examkrackers 1001 biology

[batista_123]

i have some questions about #268
the answer says when arteries widen, there is more room for blood to flow, which decreases resistance
but if i remember correctly, there is a physics equation that says when area gets bigger, resistance decreases, its like R= something over A??
also, it says when resistance is decreased, the pressure is reduced. how do they know this? which equation did they use?

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[LeftyUM]

I take it you might be thinking bout P = F/A.....as the arterial cross area increases, the pressure on the surrounding walls decreases.

 

[batista_123]

I take it you might be thinking bout P = F/A.....as the arterial cross area increases, the pressure on the surrounding walls decreases.

i am talking about the equation that says when you have a pipe and you run a liquid through it, Q=AV, and Resistance= something / Area

when the area is increased, the velocity decreases (Q=AV)
when velocity decreases, pressure increases
all of this contradicts what the answer says

 

[reddyjs]

i am talking about the equation that says when you have a pipe and you run a liquid through it, Q=AV, and Resistance= something / Area

when the area is increased, the velocity decreases (Q=AV)
when velocity decreases, pressure increases
all of this contradicts what the answer says

When flow speed increases, pressure decreases. I think with arteries what they are trying to say is that if you hold flow speed constant and then increase the size of the artery than pressure will decrease. So with bigger arteries you will have less pressure with a bigger artery because flow speed is held constant.

 

[zayka]

Poiseuille's Law for fluid flow is the equation that relates pressure with resistance and radius.

 

[Alvarez13]

i have some questions about #268
the answer says when arteries widen, there is more room for blood to flow, which decreases resistance
but if i remember correctly, there is a physics equation that says when area gets bigger, resistance decreases, its like R= something over A??
also, it says when resistance is decreased, the pressure is reduced. how do they know this? which equation did they use?

R=rho*L/A; rho is your resistivity. Think about some PVC pipe. The long, narrow pipe has a ton of surface area for the fluid to attach too and get slowed down by...basically lots of friction/drag. Now, if we take a fat, short pipe, there's not so much friction to result in the head loss of the system. Always remember: resistance will increase when L increases or A decreases (see that first eqn for resistance). This is true for fluids, circuits, and even heat transfer.

For the next part of that, the artery is opening up, it's increasing the area. Pressure = F/A...The pressure is decreasing. Remember that when you are bleeding, your vessels will constrict to try and keep your blood perssure constant...that's what I think of.

 

[link2swim06]

When flow speed increases, pressure decreases. I think with arteries what they are trying to say is that if you hold flow speed constant and then increase the size of the artery than pressure will decrease. So with bigger arteries you will have less pressure with a bigger artery because flow speed is held constant.

This confusing because how can the size change and not the flow speed.

 

[rHinO1]

i have some questions about #268
the answer says when arteries widen, there is more room for blood to flow, which decreases resistance
but if i remember correctly, there is a physics equation that says when area gets bigger, resistance decreases, its like R= something over A??
also, it says when resistance is decreased, the pressure is reduced. how do they know this? which equation did they use?

I'm not sure what your confused about. When an artery widens, then the area inside increases (area gets bigger). Where is the conflict in what you just stated above. In both cases as the area increases the resistance decreases.

If you have a garden hose and you pinch it off, then the pressure builds due to an increased resistance where the pinch is located. If you let the hose go, the pinched area increases and both resistance and pressure decrease. If you can relate these problems to practical examples it is much easier than memorizing a million equations.