27-10-21 - Endocrine System Flashcards
Learning outcomes
- Explain the difference between an exocrine and an endocrine gland
- Distinguish between three classes of hormones
- Describe the relationship between the hypothalamus and pituitary gland in the control of the endocrine system
- Explain the regulation of the endocrine system, including the concept of negative feedback loops
- Describe the anatomical location, structure and function of the following endocrine glands: pituitary, thyroid, parathyroid, adrenal (suprarenal) & pancreatic islets (islets of Langerhans)
What is the endocrine system?
What is it secondary to?
How long can it take to respond?
What are the 5 things the endocrine system is responsible for?
- The endocrine system is a system of loads of hormone producing glands that are scattered around the body, which use hormones as chemical messengers
- The endocrine (hormone) system is the second integrative control System of the body – slower than the nervous system
- Hormones of the endocrine system can have a lag of seconds, weeks or months before they work
• The endocrine system is responsible for:
1) Cellular metabolism
2) Growth and development
3) Sexual development
4) Homeostasis e.g Ca++
5) Behaviour e.g mood/sleep
What are endocrine glands?
Why do they have a rich blood supply?
What does endocrine hormone mean?
Where are the typical target tissues of endocrine glands located?
How do hormones act on target tissues?
What concentrations are hormones released in?
How can hormones elicit such a big response?
What other hormone systems is the endocrine system in line with?
What is an example?
- Endocrine glands are ductless glands with a rich blood supply, that secrete hormones into the bloodstream
- Having a rich blood supply ensures rapid movement of hormones into the blood and around the body
- Endo = internal, Crine = secretion, hormone = to excite/set in action
- Endocrine glands typically act on tissues distant from the source
- Hormones are able to act on target tissues, as these tissues have specific receptors that re complementary to the hormone
- Hormones are released in very small concentrations, but can elicit a huge response, because target tissue receptors have every high affinity for their hormones
- The endocrine system is also alongside other local hormonal systems, such as the paracrine and autocrine systems e.g. testosterone produced in testes and acting on testes is paracrine
What do exocrine glands may or may not have?
How do exocrine and endocrine glands differ?
What are 3 examples where exocrine glands are used?
• Exocrine glands may or may not have ducts, where whatever substance is secreted travels up the ducts (typically have ducts)
• Exo refer to the fact that secretions come out of the gland onto an external epithelial layer, rather than internally into the blood stream or extracellular fluids, like endocrine glands
• 3 places where exocrine glands are used:
1) Tongue – saliva
2) Skin – sebum/sweat
3) GI tract – digestive enzymes
What are the 3 different classes of hormones?
1) Proteins/peptide hormones
2) Steroids
3) Amino acid derivatives
What are the typical sizes of protein/peptide hormones?
How are they usually transported?
What is their half life like?
How do they act as chemical messengers for cells?
What does this initiate?
How are they degraded?
When might they be used?
- Peptide hormones vary in size (i.e 3 amino acids to 30 kDa glycosylated proteins)
- Peptide hormones are readily transported in the blood, as they ae usually water soluble
- They have a short half life of a few minutes, as they are not attached to transport proteins
- Peptide hormones bind to plasma membrane receptors on plasma membrane cells
- This initiates a cascade response in the cell, which can initiate cell changes, such as in gene expression or phosphorylation of proteins
- Peptide hormones are degraded via proteolysis
- Peptide hormones can allow generation of short term elicited response in an immediate crisis
What are steroids synthesised from?
What are they soluble in?
What do they need to be transported and why?
How does this affect their half-life?
How do steroid hormones get into cells?
How do steroids target cells?
When is the steroid hormone biologically active?
- Steroids are synthesised from cholesterol
- They are lipid soluble
- Steroids require specific transporter proteins to move through the blood, due tot being lipophilic
- This increases their half life
- Hormones can readily diffuse into and out of the cell through the plasma membrane
- Steroids can then bind to intracellular receptors/transcription factors and modify gene expression e,g hormone can bind to a receptor, and the complex becomes a transcription factor
- Only the free hormone is biologically active
What are amino acid derivative hormones derived from?
What are the 2 types?
What are the example of each type?
How are they similar?
What 2 ways do they differ?
