Regulation of Arteriolar Resistance Flashcards
What is varying the radius of resistance vessels used to control?
- Control flow and redirect blood
- Control TPR and regulate MAP
What can high MAP cause?
Damage to capillaries
What can low MAP cause?
Syncope
What happens to the flow through a vascular bed if the resistance of the vascular bed is decreased?
Increases flow
What effect does reducing TPR have on MAP?
Reduces MAP
How is blood flow to a vascular bed kept sufficient and MAP kept in range?
2 levels of control over smooth muscle surrounding arterioles:
- Intrinsic mechanisms
- Extrinsic mechanisms
What are intrinsic mechanisms concerned with?
Ensuring the selfish needs of each individual tissue
What are extrinsic mechanisms concerned with?
Ensuring that the TPR of the whole body stays in the right ball park
What type of control is extrinsic control?
- Neural
- Hormonal
How is the sympathetic system involved in extrinsic control?
- Releases norepinephrine
- Binds to a1 receptors
- Causes arteriolar constriction
- Therefore decreased flow through that tissue and tends to increase TPR
How is the parasympathetic system involved in extrinsic control?
Usually no effect
How is epinephrine involved in extrinsic control?
- Released from adrenal medulla
- Binds to a1 receptors
- Causes arteriolar constriction
- Therefore decreased flow through that tissue and tends to increase TPR
What does epinephrine do in tissues such as skeletal and cardiac muscle?
- Activates B2 receptors
- Causes arteriolar dilation
- Therefore increases flow through that tissue and tends to decrease TPR
How is angiotensin involved short term control of BP?
- Produced in response to low blood volume
- Causes arteriolar constriction
- Therefore increases TPR
How is vasopressin involved in short term control of BP?
- Released in response to low blood volume
- Causes arteriolar constriction
- Therefore increases TPR
How are ANP and BNP involved in the short term control of BP?
- Released in response to high blood volume
- Causes arteriolar dilation
- Therefore decreases TPR
What local intrinsic controls are there?
- Active metabolic hyperaemia
- Pressure flow auto regulation
- Reactive hyperaemia
- The injury response
Describe the active metabolic hyperaemia response?
- Increase in metabolic activity causes increase in con. metabolites
- Triggers release of EDRF
- Causes arteriolar dilation
- Increase flow to wash out metabolites
- An adaptation to match blood supply to the metabolic needs of the tissue
Describe pressure flow auto-regulation.
- Decrease in MAP, decreases flow
- Metavolites accumulate
- Triggers release EDRF
- Arterioles dilate and flow is restored to normal
- An adaptation to ensure that a tissue maintains its blood supply despite changes in MAP
Describe reactive hyperaemia?
- Occlusion of blood supply causes a subsequent increase in blood flow
- An extreme version of pressure auto-regulation
Describe the injury response.
- Substance increases the degranulation of mast cells which release histamine
- Arteriolar dilation, increases blood flow and permeability
- Aids delivery of blood born leucocytes to injured area
Why is the coronary circulation opposite from the systemic circulation?
- Each time the heart contracts it cuts off its own blood supply
- Blood flow to the heart during systolic phase is low
- It gets more blood during diastole
- This is the opposite to the rest of the body
Why is the coronary circulation considered a special area?
- Blood supply is interrupted by systole
- But still has to cope with increased demand during exercise
- Shows excellent active hyperaemia
- Expresses many B2 receptors
- These swamp any sympathetic arteriolar constriction
Why is the cerebral circulation a special area?
- Needs to be kept stable, whatever
- Shows excellent pressure auto-regulation
Why is the pulmonary circulation considered a special area?
- Decrease in O2 causes arteriolar constriction
- The opposite response to most tissues
- This ensures that blood is directed to the best ventilated parts of the lung
Why is the renal circulation considered a special area?
- Main function is filtration which depends on pressure
- Changes in MAP would have big effects on blood volume
- Shows excellent auto-regulation
