Regulation of arteriolar resistance Flashcards
How does Poiseuille’s Law relate control of resistance and blood flow?
(ΔPr^4 π)/ηL8
How is mean arterial pressure related to cardiac output and total peripheral resistance?
MAP = CO xTPR
Varying the radius of resistance vessels is used o control TPR, so that regulates MAP
What are the two levels on control over smooth muscles surrounding arterioles? Why does this happen?
Local to allow more blood supply: intrinsic mechanisms
Global to allow the whole system to have enough blood: extrinsic mechanisms
Need to keep blood flow to each vascular bed sufficient and keep mean arterial pressure in the right range
What do intrinsic mechanisms of arteriole blood supply do?
Meet selfish needs of each individual tissue
What do extrinsic mechanisms of arteriole blood supply do?
Ensure the total peripheral resistance of the whole body stays within a normal range
What are the three local controls of arteriole blood pressure?
Active hyperaemia
Pressure autoregulation
Reactive hyperaemia
What is active hyperaemia?
Increase metabolic activity causes increased concentration of metabolites -> skeletal muscle functioning at a constant rate
Triggers release of EDRF (Usually NO) aka paracrines
Cause arteriolar dilation
Increased flow to wash out metabolites
Adaptation to match blood supply and the metabolic needs of that tissue
What is pressure autoregulation?
Decreased MAP causes decreased flow
Metabolites accumulate
Triggers release of EDRF
Arterioles dilate and flow is restored to normal
Adaptation to ensure that a tissue maintains its blood supply despite changes in MAP
What is reactive hyperaemia?
Occlusion of blood supply causes a subsequent increase in blood flow
An extreme version of pressure autoregulation -> apply a blood pressure cuff for a very long time
What is the basis of the injury response?
Norireceptors connect to C-fibres
C fibres are branches: antidromic activation, substrate P signals a mast cell -> histamines signalled -> swelling -> aids delivery of blood born leucocytes etc to injured area
What are the different methods of extrinsic control?
Sympathetic nerves Parasympathetic nerves Angiotensin II Vasopressin Atrial natriuretic peptide and brain natriuretic peptide
How do sympathetic nerves increase TPR?
Release norepinephrine
Binds to α1-receptors
Cause arteriolar constriction
A decreased flow through that tissue and this tends to increase TPR
How do sympathetic nerves decrease TPR?
In some tissues e.g. skeletal and cardiac muscle
β2-receptors activated
Cause arteriolar dilation
Increases flow through that tissue and tends to decrease TPR
How does the parasympathetic system affect TPR?
Usually no effect
How does angiotensin II affect TPR?
Produced in response to low blood volume
Causes arteriolar constriction
-> increases TPR
How does vasopressin affect TPR?
AKA antidiuretic hormone
Released in response to low blood volume
Causes arteriolar constriction
Increase in TPR
How does atrial natriuretic peptide and brain natriuretic peptide affect TPR?
Released in response to high blood volume
Causes arteriolar dilation
-> decreased TPR
What factors affect coronary circulation?
Blood supply is interrupted by systole
It has to cope with increased demand during exercise
Shows excellent active hyperaemia
Express many β2-receptors
These swamp any sympathetic arteriolar constriction
Why is the heart so good at active hyperaemia?
The heart is always working on the edge since it only receives blood flow during diastole
How is blood flow controlled in cerebral circulation?
Excellent pressure autoregulation: can dilate arterioles
Needs to be kept stable
How is blood flow controlled in pulmonary circulation?
Decreased oxygen causes arteriolar constriction (shunt)
Opposite response to most tissues
Ensures that blood is directed to the best ventilated parts of the lung
How is blood flow controlled in renal circulation? Why?
Excellent pressure autoregulation
Main function is filtration which depends on pressure
Changes in MAP would have big effects on blood volume