- Amino acid derivative hormones are derived from tyrosine
- There are 2 types of amino acid derivative hormones:
1) Catecholamines – neurotransmitters
• Adrenaline
• Noradrenaline
• Dopamine
2) Thyroid hormones
• Throxine (T4)
• Tri-iodothyronine (T3)
- Although catecholamines and thyroid hormones have a common pre-cursor, they behave very different.
- Catecholamines have the shortest half-life of all hormones (can be a few seconds e.g adrenaline released instantly, but degraded very quickly)
- Thyroid hormones have the longest half-life of all hormones, and can stay around for a few days
- Catecholamines (like peptide hormones) bind to plasma membrane receptors, which triggers secondary messenger signal responses in the cell
- Thyroid hormones (like steroids) diffuse across the plasma membrane, and bind to receptors inside the cell.
How do all hormones act?
What can make response vary?
What changes will all of them bring about?
What 3 hormones use plasma membrane receptors?
What 2 hormones use intracellular receptors?
- All hormones act by binding to receptors on target cells
- Cellular response can vary depending on the numbers of receptors present.
- All hormones bring about changes in gene expression
• Hormones that use plasma membrane receptors:
1) Peptides
2) Glycoproteins (peptide hormones)
3) Catecholamines
• Proteins that use intracellular receptors:
1) Steroids
2) Thyroid hormones
Why are most hormones secreted?
What are the 3 different rhythms hormones can be secreted?
- Most hormones are secreted in response to stimuli
- The 3 patterns of hormone secretion:
1) Episodic
• In response to physiological demand e.g insulin
2) Diurnal
• Predictable pattern over 24 hours e.g cortisol
• May also respond to episodic stimuli
3) Constant
• Fairly constant blood concentrations for normal physiological function e.g thyroxine, which is involved in BMR
What are the 3 stimuli for endocrine glands/cells to release their hormones?
What is an example of each?
10) What are the 10 main endocrine glands of the endocrine system?
Where is the pineal gland found?
What is it responsible for?
- The pineal gland is found in the centre of the brain, between the 2 hemispheres of the thalamus
- It is responsible for the circadian sleep cycle.
What does the hypothalamus link?
What is the role of the hypothalamus and pituitary gland?
What do they provide?
What does the hypothalamus release?
What does the pituitary then release?
How is the hypothalamus connected with the pituitary gland?
What 3 things is the hypothalamus responsible for?
- The hypothalamus links the nervous system and the endocrine system together via the pituitary gland
- The hypothalamus, along with the pituitary gland, coordinate the endocrine system, and orchestrate the activity of other endocrine glands
- They provide signal amplification through cascades and fine control
- The hypothalamus releases and inhibits hormones that have an effect on its target cells: the pituitary gland cells
- This stimulates the pituitary gland to release hormones which stimulates/controls many other endocrine glands.
- The hypothalamus is connected to the pituitary gland by a stock that receives the neuronal and hormonal and stimuli from the hypothalamus
- The hypothalamus is responsible for:
- Circadian rhythms (wake/sleep pattern)
- Reproduction
- Growth
What does the hypothalamus respond to?
Describe 4 stages of the endocrine axis using the thyroid as an example.
How is this system regulated?
- The hypothalamus responds to neuronal and hormonal stimuli from circulating hormones
- The endocrine axes (thyroid example):
1) Hypothalamus releases hormone through the stock that targets the pituitary gland cells e.g TRH (Thyrotropin releasing hormone)
2) TRH stimulates the target cells of the anterior lobe of the pituitary gland to produce and release TSH (throxine-stimulating hormone)
3) The TSH travels towards the thyroid through systemic circulation and stimulates the thyroid cells to produce Thyroxine
4) Thyroxine then goes into systemic circulation towards the target cells
• This system is regulated by a negative feedback control, where products can inhibit hormones produced earlier in the chain, stimulate glands to produce inhibitory hormones, or inhibit glands from producing hormones
What does the hypothalamus do with the 2 lobes of the pituitary gland?
What are the 2 types of hormones the hypothalamus releases?
- The hypothalamus uses hormones to stimulate the production and secretion of hormones in the anterior pituitary lobe
- The hypothalamus synthesises hormones (oxytocin and anti-diuretic hormone) which are stored in the posterior lobe of the pituitary gland
- The hypothalamus releases:
1) Releasing hormones
• E.g growth hormone releasing hormone (GHRH) which stimulates the anterior pituitary to release Growth hormone
2) Inhibitor hormones
• E.g growth hormone inhibiting hormone (GHIH)
What time of day are growth hormone levels highest?
