Biology conceptual question

[paragon1111]

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Hi all,

I understand that increasing the diameter of an axon can increase the rate of saltatory conduction due to a decrease in resistance. However, I'm confused as to why we cannot apply the same logic to blood velocity under vasoconstriction/vasodilation.

Actually, i'm just confused as to the effects of vasoconstrictin/dilation on velocity. Does vasoconstriction increase or decrease blood velocity and why?

Thanks!

 

[Cope93]

Hi all,

I understand that increasing the diameter of an axon can increase the rate of saltatory conduction due to a decrease in resistance. However, I'm confused as to why we cannot apply the same logic to blood velocity under vasoconstriction/vasodilation.

Actually, i'm just confused as to the effects of vasoconstrictin/dilation on velocity. Does vasoconstriction increase or decrease blood velocity and why?

Thanks!

In blood we talk about flow (Q). Q = P/r where P= pressure and r = resistance, when vasoconstriction occurs the resistance increases and Q (flow) decreases. However, if vasodilation occurs then resistance is decreased and flow (Q) increases. Hope this helps

 

[NavyDDS1990]

think of water running through the pipe. larger diameter, greater flow due to lower resistance and vice versa

 

[embrojam]

I think you should read this page http://www.cvphysiology.com/Hemodynamics/H013.htm

You need to make the distinction between flow and velocity, as they're not the same entity.

If flow stays constant and the radius DECREASES then velocity INCREASES as radius and velocity are on the same side of the equation, and therefore are inversely related. The reason is, that if the total amount of blood moving is the same, if you make the pipe smaller (radius dec) the velocity would need to increase to keep flow constant.

 

[paragon1111]

Thank you everyone!

So er, are both the equations Q=P/R (Q=flow, P=pressure, R=resistance) and F=VR^2 (F=flow, V=velocity,R=resistance) valid?

 

[embrojam]

Thank you everyone!

So er, are both the equations Q=P/R (Q=flow, P=pressure, R=resistance) and F=VR^2 (F=flow, V=velocity,R=resistance) valid?

Yes. But I would more focus on understanding them conceptually rather then memorize the equations. The only biology equations of the heart I would memorize for the DAT are for cardiac output and TPR (resistance)

 

[rapidy]

Yes. But I would more focus on understanding them conceptually rather then memorize the equations. The only biology equations of the heart I would memorize for the DAT are for cardiac output and TPR (resistance)

Do you think for the DAT we need to know the Frank Starling preload / afterload calculation stuff? It kind of relates to CO, TPR etc.

 

[Frank22]

Hi all,

I understand that increasing the diameter of an axon can increase the rate of saltatory conduction due to a decrease in resistance. However, I'm confused as to why we cannot apply the same logic to blood velocity under vasoconstriction/vasodilation.

Actually, i'm just confused as to the effects of vasoconstrictin/dilation on velocity. Does vasoconstriction increase or decrease blood velocity and why?

Thanks!

The velocity of an action potential manifests itself in the movement of electricity across biological material, whereas blood flow refers to the movement of a fluid encased within vessels, which is why they operate under different principles.

Decreasing the diameter of a blood vessel causes the liquid to travel more rapidly because fluids always travel to the area of least pressure (this is why constricting the afferent arteriole of a nephron decreases the rate of glomerular filtration, while constricting the efferent arterioles increases the amount of filtrate). However, although the liquid travels faster through the arteries, the amount of blood that can travel through an artery decreases, ultimately decreasing blood flow.

While arteries can transport blood through the circulatory system via the aid of the heart, which has a left ventricle through which to pump the blood to the aorta, veins primarily transport blood back to the heart by the aid of contracting muscles, and valves which prevent the backward flow of blood. Vasoconstriction and vasodilation are therefore more important with these vessels because they do not have the aid of the heart to transport blood.

Also, remember that hypotension is a more serious condition than hypertension.