Introduction to the Endocrine System Flashcards
What is the endocrine system?
A system of specialised organs/glands throughout the body which secrete hormones into the blood or extracellular fluid where they act as chemical messengers
Nervous system: uses neurotransmitters
Endocrine system: uses hormones
Why is endocrine system needed? How does it work?
the endocrine system is one of the key systems in the maintenance of homeostasis
Homeostasis
the maintenance of relatively constant states within the body
Target cells
cells possessing the receptors for a specific hormone and so are receptive to their effects
Target organ/tissue
organ/tissue containing the target cells for a specific hormone
Signal transduction
process by which the chemical signal is detected by receptor on target cell
Some cells have receptors for hormones (endocrine system) and neurotransmitters (nervous system)
Neurotransmitters
– Travel short distances using synapses
– Effects are rapid to appear but are short-lived
Hormones
– Travel longer distances using the blood
– Effects are slower to appear but are long-lasting
Glands
- Organs which produce and secrete substances
* Two key types
Endocrine glands
– Secrete hormones
– Released directly into the blood / extracellular fluid
– They do not have ducts – referred to as ductless glands
– Either glandular epithelial cells or neurosecretory cells
Exocrine glands
– Secrete, e.g. sweat and tears
– Exo: external
– Release secretions out of the body via ducts
– Referred to as ducted glands
– Exocrine glands are not part of the endocrine system
– Endocrine glands are part of the endocrine system
Hormones features
Chemicals produced by glands or specialised cells of certain organs
Secreted into the blood or extracellular fluid where they act as chemical messengers
Hormones primarily serve to regulate the metabolic function/cellular activity of other cells
For a hormone to have an effect the cell must possess receptors for that specific hormone
Some common hormone effects
– Alteration of plasma membrane permeability/electrical state
– Stimulation of protein/enzyme synthesis
– Activation/deactivation of enzymes
– Stimulation of secretions
– Stimulation of mitosis
Tropic hormones
Target other endocrine glands
Sex hormones
Target reproductive tissues
Anabolic hormones
Bring about anabolism in their target cells
Classification based on hormone structure
- Steroid hormones
non-steroid hormones
- Protein / polypeptide hormones
- Modified amino-acid hormones
Hormone solubility
- Steroid hormones are lipid-soluble
- Protein/polypeptide hormones are water-soluble
- Majority of modified amino-acid hormones are watersoluble. Exceptions: thyroid hormones – lipid soluble
- Receptors for hormones can be situated either inside or on the surface of the target cell
• Receptor location depends on the solubility of the
hormone …
Some hormones are water-soluble
- Are unable to cross the phospholipid bilayer
- Bind to receptors on surface of target cell
- As they cannot pass the plasma membrane, water-soluble hormones do not affect gene transcription
- They instead rely on a signalling cascade via a second messenger molecule to bring about their effects
Some hormones are lipid-soluble
- Are able to cross the phospholipid bilayer
- Bind to receptors inside the target cell
- As they can pass the plasma membrane, lipid-soluble hormones are able to alter gene transcription
- They are therefore able to cause alteration of cellular function in order to bring about their effects
Steroid hormones
- Steroid hormones are lipid-soluble
- Secreted by, e.g. adrenal cortex, ovaries and testes
- Steroid hormones are transported through the blood by specialised carrier proteins
- They are mostly derived from cholesterol
- Cholesterol serves as the precursor for the two main classifications of steroid hormones.