When do growth hormone levels peak?
What causes Gigantism?
What might cause gigantism?
What causes acromegaly?
What does it lead to?
What causes dwarfism?
- GH levels are highest at their highest in the evening and during sleep
- Growth hormone levels peak in adolescence at 15 when bones are undergoing massive growth (long bones undergo loads of growth – GH targets epiphyseal growth plate)
- Gigantism is caused by hypersecretion of GH in pre-pubescent children.
- Gigantism may be caused be an anterior pituitary tumour, which could only be removed surgically
- Acromegaly is caused by excess GH, and leads to excessive bone growth in the extremities e.g hands, feet and face
- Dwarfism is caused by hyposecretion of GH in children
What are the 2 lobes of the anterior pituitary glands?
What are they also referred to as?
What are they each derived from?
How are they each linked to the Hypothalamus?
Where are they located?
- Anterior pituitary (adenohypophysis)
- Oral ectoderm derived
- Vascular connection to the hypothalamus
- Posterior pituitary (neurohypophysis)
- Neuroectoderm derived
- Direct neural link to hypothalamus
- They are both located in a hollow of the sphenoid bone
What does the hypothalamus stimulate in the anterior lobe (adenohypophysis) of the pituitary gland?
What does the anterior lobe consist of?
What 6 hormones does the anterior lobe synthesise and secrete in response to stimuli from hypothalamus?
How are they released?
- Hormones from the hypothalamus stimulates the anterior lobe to synthesise its own hormones, which it then released into circulation
- The anterior lobe consists of epithelial cells
• The 6 hormones synthesised and secreted by the anterior pituitary in response to stimuli from hypothalamus:
1) Growth hormone (GH)
2) Thyroid stimulating hormone (TSH)
3) Adrenocorticotropic hormone (ACTH)
4) Luteinizing hormone (LH)
5) Follicle stimulating hormone (FSH)
6) Prolactin
- 1-5 are secreted by releasing hormones
- Prolactin is released following inhibition of inhibiting hormones from the hypothalamus
What do each of the 6 hormones releases from the anterior lobe of the pituitary gland do?
- Growth hormone (GH)
- Encourages growth
- TSH – Thyroid stimulating hormone
- This stimulates production of thyroid hormones
- These hormones are triiodothyronine (T3), Tetraiodothyronine (T4), Calcitonin
- These target virtually every cellular tissue e.g metabolism, express proteins for cellular metabolism and growth
- ACTH – adrenocorticotropic hormone
- Stimulates adrenal glands to produce glucocorticoids, such as cortisol and weak androgens
- Also stimulates adrenal glands to produce mineralocorticoids
- LH and FSH – luteinizing hormone and follicle stimulating hormone
- Involved in the synthesis of steroids and gametes in males and females
- Prolactin
- Used for the production of breast milk in woman
- Enhances LH receptors in Leydig cells of men, which leads to testosterone secretion, which lead to increased levels of spermatogenesis
What does the posterior lobe (neurohypophysis) the pituitary gland consist of?
What is its purpose?
What are these 2 hormones?
How are these hormones transported into the posterior lobe?
Why is the posterior lobe technically not an endocrine gland?
- The posterior lobe consists of neural tissue
- The posterior lobe stores hormones secreted by neurosecretory cells in hypothalamus
It stores:
• Anti-diuretic hormone (ADH)
• Oxytocin
These hormones are transported via neuronal axons to the posterior lobe
• The posterior lobe of the pituitary is technically not an endocrine gland, as it does not synthesise its own hormones
What is secretion from the hormones in the posterior lobe of the pituitary part of?
What do these hormones act on?
What do ADH and oxytocin do?
• Secretion of hormones from the posterior pituitary is part of a neuroendocrine reflex that acts on smooth muscles to initiate muscular contraction
- Anti-diuretic hormone (ADH)
- ADH released in respond to low blood volume or increased solute concentrations
- ADH
- Maintains water in the body to encourage the kidneys to reabsorb more water from the tubules
- Oxytocin
- Responsible for muscular contraction of uterus during childbirth
- Stimulates breast cells to produce milk
- Works on positive feedback mechanism – the more stimuli received from feeding; the more milk is produced.
What does the thyroid gland consist of?
Where is it found?
How big is it?
Why does size of the thyroid vary?
