Endocrine System Flashcards
Endocrine System
Part 1
Hormones and Endocrine Glands
In the body there are 2 types of glands
1) Exocrine glands: secrete their
products into duct (e.g., sweat or
the intestines)
2) Endocrine glands: ductless and
release hormones into the blood
The endocrine system is one of
the body two major
communication systems
- Consists of glands and organs that
secrete hormones - A single gland may secret multiple
hormones
Hormones are chemical
messengers carried by the blood to target cells
Hormones
Releases by glands to elicit a response
- Enhance or inhibit cellular reactions
Present at v. low concentrations
- Much lower than other similar molecules
Response to small amount is possible due to the way the cell
‘behaves’
- This means that although a given hormone travels throughout the
body in the blood, it affects only specific target cells.
Function of Hormones
- Hormones help regulate:
Chemical composition and volume of internal environment (e.g., interstitial fluid)
Metabolism and energy balance
Contraction of smooth and cardiac muscle fibers
Glandular secretions
Some immune system activities - Control growth and development
- Regulate operation of reproductive system
- Help establish circadian rhythm
Hormones
Operate in synchrony with the nervous system
- Endocrine = chemical messengers
Act (relatively) more slowly
Often longer lasting effects
- Nervous system = electrical conduit system
Instantaneous
Short-lived
Hormone structures and synthesis
Hormones fall into three major structural classes:
1. Amines
E.g., Thyroid hormones, Dopamine, Catecholamines (Epinephrine & Norepinephrine)
Derivatives of the amino acid tyrosine
Secreted by the adrenal medulla and the hypothalamus
2. Peptides and proteins
E.g., Insulin
The majority of hormones are polypeptides
Many peptide hormones are synthesised as large, inactive molecules that are cleaved into
active fragments.
3. Steroids
E.g., Aldosterone, Cortisol, Androgens (e.g., testosterone), Estrogens
Produced from cholesterol by the adrenal cortex and the gonads
Hormone transport, metabolism and excretion
Peptide and all catecholamine hormones
are water soluble and therefore circulate
dissolved in the plasma.
Some peptide hormones bind to plasma
proteins
Steroid and thyroid hormones circulate
mainly bound to plasma proteins.
Hormone transport, metabolism and excretion
The liver and kidneys are the major organs that remove hormones
from the plasma by metabolizing or excreting them.
Liver = major organ responsible for the metabolic inactivation
(otherwise called metabolism or biotransformation)
Kidneys = filter the blood, removing waste products, including
hormones and their metabolites.
Peptide hormones and catecholamines are rapidly removed from the blood
Steroid and thyroid hormones are removed more slowly
Because they circulate bound to plasma proteins
After their secretion, some hormones are metabolised to more active molecules in their
target cells or organs.
Mechanisms of hormone action
Public transport (blood)– specific destinations (target tissues)
The presence of specific receptors for those hormones on or in the target cells
necessary for response
Hormone receptors
For lipid-soluble steroid and thyroid
hormones, the majority of receptors are
inside target cells
affect cell function by altering gene expression
For water-soluble peptide hormones and
catecholamines, receptors are on the
plasma membrane
peptide hormones and catecholamines may
exert both rapid (nongenomic) and slower (gene
transcription) actions on the same target cell
Mechanisms of hormone action
The responsiveness of a target cell to a hormone depends on
(1) the hormone’s concentration in the blood,
(2) the abundance of the target cell’s hormone receptors, and
(3) influences exerted by other hormones.
Hormonal interactions can have three types of effects:
Permissive: action of one hormone enhances the responsiveness or activity of
another hormone
e.g., epinephrine & thyroid hormones (T3 and T4) stimulation of lipolysis
Mechanisms of hormone action
The responsiveness of a target cell to a hormone depends on
(1) the hormone’s concentration in the blood,
(2) the abundance of the target cell’s hormone receptors, and
(3) influences exerted by other hormones.
Hormonal interactions can have three types of effects:
Permissive: action of one hormone enhances the responsiveness or activity of
another hormone
e.g., epinephrine & thyroid hormones (T3 and T4) stimulation of lipolysis
Synergistic: the effect of two hormones acting together is greater or more
extensive than one hormone acting on its own.
e.g., follicle-stimulating hormone & estrogens
Or Antagonistic: one hormone opposes the actions of another hormone
e.g., insulin & glucagon
Negative feedback systems regulate the secretion of many hormones.
Inputs that control hormone secretion
ormone secretion controlled by:
1. Plasma concentration of an ion or nutrient that the hormone regulates
2. Neural input to the endocrine cells
3. Other hormones
Inputs that control hormone secretion
- Plasma concentration of an ion or
nutrient that the hormone regulates
E.g., Insulin secretion
https://www.youtube.com/watch?v=OYH1deu7-4E&t=8s
Inputs that control hormone secretion
- Neural input to the endocrine cells
The autonomic nervous system
controls hormone secretion via the
adrenal medulla and other
endocrine glands.
Neurons in the hypothalamus also
secrete hormones
Neural input from the autonomic
nervous system controls the
secretion of many hormones.