Corticosteroids
– Cortisol (a glucocorticoid)
• Produced and secreted by adrenal cortex (i.e. cortex or the adrenal gland)
• Roles in stress response (physical and psychological stress). Fight or flight
– Aldosterone (a mineralocorticoid)
• Produced and secreted by adrenal cortex
• Serves to decrease urine volume and increase blood pressure
Sex steroids
– Oestrogens & progestogens (ovaries), androgens (testes)
• Primarily produced and secreted by the gonads. Roles in sexual development
Protein / polypeptide hormones - (Non-steroid hormones)
- Protein/polypeptide hormones are water-soluble
- Secreted by, e.g. pituitary, pancreas
- The majority of hormones are of the protein / polypeptide classification
• Remember:
– Peptides: two or more amino acids
– Polypeptides/proteins: long chains of amino acids
– Proteins: consisting of one or more polypeptides with a molecular weight of >10,000
Protein / polypeptide hormones examples
– Insulin & Glucagon
• Produced and secreted by Islets of Langerhans cells of the pancreas
• Responsible for management of blood sugar
– Human growth hormone
• Produced and secreted by pituitary gland
• Stimulates cellular reproduction and development
– Follicle-stimulating hormone
• Produced and secreted by pituitary gland
• Key roles in sexual development of ovaries and testes
– Antidiuretic hormone
• Produced by the hypothalamus but stored and released by the pituitary
• Serves to decrease urine volume and increase blood pressure
Modified amino-acid hormones - (Non-steroid hormones)
• Majority of modified amino-acid hormones are water soluble. Exceptions: thyroid hormones – lipid soluble
- Secreted by, e.g. thyroid gland, pineal gland
- They are a small group of hormones
- Their names mostly end in ‘ine’
- Majority based on the amino acids tryptophan and tyrosine
Modified amino-acid hormones - Tryptophan derived example:
– Melatonin
• Produced and secreted by pineal gland
• Has roles in the sleep cycle
• It is the only hormone known to be produced by the pineal gland
Modified amino-acid hormones - Tyrosine derived examples
– Thyroxine (T4), triiodothyronine (T3)
• Produced and secreted by thyroid
• Have roles in metabolic rate
– Epinephrine (adrenaline), norepinephrine (noradrenaline)
• Produced and secreted by medulla of adrenal gland
• Have roles in cardiac output and blood sugar levels. Fight or flight response
Steroid hormone action
steroid hormones = lipid soluble
• They can diffuse into target cells
• This gives you some clues as to their mode of action
• If a hormone can cross the phospholipid bilayer of the plasma membrane then the receptors for that hormone are located inside the cell
• If it can get inside, receptors are inside (steroids)
• If it can’t get inside, receptors outside (most non-steroids)
• Steroid receptors are located within the cell
Non-steroid hormone action
- Protein / polypeptide hormones = water soluble
- Modified amino acid hormones = majority are water soluble
- The vast majority of non-steroid hormones are therefore unable to diffuse across the phospholipid bilayer of the plasma membrane
Non-steroid receptors located on surface of plasma membrane
intracellular second messengers - Non-steroid hormone action
First messenger: the initial signal molecule. In this case, a non-steroid hormone
• Second messenger: intracellular molecules produced in response to a signal from the first messenger, the second messenger transmits the signal from receptor to target
Two most understood second messenger mechanisms
Cyclic adenosine monophosphate (cAMP)
Calcium-calmodulin
Hypothalamus
Situated at base of brain, on top of pituitary gland
Has both neural and endocrine function
- Links the nervous and endocrine systems
- Key role in homeostasis
Primary roles of Hypothalamus
– Signals pituitary to start/stop hormone production
– Produces a number of its own hormones
Communicates with pituitary gland via hormones
hormonally-controlled aspects by hypothalamus
– Temperature regulation – Thirst/water intake – Hunger – Behaviour – Sleep cycles
Thyrotrophin Releasing Hormone
- Triggers Thyroid Stimulating Hormone (TSH) in pituitary
- Secretion of thyroxine from thyroid
Corticotrophin Releasing Hormone
- Stimulates adrenocorticotrophic hormone (ACTH) in pituitary
- Secretion of glucocorticoid hormones from adrenal cortex
Growth hormone releasing hormone
- Stimulates human growth hormone
- Stimulates growth of bones, muscles etc.
