Relation between BP, TPR and Tissue Blood Flow
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E.S.Prakash
Sep 26, 2010, 8:34:01 PM9/26/10
to Medical Physiology at SOM, AIMST University
Dr Prakash:
What does variation in peripheral resistance affect more - blood
pressure or tissue blood flow? In your lecture, you state that a
decrease in peripheral resistance causes a drop in BP. But if we
follow the equation, there should be a rise in tissue blood flow.
Confused.
Allen Mugialan
Year 2 Student
************************
What does variation in peripheral resistance affect more - blood
pressure or tissue blood flow? In your lecture, you state that a
decrease in peripheral resistance causes a drop in BP. But if we
follow the equation, there should be a rise in tissue blood flow.
Confused.
Allen Mugialan
Year 2 Student
************************
E.S.Prakash
Sep 26, 2010, 11:32:50 PM9/26/10
to Medical Physiology at SOM, AIMST University
Allen,
The confusion you describe occurs because you are mixing up two very different things: vascular resistance in a tissue (LVR) and total peripheral resistance (TPR).
Tissue Blood Flow (TBF) = Mean arterial Pressure (MAP) / Local Vascular Resistance (LVR)
It follows that TBF can be affected or regulated by varying MAP or LVR or both.
What determines LVR?
LVR (in a tissue) depends on the tone of arteriolar smooth muscle and precapillary sphincters in that tissue (not any other tissue).
For example, when cerebrovascular resistance is increased, blood flow to the brain will decrease if MAP remains constant but one can't tell if this will raise TPR.
If coronary vascular resistance (CVR) is increased then coronary blood flow will reduce provided MAP is constant, but again one can't tell what this rise in CVR will cause to TPR unless one measures both MAP and cardiac output.
Note Local Vascular Resistance and total peripheral vascular resistance TPR (or Systemic Vascular Resistance) are not the same.
Mean Arterial Pressure = Cardiac Output times Total Peripheral Resistance (TPR)
That is, TPR is a composite of all systemic vascular resistances (which are in parallel with each other) and is calculated from measurements of MAP and Cardiac Output.
In many situations, we find vascular resistance decrease in one tissue (such as exercising skeletal muscle due to accumulation of products of metabolism) and vasoconstriction occur in others (kidneys, GIT, inactive skeletal muscle). What happens to TPR - one cannot easily predict (it might decrease or increase or change little); similarly one can't predict what happens to BP - we need to measure MAP as it depends on cardiac output as well as TPR. So we use the classic scientific method - observe and then offer potential explanations.
Prakash
**********************
The confusion you describe occurs because you are mixing up two very different things: vascular resistance in a tissue (LVR) and total peripheral resistance (TPR).
Tissue Blood Flow (TBF) = Mean arterial Pressure (MAP) / Local Vascular Resistance (LVR)
It follows that TBF can be affected or regulated by varying MAP or LVR or both.
What determines LVR?
LVR (in a tissue) depends on the tone of arteriolar smooth muscle and precapillary sphincters in that tissue (not any other tissue).
For example, when cerebrovascular resistance is increased, blood flow to the brain will decrease if MAP remains constant but one can't tell if this will raise TPR.
If coronary vascular resistance (CVR) is increased then coronary blood flow will reduce provided MAP is constant, but again one can't tell what this rise in CVR will cause to TPR unless one measures both MAP and cardiac output.
Note Local Vascular Resistance and total peripheral vascular resistance TPR (or Systemic Vascular Resistance) are not the same.
Mean Arterial Pressure = Cardiac Output times Total Peripheral Resistance (TPR)
That is, TPR is a composite of all systemic vascular resistances (which are in parallel with each other) and is calculated from measurements of MAP and Cardiac Output.
In many situations, we find vascular resistance decrease in one tissue (such as exercising skeletal muscle due to accumulation of products of metabolism) and vasoconstriction occur in others (kidneys, GIT, inactive skeletal muscle). What happens to TPR - one cannot easily predict (it might decrease or increase or change little); similarly one can't predict what happens to BP - we need to measure MAP as it depends on cardiac output as well as TPR. So we use the classic scientific method - observe and then offer potential explanations.
Prakash
**********************
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