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What are the chief resistance vessels of the systemic circulation?
Arterioles
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What is basal tone?
The natural degree of tone arterioles (and some larger vessels) have in their vascular smooth muscle
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What is the symbol for blood flow?
Q
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What two major categories influence basal tone?
Local factors and extrinsic factors
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What type of factors (local or extrinsic) dominate over a) critical organs eg brain b) non-critcal organs e.g. kidney
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What organ are both local and extrinsic factors important in?
Skin
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List the 5 local control factors and mechanisms
- Temperature
- Compression
- Myogenic response
- Local metabolites
- Autocoids
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What effect does heating/cooling have on cutaneous arterioles?
- Heating - vasodilation
- Cooling - vasoconstriction
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What effect does mechanical compression have on blood flow?
It stops blood flow
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What is the myogenic response?
A rise in internal pressure distends blood volume but most arterioles react by contracting
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Many byproducts of normal tissue metabolism cause vascular relaxation or contraction? Why?
Relaxation - this increases blood flow and increases local tissue perfusion to organs with a higher metabolic rate
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Give examples of local metabolites that cause vascular relaxation
Acidosis, ATP breakdown and K+ released by muscle
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What are autocoids? Give an example of an autocoid
- Vasoactive chemicals produced, released and acting locally
- Histamine, serotonin, bradykinin, prostoglandin, nitrous oxide
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Where is adrenaline and noradrenaline secreted from?
The adrenal medulla
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What are situations adrenaline and noradrenaline secreted in response to?
Fight, flight, fear, exercise, hypotension and hypoglycaemia
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Where are baroreceptors located?
In the aortic arch and carotid sinus
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Afferent fibres from which cranial nerves are in the carotid sinus and aortic arch?
IX (glossopharyngeal) and X (vagus)
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What do baroreceptors do?
They detect and respond to changes in blood pressure
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What type of receptors are baroreceptors?
Mechanoreceptors
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Increased pressure in the artery causes distention - what effect does this have on baroreceptors?
This excites the receptors
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What effect does the following have on the heart? a) increased sympathetic activity b) decreased parasympathetic activity
- a) increased HR and contractility
- b) increased HR
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What effect does increased sympathetic activity have on blood vessels?
Arteriole vasoconstriction
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Why can baroreceptors not regulate blood pressure in the long term?
As baroreceptors adapt to the prevailing level of arterial pressure e.g. in animals that are hypertensive long term the baroreceptors come to accept this as normal and regulate pressure at the elevated level
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What are the three other ways the baroreceptor reflex tries to increase blood volume by?
- Increasing the sensation of thirst
- Increasing ADH which decreases urine flow from the kidneys
- Increases renin which acts to produce aldosterone which decreases Na excretion
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