Vasodilation Help

remedy23

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    Hey guys,

    So I know vasodilation decreases resistance, and blood pressure = blood flow (Cardiac output) x resistance
    However, doesn't vasodilation also increase blood flow/stroke volume?

    Ex. in exercise, vasodilation occurs, and blood pressure increases. But, if the effect of resistance is greater than cardiac output, then blood pressure decreases?

    Or should I ignore the effect of vasodilation/vasoconstriction on blood flow since it can vary?
     

    Mad Jack

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      Vasodilation without an increase in cardiac output will result in diminished blood flow, for the reasons stated above. In the case of exercise, you will have an increase in vasodilation in skeletal muscle and the cardiopulmonary system, as well as increased blood flow through the brain and an increase in cardiac output. Tis selective vasodilation sends more blood to where it needs to go (skeletal muscle, the heart and lungs, the brain) while shunting it away from areas it doesn't need to be (the digestive system, etc).

      Basically, it's complicated.
       
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      remedy23

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        Thank you guys. So the only SURE thing about vasodilation is that it decreases resistance, and ASSUMING cardiac output/flow remains constant, blood pressure decreases (makes sense)

        I'm not sure if this is right, but from what I've reviewed, in the equation Pressure = Cardiac Output (Flow) x Resistance:
        - vasodilation/vasoconstriction most directly affects the resistance variable
        - Sympathetic/parasympathetic nervous input most directly affects the cardiac output variable (SNS increases cardiac output by increasing heart rate & stroke volume to increase pressure, PNS decreases cardiac output by decreasing heart rate to decrease pressure)
        - Sympathetic/parasympathetic do a combination of vasoconstriction/vasodilation on different systems, so it's hard to directly state the effect of SNS/PNS on resistance, and vasoconstriction/vasodilation can affect cardiac output variably, so it's hard to state the effect of vasoconstriction/vasodilation on cardiac output directly

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        Mad Jack

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          Sympathetic innervation causes vasoconstriction and an increase in resistance in most tissues in an exercise or fight-or-flight reaction via the effects of the alpha 1 receptor. This is counteracted in certain tissues by local mediators, such as the substantially lowered pH in muscle that functions as a potent vasodilator. This results in decreased flow everywhere you don't need it, increased resistance and subsequently increased flow to where you do need it, particularly when coupled with the effects of sympathetic stimulation on the beta receptors of the heart. The PNS doesn't have much of an effect on the vascular system, though certain parasympathetic responses (vagal response) can temporarily lower CO and thus BP. Most of the vasodolation is controlled by a combination of metabolic byproducts (CO2, lactic acid, etc) and local mediators (NO and the like).

          If I'm wrong on any of that, someone please feel free to correct me. Hemodynamics was many months ago and I haven't touched it since.
           
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          remedy23

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            Sympathetic innervation causes vasoconstriction and an increase in resistance in most tissues in an exercise or fight-or-flight reaction via the effects of the alpha 1 receptor. This is counteracted in certain tissues by local mediators, such as the substantially lowered pH in muscle that functions as a potent vasodilator. This results in decreased flow everywhere you don't need it, increased resistance and subsequently increased flow to where you do need it, particularly when coupled with the effects of sympathetic stimulation on the beta receptors of the heart. The PNS doesn't have much of an effect on the vascular system, though certain parasympathetic responses (vagal response) can temporarily lower CO and thus BP. Most of the vasodolation is controlled by a combination of metabolic byproducts (CO2, lactic acid, etc) and local mediators (NO and the like).

            If I'm wrong on any of that, someone please feel free to correct me. Hemodynamics was many months ago and I haven't touched it since.

            Thank you, that was helpful. I'm just confused about the cardiac output equation with respect to vasodilation & vasoconstriction:
            Cardiac Output = Heart Rate x Stroke Volume
            Blood Pressure = Cardiac Output x Resistance

            So for the SNS, vasodilation occurs in arterioles leading to skeletal muscle & cardiac muscle. But in vasodilation, heart rate should decrease, and stroke volume increases in vasodilation - so cardiac output could either increase or decrease depending on these effects of heart rate & stroke volume? We can be sure that resistance decreases in vasodilation, but what about cardiac output?

