Blood flow with increased dilation

[ieatshrimp24]

Why does blood flow increase to the muscle if arteriolar dilation increases? Isn't it Q = Av so if A increases, then v decreases and Q remains constant? Simple question but I feel I'm missing something here. Come to think of it, does blood flow to the muscles increase while blood flow back to the heart (through the veins) decrease in order to have Q constant? Thanks.

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[justalittlestar]

I've wondered about this as well and found myself confused last week. I think this is the case because there's a difference between flow rate and flow speed. The same concept doesn't apply for both.

 

[AwayFromReality]

Q is flow rate and v is the flow speed. I'm not absolutely sure about this but I think when the arteries are dilated in order to supply more blood to certain parts of the body (for example in the muscles when stimulated by sympathetic system), the cross sectional area increases while the flow speed (v) remains the same. This will allow Q (flow rate) to increase based on the formula Q=Av.

Also I would imagine, in this case, Q back to the heart will also increase since the deoxgenated blood would have to quickly get back to the heart to get oxygenated and return to the crucial muscle.

I think the MCAT would most likely provide enough explanation about something like this in the passage since we learned about blood flow at a very basic level for the MCAT

 

[ieatshrimp24]

Q is flow rate and v is the flow speed. I'm not absolutely sure about this but I think when the arteries are dilated in order to supply more blood to certain parts of the body (for example in the muscles when stimulated by sympathetic system), the cross sectional area increases while the flow speed (v) remains the same. This will allow Q (flow rate) to increase based on the formula Q=Av.

Also I would imagine, in this case, Q back to the heart will also increase since the deoxgenated blood would have to quickly get back to the heart to get oxygenated and return to the crucial muscle.

I think the MCAT would most likely provide enough explanation about something like this in the passage since we learned about blood flow at a very basic level for the MCAT

The TPR Biology book also says that increasing the flow rate to the muscles would increase the flow rate back to the heart. So yes you are correct. I guess I didn't read enough into the explanation in the book. Thanks for the help.

 

[Hadi7183]

Isn't it Q = Av so if A increases, then v decreases and Q remains constant?

Your reasoning is incorrect. The continuity equation assumes that the flow rate is constant; that is, "Q remains constant" is an assumption for "if A increases, then v decreases" and not vice versa.

What you need to look at is the flow-resistance equation (delta P= Q*R). When the vessel is dilated, R is reduced and since the pressure supplied by the heart is constant, Q is increased.

 

[AwayFromReality]

Your reasoning is incorrect. The continuity equation assumes that the flow rate is constant; that is, "Q remains constant" is an assumption for "if A increases, then v decreases" and not vice versa.

What you need to look at is the flow-resistance equation (delta P= Q*R). When the vessel is dilated, R is reduced and since the pressure supplied by the heart is constant, Q is increased.

That makes a lot more sense than what I had.

But lets say the heart rate also increases, like in a sympathetic system, then how would we explain the increased flow rate since both Delta P and Q are now increased with resistance decreasing.

 

[Hadi7183]

I am not sure what exactly you mean by "how would we explain the increased flow rate [that is Q] since both Delta P and Q are now increased."

I assume you are asking what happens when delta P is increased? Again, looking at delta P= Q*R, if delta P is increased and R is decreased, Q is increased (Q=delta P/R).

 

[AwayFromReality]

I am not sure what exactly you mean by "how would we explain the increased flow rate [that is Q] since both Delta P and Q are now increased."

I assume you are asking what happens when delta P is increased? Again, looking at delta P= Q*R, if delta P is increased and R is decreased, Q is increased (Q=delta P/R).

Yeah my bad, I didn't look at the equation correctly. Thanks

 

[ieatshrimp24]

Your reasoning is incorrect. The continuity equation assumes that the flow rate is constant; that is, "Q remains constant" is an assumption for "if A increases, then v decreases" and not vice versa.

What you need to look at is the flow-resistance equation (delta P= Q*R). When the vessel is dilated, R is reduced and since the pressure supplied by the heart is constant, Q is increased.

Thank you. I forgot about the delta P = QR equation. That really helps.