Pituitary
Situated at base of brain, below the hypothalamus
Controlled by hormonal signals from hypothalamus
Primary role of Pituitary
Causes hormone production and secretion which signal/control other organs of the endocrine system e.g thyroid gland, adrenal gland and gonads
• pituitary gland is often referred to as the master gland of the endocrine system
Pituitary - two key lobes
- Anterior (i.e. front) pituitary gland
- Posterior (i.e. back) pituitary gland
- Both regions produce hormones
- The anterior and posterior lobes are connected by the intermediate lobe
Some hormones secreted by anterior lobe
– Adrenocorticotropic hormone (ACTH)
• Maintains growth and development of the adrenal gland
• Stimulates adrenal gland cortisol and epinephrine production
– Follicle-stimulating hormone (FSH)
• Involved in sexual development (ovaries and testes)
• Control of menstrual cycle and control of sperm production
– Luteinising hormone (LH)
• Involved in sexual development (ovaries and testes)
• Control of menstrual cycle and control of sperm production
Some hormones secreted by posterior lobe
– Antidiuretic hormone (ADH)
• Produced by hypothalamus but stored & released by pituitary
• Serves to decrease urine volume and increase blood pressure
– Oxytocin
• Uterus contraction during childbirth
• Stimulates lactation
melatonin
– Only hormone known to be produced by pineal gland
– Melatonin is derived from serotonin
– Role of melatonin is not fully understood but is known to control our circadian (i.e. 24hr) rhythm
– Melatonin also plays a role in enhancing sleep patterns
Thyroid
- Butterfly-shaped gland located in the neck.
- It is anterior to the trachea, inferior to the larynx
- Composed of two lateral lobes connected by an isthmus
- Associated with 4 parathyroid glands in calcium deposition
Thyroid key functions
– Regulation of metabolism
– Growth and development
– Calcium deposition in bone
Thyroid follicles
– Structural and functional units of the thyroid
– Round with a wall of densely packed follicular cells
– Follicular cells: Site of thyroid hormone production
Higher TH
higher metabolic rate
Lower TH
lower metabolic rate
which disorders can come from abnormal thyroid function and hormone release
Hypothyroidism = under-production of thyroid hormones
– Associated with weight gain, tiredness, impaired mental function
• Hyperthyroidism = over-production of thyroid hormones
– Associated with weight loss, irritability, increased body temperature
Hormones produced by the thyroid gland
- Tetraiodothyronine (T4)
- Triiodothyronine (T3)
- Calcitonin (CT)
calcitonin (CT)
– CT serves to decrease blood calcium levels & increase calcium deposition in bone
parafollicular cells
aka c cells
– ‘para’ can mean next to / adjacent to
– Called parafollicular because adjacent to thyroid follicles
how many parathyroid glands are there
4
parathyroid hormone (PTH)
raises blood calcium levels by removing calcium from bone
The two hormones work together to maintain the balance (homeostasis) of blood calcium levels
Thymus
- Located in the chest between the lungs
- Posterior to the breastbone (sternum)
- Anterior to the primary blood vessels
- Composed of two lobes, left and right
- Grows and is most active from childhood to puberty, where it then decreases in size to the age of ~50
- Produces a range of polypeptide hormones (e.g thymopoietin)
T-cells
plays a key role in protecting us from viruses
- We have all of our T-cells by the time we reach puberty
- Maintenance of T-cell numbers beyond this point are typically the result of division of existing, mature T-cells
Myasthenia gravis
– Rare autoimmune disorder affecting neuromuscular system
– Affects range of the body but commonly: muscles controlling eyes and eyelids, expressions, swallowing and speech
– Can cause drooping eyelids, visual impairments, facial expressions as well as difficulty breathing in serious cases
Pure Red Cell Aplasia (PRCA)
– Another rare autoimmune disorder, affecting production of red blood cells (erythrocytes) in the bone marrow, causing anaemia
– Typical symptoms: lack of energy, frequent headaches, dizziness, difficulty breathing
Pancreas
- Situated within the abdomen, posterior to the stomach
- Long and flat organ 12-20 cm in length
- Has endocrine and exocrine function
Exocrine function of Pancreas
– Key role in digestion where is secretes digestive enzymes into the start of the small intestine (called the duodenum)
– Enzymes for carbohydrate, lipid and protein breakdown
Endocrine function of Pancreas
Primarily concerned with production of the hormones insulin and glucagon in regulation of blood glucose levels
5 key cell types
alpha beta gamma delta Epsilon
Alpha cells
Produce glucagon (a peptide hormone).
raises blood glucose level by triggering breakdown of glycogen stores in the liver into glucose
Beta cells
Produce insulin (a peptide hormone).
decreases blood glucose level by promoting the absorption of glucose by muscle and fat cells.