What is blood supply and innervation like in the thyroid?
Why is it this way?
How does the thyroid develop in the embryo?
- The thyroid gland consists of 2 lobes covering the anterolateral surface of the trachea
- It is found within the visceral compartment of the neck surrounded by pre-tracheal fascia
- It is the largest pure endocrine gland in the body at about 20g
- The size of the thyroid varies based on diet/iodine availability and physiological demands for thyroid hormones
- The thyroid has a rich blood supply, which allows for quick release of hormones into the blood
- The thyroid is innervated by the parasympathetic and sympathetic nervous system
- This allows the thyroid to respond to more immediate neuronal stimuli, as well as hormones
- In the embryo, the thyroid develops as a down growth of the tongue epithelium
What are the structural and functional units of the thyroid?
What do these units consist of?
What is produced in the cells of these units?
Where are they stored?
What 2 hormones are these products used to form?
Where do these hormones then go?
What are other cells present in the thyroid?
What hormones do they produce?
What is this hormone for?
What is the hormone that opposes this?
- The structural and functional units of the thyroid are called follicles
- Follicles consists of a lumen filled with colloid surrounded by simple columnar folicular cells
- Thyroglobulin is a glycoprotein produced by follicular cells and stored in the lumen of the follicle
- Thyroglobulin combines with iodine and is endocytoses back into the follicular cells and is cleaved to generate Thyroid hormones Tri-iodothyronine (T3) and Thyroxine (T4)
- Once cleaved from thyroglobulin , T3 and T4 diffuse into the rich blood supply of the thyroid
- C-cells (parafollicular cells) produce and release calcitonin
- Calcitonin respond to high blood calcium levels, and inhibits osteoclasts to decrase the blood calium levels
- Parathyroid hormone works oppositely to Calcitonin
• Thyroxine regulates:
1) Rate of metabolism (energy use)
2) Protein production – growth and development
3) Sensitivity of cells to other hormones
What are 3 thyroid disorders?
What are they caused by?
What symptoms do they have?
- Hypothyroidism
- Caused by iodine-poor diet
- Generates non-toxic goitre(swelling)
- Hyperthyroidism (graves’ disease)
- Caused by autoimmunity or poor diet, which results in excess hormone production
- Can be caused by anti-bodies being released and mimicking Thyroid stimulating hormone (TSH)
- Can generate toxic goitre
- Causes oedema (swelling due to build-up of fluid), which can push the guy out
- Leads to increase bmr, which causes weight loss, irregular heartbeat, and anxiety
- Severe hypothyroidism
- Caused by iodine deficiency
- Leads to neurological and physiological defects, such as low IQ
- HRT can be given if caught early, but damage is not reversible
What does the parathyroid gland consist of?
Where is it found?
What does it develop?
What does the parathyroid gland produce?
What 2 things is this hormone responsible for?
- The parathyroid gland consists of two pairs of glands embedded on the posterior aspect of the thyroid gland
- It develops the wall of the pharynx
- The parathyroid gland produces parathyroid hormone, which opposes calcitonin
• Parathyroid hormone is responsible for:
1) Regulating calcium homeostasis – stimulates osteoclasts to breakdown bone matrix and increase blood Ca++ levels
2) Promotes conversion of inactive vitamin D to active form, which is required for absorption of Ca++ from diet.
What are the adrenal/suprarenal glands?
Where are they found?
What 2 tissues are they comprised of?
What are these tissues comprised of?
What does each region produce?
• The adrenal/suprarenal glands are paired glands superior to the kidneys
- The 2 tissues that make up the adrenal glands:
- Cortex
- Glandular tissue derived from mesoderm
- Medulla
- Nervous tissue derived from neural crest (neuroectoderm)
- The cortex surrounds the adrenal medulla
• Each region produces their own set of hormones that allow the body to immediately respond to stress e.g exercise, injury
What hormones does the adrenal cortex produce?
What is a common pre-cursor?
What are the 3 different layers of the adrenal cortex from top to bottom?
What kind of steroids do they each produce?
What is an example of each?
What do these examples do?
