Regulation of Arteriolar Resistance Flashcards
What does Poiseuille’s Law state?
That the resistance to blood flow is directly proportional to the length of the vessel through which it flows and to the viscosity of the fluid
Inversely proportional to he fourth power of the tubing radius (radius increases = resistance decreases)
What allows resistance in the arterioles to be variable?
Smooth muscle in the arteriolar wall
What is the equation of Darcy’s Law?
Difference in pressure / resistance
What is used to vary total peripheral resistance?
Radius of vessel
What does varying the radius of vessels affect?
Total peripheral resistance and therefore regulate MAP (MAP = TPR X CO)
What does arteriolar radius affect/
Flow though individual vascular beds
MAP
What are the two controls over the smooth muscle surrounding arterioles?
Intrinsic mechanism
Extrinsic mechanism
What are the intrinsic mechanisms concerned with?
Meeting the selfish needs of each individual tissue
What are the extrinsic mechanisms concerned with?
Ensuring that the TPR of the whole boy stays at the right level
What are the extrinsic mechanisms for controlling arteriolar constriction?
Sympathetic innervation Hormal control (adrenaline, angiotensin II, vasopressin, natriuretic peptides)
Describe how sympathetic innervation affects the constriction of arterioles
Release of noradrenaline which binds to a1 receptors causing arteriolar constriction -> decrease in flow and increase in TPR
Does the parasympathetic system have an effect of arteriolar constriction?
No
How does adrenaline affect arteriolar radius?
Binds to a1 receptors and causes arteriolar constriction (decrease blood flow and increase TPR)
However, can activate B2 receptors in skeletal and cardiac muscle causes arteriolar dilation (decrease TPR)
How does angiotensin II affect arteriolar constriction?
Produced in response to low blood volume and causes arteriolar constriction, increasing TPR
How does vasopressin (antidiuretic hormone) affect arteriolar constriction?
Released in response to low blood volume and causes arteriolar constriction, increasing TPR
How do atrial and brain natriuretic peptides affect arteriolar constriction?
released in response to high volume and cause arteriolar constriction, increasing TPR
What are the intrinsic contol mechanisms for arteriolar constriction?
Active (metabolic) hyperaemia
Pressure (flow) auto-regulation
Reactive hyperaemia
The injury response
Describe the process of metabolic hyperaemia?
- Increased metabolic activity increase conc of metabolites
- Triggers release of EDFR which causes arteriolar dilation
- Increase flow to wash out metabolites
Describe the process of pressure (flow) autoregulation?
- Decrease MAP = decrease flow, causing metabolites to accumulate
- High conc. triggers release of EDRF causing arteriolar dilation to restore flow
Describe the process of reactive hyperaemia?
Increase in blood flow following occlusion
Describe the injury response?
Injured tissue mast cells release histamine, causing the surrounding blood vessels to dilate and increase in permeability, which allow leucocytes to move from bloodstream to tissue
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?
Active hyperaemia
Expresses many B2 receptors (allows more blood flow)
What sort of arteriolar regulation do the arterioles in the brain show?
Pressure auto-regulation
What happens to the arterioles in the lungs when oxygen conc. decreases?
Arteriolar constriction
Opposite to the systemic response to hypoxia, but it ensures that blood dis directed to the best ventilated parts of the lung
What sort sort of arteriolar constriction are the arterioles in the kidneys good at?
Pressure autoreagualtion