triggers formation of glycogen (from glucose) in the liver
Delta cells
Produce somatostatin (a peptide hormone). Also known as growth hormone inhibiting hormone.
regulates the system as it inhibits insulin and glucagon secretion
Gamma cells (PP Cells)
Produce pancreatic polypeptide (a polypeptide hormone).
regulates pancreatic function through reduction of appetite by inducing feelings of
Epsilon cells
Produce ghrelin (a peptide hormone).
induces feelings of hunger.
inhibits insulin secretion - allows blood sugar level to rise after a meal
Pancreas role in blood glucose regulation
alpha cells detect a drop in blood glucose - release glucagon - circulates to the liver via the bloodstream, signalling - breaks down glycogen into glucose - Blood sugar level rises
beta cells detect a rise in blood glucose - release insulin - Insulin circulates throughout the body via the bloodstream, binding to insulin receptors present on the surface of cells - causes special proteins for transporting glucose to be relocated from the cell cytoplasm to the plasma membrane
- Glucose moves into cells via facilitated diffusion
- Blood sugar level decreases
Adrenal
- Adrenal glands are situated above the kidneys
* One adrenal gland above each kidney
Each adrenal gland has two primary regions
– Adrenal cortex (the outer region)
– Adrenal medulla (the inner region)
Adrenal cortex
– Produces steroid hormones such as cortisol
– Cortisol plays a key role in the stress response (physical and psychological stress). Fight or flight response
– Promotes glucose production from fat & protein metabolism
– Glucose release into blood → increased blood sugar level
aldosterone
plays a crucial role in the salt:water balance of
the body where it targets the nephrons of the kidneys
promotes reabsorption by the body of sodium and water from the tubules of the nephrons
– Net effects:
• Decrease urine volume
• Increase blood pressure
Adrenal medulla
– Produces the amino acid hormones epinephrine
(adrenalin) and norepinephrine (noradrenalin)
– Epinephrine has a key role in fight or flight response.
– It is released in response to perceived threat and also
when excited
– It primes the body for a rapid response to any threat
– Cardiac output is increased and blood sugar level rises
disorders of the adrenal gland
Adrenal gland tumours
Addison’s disease
Cushing’s syndrome
Ovaries
- Ovaries are part of the reproductive system in females
- They are oval in shape and are situated either side of the uterus (womb)
- Females have two ovaries which produce and release eggs (termed oocytes)
- The ovaries are also responsible for producing the hormones oestrogen and progesterone and their release is controlled by the hypothalamus
Oestrogen
– Stimulates breast development
– Development of reproductive organs;
– Secreted by ovarian follicle
– Stimulates repair and thickening of the endometrium (lining of womb) in preparation for fertilisation
Progesterone
– maintains the endometrium between the points of ovulation and menstruation
– During pregnancy, increased production decreases
menstrual cycles. prevents muscle contraction in the uterus which would otherwise serve to reject an egg
– Elevated levels during pregnancy also prevent ovulation
Testes
- part of the reproductive system in males
- male equivalent of the ovaries
- oval in shape
located within the scrotum where they are suspended by the spermatic cord
• The testes produce and release sperm
Testosterone
– Male sex hormone. Secreted by Leydig cells of the testes
– Promotes development of male reproductive organ
– Influences sexual behaviour
– Stimulates the production of public and chest hair
– Increases retention of calcium in bones (also bone strength)
– Stimulates protein synthesis (production of protein)
– Increases metabolism
– Promotes muscle growth and development
3 main types of stimuli for hormone release:
– Humoral
– Neural
– Hormonal
Hormonal
Stimulation by presence of other hormones
Humoral
Stimulated by changing levels of ions/nutrients
These chemicals cause hormonal release or inhibition
Neural
Hormonal release in response to a neural stimulus
Endocrine disorders are generally of two categories
resulting from hypersecretion or hyposecretion of hormones
resulting from tumours or lesions which do not always have an effect on hormone levels
Hypersecretion
Too much of a hormone produced
Hyposecretion
Too little of a hormone produced
Impacts of hyper- and hypo- secretion
Diabetes mellitus
Hyperinsulinism
Menopause
