Physiological Homeostatis Flashcards
What is homeostasis?
‘The dynamic maintenance of physiological variables within a predictable range’
What does dynamic mean?
The variable may fluctuate, but still within a ‘normal’ range and the average value will be predictable over longer periods of time
What does the size and frequency of the fluctuation depend on?
A given variable
What is the short term purpose of homeostatis?
Immediate survival
What is the medium-long term purpose of homeostatis?
Health and well-being
Reproductive capability
What is the hierarchy of importance of physiological variables?
A variable that is of greater immediate importance may be maintained at the expense of other variables that are important in the long term
What do Homeostatic control mechanisms ensure?
That changes in the physiological variables are normalised
What is negative feedback also known as?
Reflex arc
What is negative feedback?
A change in the variable being regulated is compared against a set-point, causing a response that tends to move the variable back to the set-point
What happens to the set point when physiological circumstances change?
Changed (e.g. fever) or over-ridden (e.g. exercise)
What are the types of negative feedback?
Neuronal
Endocrine (hormonal)
Local (chemical/ physical)
What is a feed forward mechanism?
Anticipation of a change brings about the response to that change before the change can be detected by negative feedback sensors
What is positive feedback?
Change in a variable triggers a response that causes further change in that variable (amplification rather than normalisation)
What are the common components of a negative feedback reflex arc?
Sensors Afferent Pathway Integrating Centre Efferent Pathway Effectors
What do sensors do?
They detect changes in the physiological variable
What does the afferent pathway do?
Carries signal from the sensors to the integrating centre
What does the integrating centre do?
It compares inputs from sensors against physiological set-point and elicit a response
What does the efferent pathway do?
Carries signals from integrating centre to effectors
What do effectors do?
Produces a response that tends to normalise the physiological variable
Where are neuronal integrating centres for physiological control located?
Mid-brain
Brainstem
What is the mid-brain and brainstem comprised of?
Hypothalamus
Pons
Medulla
What is an example of a parasympathetic neurotransmitter?
Acetylcholine
What is an example of a sympathetic neurotransmitter?
Noradrenaline
What are the human endocrine organs?
Hypothalamus, Pituitary, thyroid, Adrenal, Pancreas, Ovaries, Testes
What are neuroendocrine cells?
Cells that receive neuronal input and as a consequence release of hormones
What are some examples of inhibitory hormones?
Somatostatin
Dopamine
What are some examples of releasing hormones?
GHRH
CRH
TRH
GnRH
What are the times of hormones?
Peptides
Polypeptides
Glycopeptides
What are some examples of peptide hormones?
ADH
Oxytocin
What are some examples of polypeptide hormones?
Growth Hormone
Insulin (pancreas)
What are some examples of glycoprotein hormones?
Luteinising hormone
Follicle Stimulating hormone
Thyroid stimulating hormone
What are adrenaline and thyroxine (T4) derivatives of?
Tyrosine
What are some examples of hormones derived from cholesterol?
Ovaries:Progesterone, Oestrogen
Testes: Androgen
Adrenal Cortex: Glucocorticoids, Mineralocorticoids
What is the receptor location for peptides, proteins, glycoprotein, catecholamines?
Plasma membrane
What is the mechanism of action for peptides, proteins, glycoprotein, catecholamines?
Second messengers
Rapid, transient response
What is the receptor location for steroids and thyroid hormones?
Cytoplasm
Nucleus
What is the mechanism of action for steroids and thyroid hormones?
Alter gene transcription
Slow, prolonged response
What is a feature of local homeostatic responses?
Negative feedback reflexes operating locally, within tissues (does not require brainstem or release of hormones)
How is blood flow controlled locally?
Muscles work harder Increased pO2 and lactic acid (drop in pH) Sensed by arterioles suppling the muscle Arterioles dilate Increasing O2 supply/CO2 removal
How is blood volume controlled locally?
Increased plasma volume
Macula dense sense increased renal filtration
Juxtaglomerular cells secrete less renin
Less Angiotensin II and Aldosterone
Plasma Volume Falls
What usually controls feed forward control mechanisms?
Neuronal
What happens in anticipation of physical exertion?
Anticipation of physical exertion (fight or flight)
Sympathetic Activation
Noradrenaline and Adrenaline Release
Increased Heart Rate and Increased Blood flow in muscles
Preparation for increased demand for O2 and fuel by muscles
What usually controls Positive Feedback?
Hormones: steroids
What are some examples of Positive Feedback?
Menstrual Cycle
Parturition
