Regulation of Arteriolar Resistance Flashcards
What does Darcy’s law describe?
- The flow of fluid through a porous medium
What does Poiseuille’s Law state?
Velocity of a liquid flowing through a capillary is directly proportional to the pressure of the liquid and the fourth power of the radius of the capillary
When applying Darcy’s law to the systemic circulation, what does it mean?
Mean arterial pressure = cardiac output x total peripheral resistance
What does varying the radius of resistance vessels control?
- Total peripheral resistance
- Mean arterial pressure
Explanation of Darcy and Poiseuille’s Laws in the Body
- Mr Darcy told us that Flow = D Pressure / Resistance
- Monsieur Poisieuille told us that varying radius is a powerful way of controlling resistance
- Therefore varying radius of arterioles is used to control flow through individual vascular beds
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- However, it is equally true that D Pressure = Flow x Resistance
- Therefore varying radius of arterioles (the Total Peripheral Resistance) also affects D Pressure (the Mean Arterial Pressure)
Diagram explaining Darcy and Poiseuille’s
What is the aim of resistance juggling?
- To keep blood flow to each vascular bed sufficient
- To keep mean arterial pressure in the right range
- These two are related so need this control
What are the two levels of control over smooth muscle surrounding arterioles?
- Intrinsic mechanisms
- Extrinsic mechanisms
What are intrinsic mechanisms?
Concerned with meeting the needs of each individual tissue
What are extrinsic mechanisms??
Concerned with ensuring the total peripheral resistance of the whole body stays at a good level
Why can’t all capillaries dilate to increase blood flow to all areas?
- Severe loss of pressure
- Some vessels need to constrict when others dilate
What controls extrinsic control?
- Sympathetic system
What does the sympathetic system release and what receptors does it act on to cause arteriolar constriction of the peripheral blood vessels?
- Release noradrenaline
- Binds to A1 receptors
What does the arteriolar constriction from the A1 receptors result in?
- Decreased flow
- Increased TPR
What effect do parasympathetic fibres have on blood vessels?
Usually no effect
Why does TPR increase due to innervation by the sypmpathetic system?
It happens all over the body
What hormonal regulatory systems exist for the extrinsic control of resistance juggling?
- Epinephrine released by adrenal medulla
- Binds to A1 receptors
- Arteriolar constriction
- Reduced flow
- Increased TPR
What other tissues are affected by the hormonal control of extrinsic resistance juggling?
- Skeletal and cardiac muscle
- Also activates B2 receptors
- Causes arteriolar dilation and increases flow/decrease TPR through those tissues
What is angiotensin II and Vasopressin (ADH) produced in response to?
Low blood volume
What does angiotensin II and Vasopressin (ADH) cause?
- Arteriolar constriction
- Increased TPR
What is atrial natriuretic peptide and brain natriuretic peptide released in response to?
- High blood volume
What does natriuretic peptide & brain natriuretic peptide cause?
- Arteriolar dilation
- Decreased TPR
What are the four local intrinsic controls?
- Active (metabolic) hyperaemia
- Pressure (flow) autoregulation
- Reactive hyperaemia
- Injury response
How does active hyperaemia control flow?
- Increased metabolic activity increases conc of metabolites
- Triggers release of EDRF (endothelium derived relaxing factor)
- Arteriolar dilation
- Increased flow to wash out metabolites
What type of adaptation does active hyperaemia follow?
Blood flow to match metabolic activity
Describe pressure autoregulation?
- Decreased mean arterial pressure causes decreased flow
- Metabolites accumulate
- EDRF released
- Arterioles dilate
What type of adaptation does pressure autoregulation allow?
Maintain tissue blood supply despite MAP changes
What is reactive hyperaemia?
- Occlusion of blood supply causes a subsequent increase in blood flow
- It is an extreme version of pressure autoregulation
Diagram showing the injury response
What is the goal of the injury response?
To deliver blood born leucocytes and other immune cells to injured area
What is blood flow to the coronary circulation like in systole?
Very low
Why is blood flow to the coronary arteries low during systole and high during diastole?
- Semi lunar valve occludes the ostia for entry into the coronary circulation
- Heart may have a mechanism to cut off its own blood supply during systole
How does the blood supply to the coronary arteries cope with exercise strain?
- Active hyperaemia
- Many B2 receptors
- B2 receptors swamp any sympathetic A1 constriction
How does the cerebral circulation maintain it’s stable circulation?
Pressure autoregulation
What are the arterial pressure ranges in the brain and how does it achieve this?
- 95-45mmHg
- Excellent at dilating
How does the renal system control flow?
- Main function is filtration which depends on pressure differences
- Changes to MAP could have huge effects on systemic blood volume
- Uses pressure autoregulation