Inputs that control hormone secretion
- Other hormones
Often the secretion of a particular
hormone is directly controlled by the
blood concentration of another
hormone
A hormone that stimulates the
secretion of another hormone is
often referred to as a tropic
hormone.
E.g., Thyroid-stimulating hormone
(TSH) or Follicle-stimulating
hormone (FSH)
Endocrine disorders
The wide variety of hormones and endocrine glands determines that
disorders of the endocrine system vary considerably.
Despite varied functional consequences, all endocrine diseases can be
categorised in 1 of 4 ways.
Too little hormones (hyposecretion)
E.g., type 1 diabetes
Too much hormone (hypersecretion)
e.g., gigantism
Endocrine disorders
The wide variety of hormones and endocrine glands determines that
disorders of the endocrine system vary considerably.
Despite varied functional consequences, all endocrine diseases can be
categorised in 1 of 4 ways.
Too little hormones (hyposecretion)
E.g., type 1 diabetes
Too much hormone (hypersecretion)
E.g., gigantism
Decreases responsiveness of the target cells to hormones (hyporesponsiveness)
E.g., type 2 diabetes
Increases responsiveness of the target cells to hormone (hyperresponsiveness)
E.g., elevated heart rate due to increased circulating levels of thyroid hormone
Pharmacological effects of hormones
Pharmacological administration of hormones for medical purposes
- Can result in supraphysiological concentrations and effects not typically observed
with at physiological concentrations.
For example: medication containing cortisol (e.g., Corticosteroids), which is
administered to suppress allergic and inflammation.
The risk of experiencing side effects depends on:
the type of steroid (oral corticosteroids more likely to cause side effects as they are acting
systemically).
the dose
the length of treatment
the age of the patient (children and older adults more susceptible)
Endocrine system
Part 2
The
hypothalamus
and the
pituitary gland
heading
The posterior pituitary gland
The posterior pituitary is really a neural extension of the hypothalamus
Hormones are synthesized in the
hypothalamus, axons pass down the
infundibulum, terminate in the
posterior pituitary and release
hormones
E.g., oxytocin and vasopressin
The anterior pituitary gland
The anterior pituitary gland secretes growth hormone (GH), thyroid-
stimulating hormone (TSH), adrenocorticotropic hormone (ACTH),
prolactin, and two gonadotropic hormones—follicle-stimulating hormone
(FSH) and luteinizing hormone (LH).
The anterior pituitary gland &
the hypothalamus
Secretion of the anterior pituitary
gland hormones is controlled mainly
by hypophysiotropic hormones from
the hypothalamus via the portal
vessels connecting the hypothalamus
and anterior pituitary gland.
The anterior pituitary gland &
the hypothalamus
Typical sequence by which a hypophysiotropic
hormone (hormone 1 from the hypothalamus)
controls the secretion of an anterior pituitary
gland hormone (hormone 2), which in turn
controls the secretion of a hormone by a third
endocrine gland (hormone 3)
Hormonal feedback control
Negative feedback inhibits the hormonal
response.
The thyroid gland
Thyroid hormones has diverse and
widespread effects throughout the body.
E.g. protein synthesis in follicular epithelial cells,
increases DNA replication and cell division
The thyroid gland sits within the neck in front of the trachea
The thyroid gland produces thyroxine (called T4 because it contains four
iodines) and triiodothyronine (T3, three iodines)
Most T4 converted to T3 in target tissues via enzymes, therefore T3 considered major thyroid
hormone
Control of thyroid function
Thyroid-stimulating hormone (TSH) production is
controlled by the negative feedback action
of T3 and T4 on the anterior pituitary gland and, to a
lesser extent, the hypothalamus
Note: TSH causes growth
(hypertrophy) of thyroid tissue.
Excessive exposure of the thyroid
gland to TSH can cause goiter
Actions of thyroid hormones
increase T3 & T4 levels associated with ↑ oxidative
substrate metabolism & ↑ mitochondrial enzyme
activity
↑ carbohydrate and lipid metabolism
Thus, T3 & T4 = high metabolic rate
T3 required for normal production of growth hormone from the anterior pituitary
gland.
T3 is a very important developmental hormone for the nervous system.
Cortisol
Cortisol secretion during stress is mediated by the
hypothalamus–anterior pituitary gland system
Physiological functions of cortisol
During non-stressful situations
Cortisol affects the responsiveness of smooth muscle cells to epinephrine
and norepinephrine (permissive action).
Thus, helps maintain normal blood pressure
Cortisol required to maintain the certain enzymes conc. involved in
metabolic homeostasis.
Thus, prevents plasma glucose concentration dropping too far below normal
Has anti-inflammatory and anti-immune functions
Cortisol levels throughout the day
Cortisol levels are generally high in the morning as we wake from a prolonged period of sleep, with an increase of up to fifty percent in the twenty to thirty minutes after waking. This is known as the ‘cortisol awakening response’. Then, as the day progresses, our cortisol levels naturally begin to drop in a fairly constant and regular fashion that is termed a diurnal rhythm, ending up as low in the late evening. This allows the body to keep a regular sleeping pattern, with the cortisol level dropping for periods of sleep, then replenishing during the following morning.