- The adrenal cortex produces hormones that are quite long lasting
- These are steroid hormones called corticoid steroids, with cholesterol being the common pre-cursor
- The 3 layers of the adrenal cortex from top to bottom:
- Zona glomerulosa
- Produces mineralocorticoids
- E.g aldosterone (homeostasis of Na+ and K+ and water)
- Zona fasciculata
- Glucocorticoids
- E.g cortisol (energy metabolism and glucose availability)
- Zona reticularis
- Gonadocorticoids
- E.g sex steroids – weak androgens which are converted in tissue cells to more potent forms e.g testosterone and oestrogens
What are the 4 steps of the process involved in the cortisol feedback mechanism?
What can this system be referred to as?
What is the release rhythm of cortisol?
What does cortisol respond to?
What are its 3 main effects?
1) Hypothalamus detects stress (exercise, hypoglycaemia or triggered by SNS when needed) and excites neurons to release corticotropic releasing factor
2) This stimulated the anterior pituitary gland to release adrenocorticotropic hormone (ACTH), which targets cells in the cortex of the adrenal gland (in this case fasciculata)
3) This stimulates the cells to release cortisol into circulation
4) Cortisol can negatively feedback, and can inhibit the hypothalamus from secreting hormones, and instead secrete an inhibitory hormone
- This system is the hypothalamic -pituitary – adrenal axis
- Cortisol has a clear diurnal rhythm which peaks at 6-8am and is at its lowest at midnight-2am
- Cortisol is released in response to stress
• The 3 main effects of cortisol are to try and relieve stress:
1) Glucose mobilisation for generation of ATP
2) Cardiovascular (blood volume)
3) Sensitivity to catecholamines (neurotransmitters)
What is the cause of Cushing’s syndrome?
What are 3 potential reasons for this?
What are 5 characteristic symptoms of Cushing’s Syndrome?
• Cushing’s Syndrome is caused be excessive cortisol
This may be due to:
• Tumour excessively stimulating cortisol production
• May be from an ectopic source, such as prescribed glucocorticoids for conditions like rheumatoid arthritis (RA)
• A hypersecretory condition
- Characteristic symptoms:
- Moon face
- Redistribution of fat to abdomen and neck
- Peripheral wasting
- Hypertension
- Osteoporosis
What is the adrenal medulla composed of?
What does it have a direct connection to?
What is it primed for?
What is synthesised and secreted in the adrenal medulla?
What is a precursor for these products?
What are 3 catecholamines produced in the adrenal medulla?
What are their 3 key effects?
- The adrenal medulla is composed of nervous tissue containing chromaffin cells
- The adrenal medulla has a direct connection with the sympathetic nervous system
- It is primer for immediate response – fight or flight
- The adrenal medulla is the site of catecholamine synthesis and secretion
- Tyrosine is a building block for catecholamines
- 3 catecholamines:
- Epinephrine
- Norepinephrine
- Dopamine
- 3 key effects of catecholamines:
- Increase in heart rate, blood pressure, and respiratory rate
- Diversion of blood to muscles
- Glucose mobilisation
Where is the pancreas located?
How is it developed?
Is it exocrine or endocrine?
What does it play a major role in?
- The pancreas is located partially behind the stomach
- The pancreas develops as on outgrowth of the epithelial lining of the GI tract
- It acts as both an endocrine and exocrine gland, but a majority of its tissues is exocrine
- Major role in glucose homeostasis
What are the endocrine cells of the pancreas?
Where are they found?
What mass do they make up of the pancreas?
What hormones are produced by what cells?
What are these hormones responsible for?
What is a disorder concerning these cells?
What are the exocrine secretions of the pancreas?
- The endocrine cells of the pancreas are the Islets of Langerhans, which are found embedded within exocrine tissue (acinar cells)
- These endocrine cells make up 1-2% of the total pancreas mass
- Islets of Langerhans produce hormones:
- Alpha cells produce glucagon
- Beta cells produce insulin
- These hormones are essential for the regulation of blood glucose
- A key disorder of these cells is diabetes mellitus, which is caused by the immune system killing insulin-producing beta cells
- The exocrine function of the pancreas is the secretion of digestive enzymes and bicarbonate
What are 4 other endocrine organs?
What do they produce?
- GI tract produces gastrin – stimulates HCl production in parietal cells of stomach
- Kidneys produce renin – involved in feedback mechanism of aldosterone (mineralocorticoids) which maintains blood volume and sodium concentrations (renin can indirectly raises blood pressure)
- Ovaries produce oestradiol and progesterone – oestradiol and progesterone thicken uterine wall so the egg can implant if fertilised.
- Testes produce testosterone – increases levels of spermatogenesis
