Kaplan Practice Exam 3 Question

[agurl1000]

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Hey! This is my first post and was wondering you all can explain this question to me.

I said that the blood velocity would increase, which is wrong. I was wondering if someone could explain to me why? Is it somehow related to blood flow? The explanation fails to state that. Also what else occurs during the constriction of blood vessels?

Thanks in advance.

 

[aldol16]

Well one way to look at it is that cross-sectional area is related directly to resistance by the following equation: resistance = resistivity*length/area. As area decreases, resistance through that channel increases and therefore as long as there is another path for the blood to take, flux through this channel will decrease.

Perhaps it would be more informative for you to try to explain why you thought blood velocity would change one way or another.

 

[agurl1000]

Well one way to look at it is that cross-sectional area is related directly to resistance by the following equation: resistance = resistivity*length/area. As area decreases, resistance through that channel increases and therefore as long as there is another path for the blood to take, flux through this channel will decrease.

Perhaps it would be more informative for you to try to explain why you thought blood velocity would change one way or another.

I felt blood velocity would change because the artery is getting smaller/narrower. When fluid goes from a large cross-section to a small cross-section, then the speed of the liquid increases. This is why I thought it would increase.

 

[aldol16]

I felt blood velocity would change because the artery is getting smaller/narrower. When fluid goes from a large cross-section to a small cross-section, then the speed of the liquid increases. This is why I thought it would increase.

Do you understand the reason why this is true? If you have simple pipe and you decrease the diameter of that pipe at the end, the same amount of water that enters the pipe must get out of the pipe at the smaller end in the same unit time. In other words, volumetric flow rate must be constant. You can express that as the cross-sectional area times the velocity of the fluid - that product must be a constant for this system. So as you decrease cross-sectional area, you increase velocity at the end as you say.

The problem with applying that to this system is that there isn't only one pipe. There are many. In other words, the fluid has many options. It can "choose" to flow down any of a set of a pipes. It will respond by flowing down the one of least resistance and if there are multiple pipes that have low resistance, it will flow through all of them with varying flux controlled by the relative resistance in those pipes. This is more in tune with the system we're working with here.

 

[desperadoflakes]

Aldol, is it correct in saying that though the velocity of blood through this vein decreases, the increase in total peripheral resistance will lead to an increase in stroke volume and velocity in the entire system?

 

[aldol16]

Aldol, is it correct in saying that though the velocity of blood through this vein decreases, the increase in total peripheral resistance will lead to an increase in stroke volume and velocity in the entire system?

I never said the blood velocity would increase or decrease.... nor the peripheral resistance. What's relevant here is the resistance in this particular path - we don't know anything about the peripheral resistance (at least not from the information I can see).