Regulation of arteriolar resistance Flashcards
State Darcy’s Law and apply this to cardiac output
Flow = change in pressure / resistance
CO = (MAP-CVP)/TPR
CO - MAP/TPR since CVP = 0
State Pouseille’s law
Resistance = (viscosity x length x 8) / (radius^4 x Pi)
Flow = (change in pressure xr^4 x Pi) / (viscosity x length x 8)
If flow = change in pressure/resistance, then what does pressure equal?
Apply this to cardiac output
What does this mean in terms of varying resistance of arterioles?
Pressure = flow x resistance
(MAP-CVP) = CO x TPR
MAP = CO xTPR
Varying arteriolar resistance regulates both flow and MAP
How do arterioles control blood flow to tissues?
By varying their radius (reduced radius = increased resistance and vice versa). This also affects MAP.
What is the effect of reducing the resistance in arterioles on the vascular bed?
Reducing resistance by increasing radius in arterioles increases supply to the vascular bed while also reducing MAP.
Name the two type of mechanisms that keep MAP and blood blow consistent.
State their individual functions.
Extrinsic: ensures TPR remains in right ballpark
Intrinsic: selfish needs of individual tissue
State the two types of extrinsic regulation
Neural/hormonal
Hormonal
Describe neural/hormonal extrinsic regulation
Sympathetic
Noradrenaline is released and adrenaline is released from the adrenal medulla.
They bind to alpha 1 receptors causing reduced flow through tissue and increased TPR
What is the effect of the parasympathetic system on arteriolar resistance
No effect
As well as alpha 1 receptors, what other kind of receptors does adrenaline act on in arterioles in certain tissues?
What is the effect
Activates Beta 2 receptors in skeletal and cardiac muscle, causes arteriolar dilatation, increased flow and reduced TPR
List the three types of hormonal extrinsic regulation
Angiotensin II
Vasopressin (ADH)
Atrial Natriuretic Peptide (ANP) and Brain Natriuretic Peptide (BNP)
Describe the effect of angiotensin II on arterioles
Responds to low blood volume
Causes arteriolar constriction and thus increased TPR
Describe the effect of vasopressin (ADH) on arterioles
Responds to low blood volume
Causes arteriolar constriction and thus increased TPR
Describe the effect of ANP and BNP on arterioles
Respond to high blood volume
Cause arteriolar dilatation and thus reduced TPR
List the local or intrinsic mechanisms that regulate arterioles
Active (metabolic) hyperaemia
Pressure (flow) autoregulation
Reactive hyperaemia
Injury response
Describe the effect of active (metabolic) hyperaemia
Matches blood supply to tissue’s metabolic needs
Increased metabolic activity -> increased metabolites -> paracrine EDRF release->
smooth muscle relaxation ->
arteriolar dilatation ->
increased flow to wash out metabolites (CO2, K+, H+)
Describe the effect of pressure (flow) autoregulation
Reduced MAP -> reduced flow -> metabolite accumulation -> EDRF release -> arteriolar dilatation -> restore of normal flow (but could be myogenic)
Describe the effect of reactive hyperaemia
Extreme version of pressure autoregulation
Occlusion of blood supply ->
increased blood flow
Describe the effect of injury response
Injury response involves C fibres activating substance P which activates mast cells leading to histamine release ->
arteriolar dilatation ->
increased blood flow and permeability
(also increased leucocytes)
What are the special areas of circulation where local mechanisms are either heavily employed or different?
Coronary circulation
Cerebral circulation
Pulmonary circulation
Renal circulation
Describe the local mechanisms employed by coronary circulation
Blood supply interrupted by systole - ventricles contract, squashing arterioles. Still has to cope with increased demand during exercise ->
excellent ACTIVE HYPERAEMIA: adenosine triggers EDRF release
Many Beta 2 receptors swamp any sympathetic arteriolar constriction
Describe the local mechanisms employed by cerebral circulation
Needs to be kept stable whatever the consequences are - excellent PRESSURE AUTOREGUULATION
Describe the local mechanisms employed by pulmonary circulation
When O2 is decreased -> arteriolar constriction (opposite response of most tissues - blood redirected to better ventilated areas)
Describe the local mechanisms employed by renal circulation
Filtration dependent on pressure so MAP changes could have big effects on volume - excellent PRESSURE AUTOREGULATION
