29 Homeostasis Flashcards
what is the definition of homeostasis?
the maintenance of a stable internal environment
what are the internal factors that must be controlled?
pH (arterial: 7.35 - 7.45)
temperature (36.5 - 37.5)
plasma glucose concentration (3.6 - 7.8 mmol dm^3)
plasma water potential (275 - 295 mOsm kg-1)
ion balance (Na+: 135 - 145 mmoldm-3)
blood pressure (120/80 - 140/90 mmHg)
what are the external factors that we respond to?
humidity
air temperature
light intensity
new/sudden sounds
what is the importance of homeostasis?
optimum conditions for enzyme-controlled reactions
control of rate and efficiency of metabolic pathways
outline the negative feedback mechanism
a stimulus produces a change in a variable
change detected by receptor
input info sent along afferent pathway to control centre
output info sent along efferent pathway to effector
corrective action is made
variable returns to set point
what is the set point?
the desired value that the negative feedback mechanism operates around
what is the feedback loop?
the pathway which returns an altered factor back to its set point, and informs receptors of the change to the system
outline the role of the positive feedback mechanism
to enhance the original stimulus
give an example of PFM
blood clotting cascade
voltage gated Na+ channels and action potentials
outline the role of oxytocin during labour
release of oxytocin –> increased speed and intensity of uterine contractions (due to increased prostaglandin production)
further increased secretion of oxytocin
positive feedback continues until childbirth is complete
oxytocin secretion stops
what is the role of the SAN in controlling heart rate?
generates electrical impulses/waves of excitation –> contraction of cardiac muscle
what is the role of the medulla oblongata in controlling heart rate?
two regions of cardiac centre, linked to SNA by motor neurones:
- incr. h.r. = symp. NS –> accelerator nerve -> SAN
- decr. h.r. = para. NS –> vagus nerve -> SAN
what is the function of baroreceptors?
to detect changes in blood pressure in aorta, carotid artery and vena cava
what is the function of chemoreceptors?
to detect O2/CO2 levels in plasma and plasma pH in the aorta, carotid artery and medulla oblongata
how does high CO2 blood concentration lead to and increase in breathing rate?
increased CO2 from aerobic respiration
increased blood pH (from H+ ions)
detected by chemoreceptors - impulse travels along sensory neurones to medulla oblongata
medulla oblongata sends impulse down accelerator nerve to SAN
noradrenaline released and binds to receptors on surface of SAN –> increased h.r.
increased ventilation rate to compensate and maintain efficient gradients for gas exchange
where is the adrenal gland located?
on top of the kidneys
how does adrenaline increase h.r.?
binds to specific receptors on cells of SAN
G protein activated
–> incr. cAMP production
–> incr. permeability of Ca2+ channels
SAN depolarises more quickly (lower threshold)
cardiac muscle contracts more frequently (and with more force)
what are the effects of adrenaline during the F/F/F response?
incr. h.r. ∴ blood pumped around body faster
muscles around bronchioles relax –> deeper breathing
glycogen hydrolysed to glucose and used as a respiratory substrate for aerobic respiration
vasodilation of arterioles supplying lungs and skeletal muscle –> incr. delivery of oxygen –> incr. aero. resp.
vasoconstriction of arterioles supplying the gut
what is the definition of thermoregulation?
the ability to regulate internal body temperature, irrespective of external temperature changes
where are thermoreceptors found?
hypothalamus - internal blood plasma temperature
skin (peripheral) - external temperature of skin
what are the mechanisms employed to reduce body temperature?
increased sweating - evaporation of sweat uses heat
erector pili muscles relax
vasodilation of arterioles supplying the skin
what is the definition of hypothermia?
when internal body temperature is below 35.C
what are the mechanisms employed to increase body temperature?
decreased sweating
erector pili muscles contract
vasoconstriction of arterioles supplying the skin
outline the effect of thyroxine secretion on internal body temperature
hypothalamus secretes thyrotropin-releasing hormone (TRH)
TRH stimulates production of thyrotropin (thyroid stimulating hormone, TSH) from the anterior p.g.
TSH stimulates secretion of thyroxine
thyroxine diffuses into nucleus of cells and causes transcription of specific genes
increased production of mitochondria and respiratory enzymes
increased metabolism of lipids and glucose
∴ rate of respiration increases
more thermal energy released
body temperature increases
how does secretion of ADH cause more water to be reabsorbed?
osmoreceptors shrink due to lower w.p. of blood plasma
ADH released from posterior p.g. into blood stream
binds to cells in the walls of the collecting duct
increase in selective reabsorption of water