Physiological functions of cortisol
in stressful situations
Effects on metabolism
1. Stimulation of protein catabolism in bone, lymph, muscle, and elsewhere
2. Stimulation of liver uptake of amino acids and their conversion to glucose
(gluconeogenesis)
3. Maintenance of plasma glucose concentrations
4. Stimulation of triglyceride catabolism in adipose tissue,
with release of glycerol and fatty acids into the blood
Physiological functions of cortisol
Enhanced vascular reactivity, improving cardiovascular
performance
Unidentified protective effects against the damaging
influences of stress
Inhibition of inflammation and specific immune responses
Inhibition of nonessential functions (e.g., reproduction &
growth
cortisol & exercise
Farrell et al. (1983) examined circulated
levels of cortisol in the blood pre and
immediately post 20-min running at 3
different exercise intensities
6 participants
3 males and 3 women, VO2max: 54 ±4 and
42 ±4 ml.kg.min-1, respectively.
cortisol increases during exercise drastically when at 100%V02 max
Cortisol and recovery
Massage decreases circulating cortisol levels
Compression decreases circulating cortisol levels ??
Thus,
Supports recovery as:
↑ Cortisol at rest inhibits immune system
↑ Cortisol at rest inhibits inflammatory response
↑ cortisol decreases capillary permeability in injured areas
Note: Stress response = example of the endocrine
and nervous system working synchronously
When the stress response is triggered, the Sympathetic Nervous System is
activated, triggering the release of epinephrine
At the same time, the endocrine system releases cortisol from the adrenal gland
Cortisol has a synergistic effect on epinephrine
Net result:
Faster breakdown of fuel stores
Larger increase in cardiac function
Bigger increase in ventilation
FYI: Other hormones also released during
the stress response
Hormonal Influences on Growth
The hormones most important to human growth are:
Growth hormone,
insulin-like growth factors 1 and 2
T3 (essential for growth during childhood and adolescents)
Insulin (mainly during fetal life)
Testosterone & estradiol
All these hormones have widespread effect
Growth hormone
Growth hormone secretion is stimulated by growth
hormone-releasing hormone (GHRH) and inhibited by
somatostatin (SST)
Growth Hormone
Growth hormone is the major stimulus of postnatal growth.
It stimulates the release of IGF-1 from the liver and many other cells
IGF-1 then acts locally (and also as a circulating hormone) to stimulate cell
division.
Growth hormone also acts directly on cells to stimulate protein synthesis.
Growth hormone secretion is highest during adolescence.
Testosterone
males-produced by the testes
females- Also produced in smaller
quantities in the ovaries and
the adrenal cortex
Peripheral conversion of
androgens
testosterone
promotes: muscle growth & development of male sex characteristics
Oestrogen & Progestogen
females- produced by the ovaries
post menopausal females- Some oestrogens also
produced in smaller
amounts by other tissues
(e.g., liver, pancreas, bone,
adrenal glands, skin, brain,
adipose tissue and breasts)
males- Estrogen produce when FSH
binds to FSH receptors
Oestrogen & Progestogen
romotes: development of female sex characteristics, regulates
menstrual cycle and adipose tissue growth
Oestrogens also promotes endothelia function
⇓
Protective effects
Dehydroepiandrosterone (DHEA)
DHEA, and its sulfate (DHEAS) are hormones produced by the adrenal cortex
DHEAS are precursors for sex hormones such as testosterone and estradiol
DHEA/S affect various systems of the body ⇒ Purported to be anti-ageing
DHEA and DHEA-S production peaks at age 20-30 and then declines progressively
with age
DHEA, DHEAS, age and exercise
DHEA/S increases following low and moderate intensity exercise in you but not older adults
Hormone replacement therapy (HRT) and exercise
In women, oestrogen-containing HRT, improves muscle function, maintains muscle mass and
prevents fat infiltration into the muscle compartment.
Exercise and HRT can be considered counteractive treatments o age-related changes in muscle
phenotype
In men, testosterone-containing HRT seems to preserve muscle tissue and offset age-related muscle
loss, rather than cause significant gains
Summary
Hypothalamic hormones (i.e., hypophysiotropic hormones) stimulate or inhibit
the release of pituitary hormones
The posterior pituitary gland secretes oxytocin and vasopressin hormones
The anterior pituitary gland secretes GH, TSH, SCTH, prolactin, FSH and LH
hormones
Thyroid hormones: affect metabolism, important in the development of the
nervous system
Cortisol: affects vascular responsiveness, involved in metabolic processes, has
anti-inflammatory and anti-immune functions, is also an important
developmental hormone during fetal and neonatal life
Growth hormone: major stimulus of postnatal growth
summary
Sex hormones (testosterone, estradiol and progestogen) are present in
both males & females, albeit at different levels.
Testosterone promotes development of male sex characteristics &
muscle growth.
Estradiols and progestogen promotes female sex characteristics,
regulates menstrual cycle and endothelial function.
Androgens and testosterone increase post exercise
This helps promote anabolic processes
