Control of Blood Vessel Function Flashcards
what do blood vessels contain in their walls that allows control of their radius
smooth muscle
active tension controls the radius of vessel
what is the chief resistance vessels of systemic circulation
arterioles
what regulates Q
local arteriolar resistance regulates Q to the tissue downstream of arteriole
the range of Q can be large
what is flow matched to
metabolic requirements
ex. tissue or organ that works harder will need more flow
what is basal tone
arterioles (and some larger vessels) have a natural degree of tone in their vascular smooth muscle
even if sympathetic innervation is removed there is some constriction
important if vessels are to be dilated
tissues with highest increased in Q (ex. skeletal muscle, salivary glands) have highest basal tone
what influences basal tone
- local factors: located in tissue (ex. tissue metabolites)
- extrinsic factors: autonomic control
which type is dominant type of control depends on tissue
what factors dominate in critical tissues
local control dominates
must have enough blood to meet metabolic needs on a second to second basis in order for the animal to survive (heart, brain, skeletal muscle)
where does extrinsic control dominate
extrinsic control dominates in tissues that can withstand a temporary decrease in Q (kidney, splanchnic, resting skeletal muscle)
where is an example where both local and extrinsic control are important
skin
what are the local control mechanisms that influence basal tone
- temperature
- compression
- myogenic response
- local metabolites
- autocoids
how does temperature influence basal tone
important in skin
heat the cutaneous arteriole and veins and the dilate (ex. reddening of skin in hot water)
cool them and they constrict
direct effect of temperature on vascular smooth muscle cells
what is paradoxical vasodilation
cool below 12 degrees and you get paradoxical vasodilation due to inhibition of neurotransmitter release –> reddeing of face in cold weather
how does compression influence basal tone
stops Q
ex. squeeze skin
can lead to ischemia and infarction
what are physiological examples of mechanical compression
- coronary Q: heart twists to push blood out of aorta –> occludes vessels –> when it relaxes it opens the vessels up and thats when heart gets oxygen supply (if diastolic period reduced too much the heart blood supply will be reduced)
- skeletal muscle contraction: will pump blood back up towards the heart
how does myogenic response influence basal tone
transmural –> high external pressure will compress BV and impair Q
raising internal pressure distends BV but most arterioles (and some arteries) react by contracting –> myogenic response
intrinsic property of the vascular smooth muscle cell
how do local metabolites influence basal tone
most important mechanism regulating vascular smooth muscle
many by-products of normal tissue metabolism cause vascular relaxation –> any increase in metabolic rate of a tissue causes arteriolar dilation and increases local tissue perfusion
metabolic vasodilation or metabolic hyperemia
what are examples of local metabolites
active factors may be acidosis (due to CO2 and lactic acid)
ATP breakdown (ADP, AMP)
K+ released by muscle –> mechanism of factors are varied and can be complex
how do autocoids influence basal tone
vasoactive chemicals produced locally, released locally and acting locally –> histamine, bradykinin, 5-hydroxytryptamine, prostaglandin, thromboxane-leukotriene group, nitrous oxide
all involved in special local responses such as inflammation and hemostasis
some cause vasodilation (bradykinin, PGE) some cause vasoconstriction (5HT, PGF)
some mixed histamine dilates arterioles, constricts veins
what are extrinsic control of peripheral Q
CNS imposes control system over circulation
what are afferent inputs for peripheral Q
sensory input –> baroreceptors
what are efferent controls of peripheral Q
ach symp: vasodilator fibres
NA symp: vasoconstrictor fibres
parasympathetic vasodilator
heart: symp and parasymp
adrenal medulla: symp
how do extrinsic and local factors work together
extrinsic predominates over “non-critical” organs (kidney, splanchnic) and controls HR and cardiac contractility
local factors regulate critical organs
so extrinsic mechanisms may increase HR and contractility this increases metabolic rate which then triggers local control mechanisms to dilate (ex. coronary arteriole and so increase coronary Q)
how do hormones control circulation
- several endocrine secretions have acute effects on heart and circulation
- normally however these are less important than neural mechanisms for short-term control of CVS
- if neural control is impaired (ex. heart transplant) or in pathological conditions (ex. hemorrhage) the endocrine secretions are important
what are the effects of adrenaline
adrenal medulla secretes adrenaline and NA in response to sympathetic stimulation
adrenaline is major component
secretions made in fight or flight, exercise, hypotension, hypoglycemia
what are baroreceptors and where are they found
sense changes in pressure
BR and non-encapsulated nerve endings in arteries at aortic arch and carotid sinus
what cranial nerve responds to changes in pressure from the carotid sinus
afferent fibres from the carotid sinus ascend in the IXth cranial nerve glossopharyngeal
what cranial nerve responds to changes in the aorta
aortic BR ascend in the X vagus nerve
what are the goals of baroreceptors
aimed at stabilized blood pressure
what do BR respond to
are mechanoreceptors responding to stretch
increased pressure in artery causes distention and this excites the receptors
what are the responses to decreased pressure
- increase sympathetic activity: increased HR, increased contractility
- decreased parasympathetic activity: increased HR
both 1 & 2 increase CO
- increased sympathetic activity: arteriole vasoconstriction (esp. kidney, splanchnic, resting skeletal muscle, non-critical organs)
leads to increase in TPR –> rediverts blood to essential organs but maintain a resistance
CO x TPR = blood pressure
increase of adrenaline from adrenal stimulates the heart
what are the response to decreased pressure
BR reflex help to compensate for increased or decreased blood pressure (not cure it)
reflex is rapid and powerful (ex. hemorrhage which would decrease BP)
BR reflex also maintains pressure during changes in posture or activity
what is BR adaptation
while BR can regulate moment to moment changes in BP they cannot regulate BP in long term
BR adapts (or are reset) to the prevailing level of arterial pressure (either up or down)
so in animals which are hypertensive over the long term the BR come to accept this as normal and regulate pressure at the elavated level
this resetting can be centrally mediated (ex. exercise the reflex is reset centrally) it can also be mediated peripherally –> happens if high BP is maintained
what are the additional effects of the BR
- sensation of thirst
- increasing ADH which decreases urine flow from kidneys
- increases renin which acts to produce aldosterone which decreases Na excretion (conserves water and maintains BP)
