Regulation of Arteriolar Resistance Flashcards
What does Darcy’s Law state, in terms of Flow?
Flow = Pressure difference / resistance
What happens to blood flow when the radius of arteries is increased?
it increases also
What is varying the radius of resistance vessels used to control?
to control flow, and redirect blood.
How do we redirect blood to areas of the body that require it, in terms of which vessels?
arterioles (resistance vessels) act like taps, dilating and constricting to alter blood flow.
What else is affected when the radius of blood vessels increase/decrease?
the total peripheral resistance (TPR), and therefore the mean arterial pressure (MAP).
What can be said about increasing the radius/resistance through one region of vessels, in relation to all vessels?
- It contributes to an increase in the TPR, and thus an increase in MAP.
- It does not only affect that specific region.
Why is it important that the MAP is maintained?
it provides the driving force that pushes blood through useful places like you brain, and is also the force driving venous return to the heart.
What equation can used to describe MAP?
MAP = CO x TPR
Arteriolar radius affects _____ through individual vascular beds, and ____________.
flow
mean arterial pressure
In order to maintain MAP whilst ensuring sufficient blood flow to vascular beds, ________ and ________ mechanisms which control _______ ______ _______ _______ is required.
intrinsic
extrinsic
smooth muscle around arterioles
What is the function of intrinsic mechanisms of controlling arteriolar constriction?
They are concerned with meeting the selfish needs of each individual tissue.
What is the function of extrinsic mechanisms of controlling arteriolar constriction?
They are concerned with ensuring that total peripheral resistance of the whole body stays in the right ball park.
What is the extrinsic neural mechanism for arteriolar smooth muscle control?
How does this work?
sympathetic innervation
- release NA
- binds to a-1 receptors
- arteriolar constriction
- reduced flow
- increased TPR
- increased MAP
Name the hormones involved in extrinsic control of arteriolar smooth muscle activity? (4)
- Adrenaline
- Angiotensin II
- Vasopressin
- Atrial natriuretic factor
Where is adrenaline released from to enter the blood direcrtly?
Adrenal medulla
Describe how adrenaline affects arteriole smooth muscle activity. (6)
- adrenaline released
- binds to a-1 receptors
- vasoconstriction
- reduced flow
- increased TPR
- increased MAP
Describe how adrenaline affects other tissues, such as skeletal and cardiac muscle.
- binds to b-2 receptors
- vasodilation
- increased flow
- reduced TPR
- reduced MAP
How can adrenalines actions be explained in the ‘fight or flight’ state?
How is MAP maintained?
- arterial constriction = directing blood flow to muscle and heart, increases TPR.
- skeletal muscle/heart vasodilation = increases flow, reduces TPR.
TPR is balanced out, therefore MAP is maintained.
Adrenaline acts on which receptors to cause vasoconstriction?
a-1 receptors
Adrenaline acts on which receptors to cause vasodilation in the skeletal and cardiac muscle?
b-2 receptors
When is angiotensin II produced?
Describe its effects.
- low blood volume (reduced MAP)
- vasoconstriction
- increases TPR (restores MAP)
When is vasopressin produced?
Describe its effects.
- low blood volume (reduced MAP)
- vasoconstriction
- increases TPR (restores MAP)
When is atrial natriuretic factor produced?
Describe its effects.
- high blood volume (increased MAP)
- vasodilation
- reduces TPR (restores MAP)
What can be said about all the extrinsic mechanisms controlling vessel radius?
They are all mainly concerned with regulating TPR, and therefore MAP.
Outline the intrinsic control mechanisms for arteriolar constriction? (4)
- active (metabolic) hyperaemia
- pressure (flow) autoregulation
- reactive hyperaemia
- Injury response
Describe the process of metabolic hyperaemia?
- metabolic activity increases = conc. of metabolites in the blood increases.
- triggers release of EDRF/NO (paracrines)
- arterioles dilate
- increased flow (washes out metabolites)
Describe the process of pressure (flow) auto-regulation?
- decrease in MAP = decrease in flow
- this reduces the movement of metabolites away from tissues = metabolites accumulate
- EDRF/NO released
- arterioles dilate
- increased flow (washes out metabolites)
Describe the process of reactive hyperaemia?
- occlusion of blood supply causes a subsequent increase in blood flow.
- extreme version of pressure autoregulation
N.B. Think of unclamping an artery during surgery, there will be a sudden, rapid increase in flow which will reach a peak before settling back to normal flow (autoregulation).
Describe the process of injury response?
- injury to an area sends an AP along C-fibre.
- substance P released.
- mast cells degranulate, releasing histamine which causes vasodilation.
- blood flow increased, permeability increased which aids the inflammatory process.
When is blood supply to the heart interrupted?
During systole
How does the heart deal with increased demand during exercise with its interrupted blood supply?
- it shows excellent active hyperaemia
- expresses many beta 2 receptors which swamp any arteriolar constriction
What sort of arteriolar regulation do the arterioles in the brain show?
excellent pressure autoregulation in order to keep a stable circulation.
What happens to the arterioles in the lungs when oxygen concentration decreases?
they constrict, hypoxaemic pulmonary vasoconstriction (HPV)
opposite to most other tissues to ensure blood gets to the best ventilated parts
What sort sort of arteriole constriction are the arterioles in the kidneys particularly good at?
pressure autoregulation, as changes in MAP would have big effects on blood volume.
