Homeostasis Flashcards
Define homeostasis and the basic process by which it occurs
Homeostasis is the existence and maintenance of a stable internal environment.
Process:
> Stimulus (change in variable) e.g too hot
v
> Receptor (detects change) e.g thermoreceptors
v
> Input (afferent pathway)
v
> Control Centre e.g cooling mechanisms activated
v
> Output (efferent pathway)
v
> Response (reduce change) e.g sweating
What are the two feedback systems
- Positive Feedback
- body exaggerates change
- when a quick response is needed (usually in potentially dangerous/ stressful processes - less common)
e. g haemostasis - break/ tear in vessel wall > platelets adhering and releasing chemicals attracting more platelets (positive feedback loop)> loop = broken when platelet plug = formed - Negative feedback
- main mechanism for homeostatic regulation
- maintenance of fixed range (minor changes = ignored)
(ranges = norm for 95% of population)
e.g. temperature control via sweating/shivering
Explain system integration and what can go wrong with homeostasis
> system integration is maintenance of homeostasis within different systems as reducing a change in one will create a change in another
- e.g sweating to reduce temperature change reduces water level in blood so heart has to work harder unless kidneys reabsorb more water
*Therefore a pathology in one system can affect homeostasis in another
in some cases (injury/infection/genetic abnormality)
the change is too great - mechanisms can’t compensate
- leading to illness/disease
- new-borns = susceptible as systems are immature - less able to cope with change
- elderly = susceptible:
a) hormones are fewer are less efficient
b) heart + circulation = less efficient
c) fewer neurons + neurotransmitters = slower response
What are the two mechanisms of homeostasis
- Autoregulation
- cells/tissues/organs adjust to changes automatically
- e.g. lack of O2 - vasodilation in area for more blood - Extrinsic regulation
a) nervous system
- sympathetic (ACh +noradrenaline)/parasympathetic (ACh only)
- fast + short lasting (crisis management - fight/flight)
- e.g. re-direction of blood from digestive system in exercise
b) endocrine system
- slower to react but longer lasting
What is the endocrine system
> glands/organs that produce hormones
typically ductless (endocrine) - secrete into interstitial fluid to then diffuse into blood stream
may also have ducts (exocrine)
hormones excite/inhibit activity of organs/tissues
e.g hypothalamus/pituitary gland/stomach/kidney/pancreas
What are the functions of the endocrine system
Regulate > blood volume - water/sodium balance > extracellular fluid concentration - calcium/phosphate levels > energy balance > control of macronutrient usage/storage > Response to stress > reproduction + sexual development > growth
Categories of hormones
- Non-steroidal (fat insoluble)
- attach to receptors on membrane - 2nd messenger mechanism
> Peptide/protein = majority e.g insulin/glucagon
- made as pre-hormones + activated as needed
- stored in secretory glands
- secreted in vesicles (exocytose from cells)
> Amino acid e.g adrenaline/dopamine (-ine suffix)
- synthesised from other amino acids - Steroidal (fat soluble)
- pass through membrane of target organ to bind with receptor in nucleus
- derived from cholesterol
- synthesised in adrenal cortex/gonads/placenta
- require binding proteins to travel in blood
e.g. testosterone/oestrogen/progesterone
Hormone pathways
> Paracrine - act on nearby receptive cells (histamine/bradykinin)
Autocrine - acts on cell it is released by
Endocrine (telecrine) - via blood to distant cells
Synaptic - produced by neurone - secreted into synapse to be taken up by nearby neurone (ACh)
Neuroendocrine - produced by neurone - secreted into synapse - travels by blood to distant cells with receptors
Hypothalamus + Pituitary gland
> Hypothalamus = situated in base of brain
- contains neurons that synthesize inhibiting/releasing hormones that act on the pituitary gland
Common releasing hormones
- Growth hormone RH (pituitary to release HGH)
- thyrotropin RH (pituitary - thyroid stimulating hormone)
- corticotropin RH (cortisol release)
- gonadotropin RH (pituitary - reproductive hormones)
* Triggered by neurotransmitters/injury/chemical mediators/neuroendocrine signals (ie. positive/negative feedback loops)
Thyroid
- position
- role
- hormones
> butterfly shaped gland on anterior part of neck
stimulates metabolism
facilitates breakdown of carbs/protein/fat - for energy/heat
produces structural enzymes/proteins/hormones
(promotes growth + development in kids)
Process
- thyrotropin RH from hypothalamus triggers thyroid stimulating hormone from pituitary - in turn triggers thyroid hormones to be released
- Thyroxine (T4)
- triiodothyronine (T3)
What happens if thyroid hormones are released
> increased glucose absorption > lipids = released from adipose tissue > Proteins= metabolised from muscle > Cholesterol = broken down in liver > Increased O2 consumption > Increased body heat production > Increased cardiac output > Increased gastric motility > Increased muscle tone + reactivity > Increased activation of cognitive processes
Causes of altered hormone function
> Impairment of hypothalamus/ pituitary gland/ endocrine gland > Too much/little hormone = secreted > Inactive hormone = produced > Inadequate receptor binding > Lack of response from target cell > Impaired negative feedback > Produced ectopically (tumour produces hormone) > Impaired metabolism/elimination
Examples of disorders of endocrine system
> acromegaly = too much HGH (thickened bone areas)
gigantism = too much hgh pre-puberty
dwarfism (achondroplasia) - too little hgh
hyperthyroidism (grave’s disease)
hypothyroidism (goiter/ myxedema)
diabetes mellitus/insipidus
hyperthyroidism
> excessive levels of thyroid hormone
- increased stimulation
- increased production of thyroid stimulating hormone
- high levels of iodine
- diseases of thyroid
> Grave’s disease
- autoimmune condition (unknown trigger)
- immunoglobulin binds to TSH receptors - thyrotoxicosis (excess thyroid stimulation)
- more common in women - 7-10x
Symptoms
- enlarged thyroid gland (goiter)
- excess metabolic rate (weight loss, muscle wastage, tremor, tachycardia)
- bulging eyeballs
(some become permanent so treatment early is key)
treated with meds/surgical removal of part or all of thyroid - may then need supplement
Hypothyroidism
Deficient thyroid hormone
> Congenital (life threatening)
- during foetal development (masked by mothers hormone)
- lack of thyroid development (less hormone produced)
- results in developmental delay + impaired growth
*If detected thyroid replacement treatment may help
> Acquired
- autoimmune disorder/iodine deficiency/surgical removal/ radiation or chemical damage to thyroid
- 10 x more common in women
Symptoms
- boggy/swollen tissue (hands/feet/face)
- muscle wastage
- weight gain
(possible goiter from gland working harder to produce deficient hormone)
Treatment
- hormone supplement
- treat other symptoms
