine
Produced by chromaffin cells
Major role in the stress response (fight-or-flight)
Effects:
–
–
–
–
Increase heart rate
Constrict major arteries
Dilate airways
Dilate blood vessels of heart, skeletal muscle, lungs &
CNS
– Increase energy availability & overall metabolism
Pancreas
Figure 18–15
Fig. 18.18
The
Pancreas
Pancreatic hormones
• Glucagon – b cells of pancreatic islets
– stimulated by low blood sugar, exercise
– Raises blood glucose by triggering glycogenolysis and
gluconeogenesis
• Insulin – a cells
– Stimulated by high blood glucose, parasympathetic n.s.,
hGH, among others
– Enhances cellular uptake of glucose, glycogenesis,
lipogenesis, and protein synthesis
• Somatostatin – d cells
– Inhibits secretion of insulin and glucagon
• Pancreatic polypeptide – F cells Inhibits somatostatin
Insulin &
Glucagon
Pancreatic Islets
Table 18–6
Effects of Diabetes Mellitus
• Results from hyposecretion or hypoactivity of
insulin
• The three cardinal signs of DM are:
– Polyuria – huge urine output
– Polydipsia – excessive thirst
– Polyphagia – excessive hunger and food
consumption
• Hyperinsulinism – excessive insulin secretion,
resulting in hypoglycemia
Effects of Diabetes Mellitus
Gonads
• Ovaries
– Produce oocytes (eggs)
– Estrogen & Progesterone:
•
•
•
•
Works with FSH & LH to control uterine cycle
Responsible for secondary sexual characteristics in females
Maintains mammaries
Maintains uterus during pregnancy
– Relaxin:
• Loosens connective tissue and dilates cervix & uterus
during delivery
– Inhibin:
• Inhibits FSH
The Testis
• Produce sperm
• Testosterone
– Responsible for secondary sexual characteristics
• Inhibin
– Controls testosterone release
Pineal Gland
• Produces melatonin
– Stimulated by darkness
– Contributes to maintaining day/cycles
– Linked to seasonal affective disorder
Thymus
• Secretes thymosin, thymic humoral factor &
thymopoietin
– These are involved with the development and
immunocompetence of T-lymphocytes
Other endocrine cells
• Erythropoietin (EPO) from kidney
– Stimulates red blood cell production
• Atrial Natruetic Peptide (ANP) from heart
– Antagonistic to aldosterone
• The placenta – produces sex hormones, human
chorionic gonadotropin (hCG), hCS and relaxin
during pregnancy
• Eicsanoids – produced by many cells as autocrine
and paracrine hormones
• GI tract – produces GIP,CCK, Gastrin
Adipose Tissue Secretions
1. Leptin:
–
–
feedback control for appetite
controls normal levels of GnRH,
gonadotropin synthesis
2. Resistin:
–
reduces insulin sensitivity
Summary of Hormones Produced by
Specific Organs
Table 18–7
Hormone Interactions
1. Antagonistic (opposing) effects
2. Synergistic (additive) effects
3. Permissive effects:
–
1 hormone is necessary for another to produce
effect
4. Integrative effects:
–
hormones produce different and
complementary results
The
Stress
Response
General
Adaptation
Syndrome (GAS)
• Also called stress
response
• How bodies
respond to stresscausing factors
Figure 18–18
General Adaptation
Syndrome (GAS)
• Is divided into 3 phases:
1. alarm phase
2. resistance phase
3. exhaustion phase
Alarm Phase
•
•
•
•
•
Is an immediate response to stress
Is directed by ANS
Energy reserves mobilized (glucose)
“Fight or flight” responses
Dominant hormone is epinephrine
7 Characteristics of Alarm Phase
1. Increased mental alertness
2. Increased energy consumption
3. Mobilization of energy reserves (glycogen
and lipids)
7 Characteristics of Alarm Phase
4. Circulation changes:
–
–
increased blood flow to skeletal muscles
decreased blood flow to skin, kidneys, and
digestive organs
7 Characteristics of Alarm Phase
5. Drastic reduction in digestion and urine
production
6. Increased sweat gland secretion
7. Increases in blood pressure, heart rate, and
respiratory rate
Resistance Phase
•
•
•
•
Entered if stress lasts longer than few hours
Dominant hormones are glucocorticoids
Energy demands remain high
Glycogen reserves nearly exhausted after
several hours of stress
Effects of Resistance Phase
1. Mobilize remaining lipid and protein
reserves
2. Conserve glucose for neural tissues
3. Elevate and stabilize blood glucose
concentrations
4. Conserve salts, water, and loss of K+, H+
Exhaustion Phase
• Begins when homeostatic regulation breaks
down
• Failure of 1 or more organ systems will
prove fatal
• Mineral imbalance
Interactions
between
Endocrine
and Other
Systems
Figure 18–19
Fig. 18.T02a
A steroid
Fig. 18.T02b
A thyroxine
Fig. 18.T02c
A catacholamine
Fig. 18.T02d
A peptide
Fig. 18.T02e
paracrines