            And for majority of the other tissues such as the Gastrointestinal tract, vasoconstriction occurs - this increases resistance, and it increases heart rate but decreases stroke volume. So do we assume cardiac output increases in vasoconstriction because heart rate has a greater effect than stroke volume?
             
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            Mad Jack

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              Thank you, that was helpful. So for the SNS, vasodilation occurs in arterioles leading to skeletal muscle & cardiac muscle, to increase blood flow (Cardiac Output) to these organs - so cardiac output increases, and resistance decreases - so overall, would we say that blood pressure increases in these fight-or-flight organs because the effect of cardiac output is greater than the effect of resistance caused by vasodilation?

              And for majority of the other tissues such as the Gastrointestinal tract, vasoconstriction occurs - this increases resistance, but here's the part that is tricky. Does it decrease cardiac output/blood flow because of the increased resistance OR do we assume that cardiac output increases here too because of the increase in heart rate by the SNS applies to all organs?
              Cardiac output is increased, and pressure is increased in all major systemic arteries. Dilation only occurs in those organs that need metabites flushed out due to pH changes and local mediator effects. This results in those areas having decreased resistance and more blood being delivered to then due to the tendency of a fluid to take the path of least resistance whenever possible. Overall you get an increase in blood going to where it needs to go and not to where it doesn't need to, such as the digestive system, which is not consuming much energy due to the lack of parasympathetic stimulation minimizing parastalsis, thus leading to no metabolites being produced, the local arterioles remaining constricted, and less blood flow being delivered. Ta-da.
               
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              remedy23

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                Cardiac output is increased, and pressure is increased in all major systemic arteries. Dilation only occurs in those organs that need metabites flushed out due to pH changes and local mediator effects. This results in those areas having decreased resistance and more blood being delivered to then due to the tendency of a cluid to take the path of least resistance whenever possible. Overall you get an increase in blood going to where it needs to go and not to where it doesn't need to, such as the digestive system, which is not consuming much energy due to the lack of parasympathetic stimulation minimizing parastalsis, thus leading to no metabolites being produced, the local arterioles remaining constricted, and less blood flow being delivered. Ta-da.

                Thank you - so this has me thinking. Is cardiac output a different concept than blood flow? Because you said that less blood is being delivered in vasoconstriction and more blood flow in vasodilation, but cardiac output would be the same in these two because it's based on the heart rate and the stroke volume of the heart. So I think blood flow is related to resistance like you said, and not the same as cardiac output?

                And 2) During the SNS, in vasodilation areas, resistance decreases but blood pressure still increases, so we assume that the effect of cardiac output overpowers the effect of resistance on blood pressure?

                You cleared a lot of my doubts - thank you so much!
                 
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                Mad Jack

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                  Thank you - so this has me thinking. Is cardiac output a different concept than blood flow? Because you said that less blood is being delivered in vasoconstriction and more blood flow in vasodilation, but cardiac output would be the same in these two because it's based on the heart rate and the stroke volume of the heart. So I think blood flow is related to resistance like you said, and not the same as cardiac output?

                  And 2) During the SNS, in vasodilation areas, resistance decreases but blood pressure still increases, so we assume that the effect of cardiac output overpowers the effect of resistance on blood pressure?

                  You cleared a lot of my doubts - thank you so much!
                  Cardiac output is total flow through the system. Resistance at the artery and arteriole level determines where that output is delivered. Total systemic vascular resistence and cardiac output combine to determine blood pressure at the systemic level. Local pressures vary based on local levels of vasodilation, except in the brain, where autoregulatory mechanisms keep pressure relatively constant.
                   
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                  remedy23

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                    Cardiac output is total flow through the system. Resistance at the artery and arteriole level determines where that output is delivered. Total systemic vascular resistence and cardiac output combine to determine blood pressure at the systemic level. Local pressures vary based on local levels of vasodilation, except in the brain, where autoregulatory mechanisms keep pressure relatively constant.

                    THANK YOU. That totally makes sense - I have a much better understanding of this now. Thank you so much for your patience :)
                     
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