The endocrine system Flashcards
Water & fat-soluble hormones
Endocrine glands secrete hormones, circulating blood delivers to target tissues
* Most water‐soluble hormones made from chains of amino acids (peptides). Receptors usually on surface of cell. Binding causes a signalling cascade within cell. A common second messenger is cyclic AMP (cAMP), synthesized from ATP.
* Lipid‐soluble hormones (thyroxine and steroids) are transported in blood by being attached to transport proteins to make them soluble. These diffuse through lipid bilayer of plasma membrane and bind to their
receptors within target cells, often in cell nucleus.
Water & fat-soluble hormones
Lipid based hormones
Steroids e.g., glucocorticoids, mineralocorticoids (adrenal gland)
Thyroid hormones
Peptide hormones
Adrenalin, noradrenalin (adrenal gland)
Insulin Glucagon
Exocrine versus endocrine glands
Exocrine glands secrete their products into ducts that carry secretions into a body cavity, into lumen of an organ, or onto outer surface of body. Sweat glands and pancreatic enzyme producing glands are examples of exocrine glands.
* Endocrine glands secrete their products (hormones) into interstitial fluid, the fluid that surrounds tissue cells. The hormones diffuse into blood capillaries, and blood carries them throughout body
Endocrine versus neural signalling Nervous System
Mediator molecules:Neurotransmitters released locally in response to nerve impulses
Site of mediator action:Close to site of release, at synapse; binds to receptors in postsynaptic membrane
Types of target cells:Muscle (smooth, cardiac, and skeletal) cells, gland cells, other neurons
Time to onset of action:Typically within milliseconds (thousandths of a second)
Duration of action:Generally briefer (milliseconds)
Endocrine versus neural signalling Endocrine System
Mediator molecules:Hormones delivered to tissues throughout body by blood
Site of mediator action:Far from site of release (usually); binds to receptors on or in target cells
Types of target cells:Cells throughout body
Time to onset of action:Seconds to hours or days
Duration of action:Generally longer (seconds to days)
Hypothalamus and pituitary gland
Hypothalamus and pituitary gland act as a unit
Posterior pituitary is an extension of nervous tissue down from brain Posterior pituitary has a “stand alone” blood supply.
Anterior pituitary supplied by blood that has already passed through capillaries in hypothalamus.
The hypothalamus releases peptides that stimulate or inhibit the hormones of anterior pituitary into these capillaries.
The blood then bathes anterior pituitary in a series of sinusoids, signalling anterior pituitary to increase or decrease secretion of hormones
Hormones of Anterior pituitary
Growth hormone,
* Thyroid stimulating hormone
* Adrenocorticotrophin hormone
* Prolactin
* Follicle stimulating hormone * Luteinising hormone
Negative feedback regulation of hormones by anterior lobe of pituitary gland
Hypothalamus senses amount of GH in blood, releases either GHRF or GHIF to bathe anterior pituitary to release more or less GH into blood
Negative feedback regulation by anterior lobe of pituitary
TRH released by hypothalamus, causes anterior pituitary to release TSH, thyroid gland releases thyroxine
Thyroxine inhibits TRH release
CRH (corticotrophin releasing hormone) released from hypothalamus, causes anterior pituitary to release ACTH (adrenocorticotrophin releasing hormone) into bloodstream, causes cortisol from adrenal glands to be released.
Hormones of anterior pituitary
growth hormone, thyroid stimulating hormone,adrenocorticotrophic hormone, prolactin, follicle stimulating hormone,Lutenising hormone,
growth hormone,
regulates metabolism promotes tissue growth especially of bones and muscles.
thyroid stimulating hormone,
stimulates growth and activity of thyroid gland and secretion of T3 and T4
adrenocorticotrophic hormone
stimulates the adrenal cortex to secrete glucocorticoids
prolactin,
stimulates growth of breast tissue and milk production
follicle stimulating hormone
stimulates production of sperm in testes,stimulates secretion of oestrogen by the ovaries, maturation of ovarian follicles, ovulation.
Lutenising hormone,
stimulates secretion of testosterone by the testes,stimulates secretion of progesterone by the corpus luteum.
Growth hormone (GH)pt2
In response to GH, cells in liver, skeletal muscles, cartilage, bones, and other tissues secrete secondary hormones called insulin-like growth factors (IGFs), which either enter bloodstream or act locally. IGFs stimulate protein synthesis, help maintain muscle and bone mass, and promote healing of injuries and tissue repair.
* The anterior pituitary releases GH in bursts that occur every few hours, especially during sleep.
* Blood glucose level is a major regulator of GHRH and GHIH secretion. Low blood glucose stimulates the hypothalamus to secrete GHRH (increasing GH). High blood glucose inhibits release of GHRH (and therefore GH)
Negative feedback for ADH in posterior pituitary
ADH causes retention of water and brings osmolality down
Also restores blood pressure as ADH causes vasoconstriction
Same thing happens after salty meal
But if this was the only homeostatic mechanism
Thyroid gland
Made up of microscopic spherical sacs called thyroid follicles. Walls of follicle comprise follicular cells, that produce two hormones: thyroxine, (T4, contains 4 I atoms) and triiodothyronine (T3, contains 3 I atoms).
* T3 and T4 known as thyroid hormones.
* Central cavity of thyroid follicles contain stored thyroid hormones.
* Most T4 converted to T3 as it enters cells. T3 more potent than T4.
* Smaller number of parafollicular cells lie between follicles . They produce the hormone calcitonin for Calcium and bone homeostasis
Actions of Thyroid Hormones
Thyroid hormones increase basal metabolic rate (BMR), the rate of oxygen consumption under standard or basal conditions.
* Thyroid hormones play important role in maintenance of normal body temperature.
* Also stimulate protein synthesis, & body growth, especially growth of nervous systems in babies.
Abnormalities of thyroid
Deficiency relatively common
* Congenital hypothyroidism, Need prompt diagnosis as can cause mental retardation. Babies screened routinely. Treated with thyroxine for life.
* Adult Hypothyroidism causes myxedema, oedema/swelling of facial tissues. Slow heart rate, low body temperature, cold sensitivity, dry hair and skin, lethargy, gain weight.
* Also caused by deficiency of iodine in diet. May cause enlarged thyroid gland (goiter) Too much thyroid hormones:
* XS thyroxine due to Graves disease, an autoimmune disorder. Antibodies mimic action of TSH, stimulating thyroid gland to grow & produce thyroxine. May get goiter and unusual swelling behind eyes causing them to protrude
Calcitonin & parathyroid hormone (parathormone)
Calcitonin producing cells in thyroid
* Parathormone in parathyroid glands (behind thyroid gland)
* Act together to control calcium levels
* Calcitonin reduces blood calcium & stimulates bone cells to store calcium * Inhibit kidney reabsorption of calcium
* Parathormone does reverse
* Release calcium from bones
* Stimulates calcium reabsorption by kidneys
Low blood calcium
Hypoparathyroidism
* Too little parathyroid hormone—causes reduced blood Ca2+, causes neurons and muscle fibres to depolarize and produce action potentials spontaneously.
* Leads to twitches, spasms, and tetany (maintained contraction) of skeletal muscle.
Adrenal medulla
Produces adrenalin and noradrenalin
* Innervated by sympathetic nervous system * Short term flight or flight response
* Increase heart rate & blood pressure
* Increasing metabolic rate
* Dilates pupils
