anonymous
  • anonymous
why does speed of blood in arteries differ from speed of blood in capillaries
Biology
  • Stacey Warren - Expert brainly.com
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SOLVED
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katieb
  • katieb
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anonymous
  • anonymous
The cross sectional area is much different between the two.
anonymous
  • anonymous
To explain further without getting too complicated, there's an equation in physics that reads \[A ^{1}V ^{1}=A ^{2}V ^{2}\] Meaning that the area of one multiplied by the velocity in that area must be equal to the area of another multiplied by its velocity. SInce you only have a certain amount of blood this formula is applicable. Your capillaries have a HUGE total cross sectional area, so they must have a smaller velocity than your arteries who do not have as big of an area. So, Arteries: smaller area, higher speed Capillaries: much larger total cross sectional area, lower speed Additionally if you think about what capillaries do, you'd want them to have a much lower speed so as to allow for more material to pass through them since they can pass material to the cells directly.
blues
  • blues
To add to Spatcher's elegant and wonderful answer, the endothelial surfaces lining the arteries and capillaries differ so the actual physical interactions between the molecules and cells in circulation and the vessel walls differs. In real life, that has much to do with it too.

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blues
  • blues
It's a shame to messy up an answer like that with biologic details. It has to do with the forces exerted on the vessel walls by the blood and the forces exerted on the blood by the vessel walls. The wiki on fluid pressure should help: http://en.wikipedia.org/wiki/Fluid_pressure
anonymous
  • anonymous
this is not any simpler
anonymous
  • anonymous
is sectional area the same as surface area
blues
  • blues
No it is not. Cross sectional area is the area you'd see if you cut across the vessel and looked down its long axis.
anonymous
  • anonymous
I found this example somewhere, i forgot to copy the source: The cube's surface area is 54 in^2 (because it has six faces, and each face has an area of 9 in^2, because each face is a square with side length 3 in) However, the cube's cross-sectional area is 9 in^2 (because if you slice the cube horizontally through the middle with a plane, then the intersection is a 3 in x 3 in square).
anonymous
  • anonymous
i'm going to type out whole question
anonymous
  • anonymous
*forgot the first paragraph Take, for example, a cube with dimensions 3 in x 3 in x 3 in.
anonymous
  • anonymous
the average speed of the blood in the arteries is 45cm/s, but the average speed in the capillaries is only 0.5mm/s what do you think causes the difference?
blues
  • blues
And as already explained, the difference in velocity is due to the difference in cross sectional area between the arteries and capillaries.
anonymous
  • anonymous
cross sectional area in arteries narrower and in capillaries they are thinner
blues
  • blues
No. In terms of size, how does the cross sectional area of big vessels like arteries compare to the cross sectional area of small vessels like capillaries?
anonymous
  • anonymous
artery bigger than capillary
blues
  • blues
Yes, the cross sectional area of arteries is bigger than the cross sectional area of capillaries. The fluid pressure of blood in the arteries and capillaries is the same - that means that in the same volume of space enclosed by arteries and veins, you have the same total volumes of blood. Is that clear?
anonymous
  • anonymous
are you familiar with the book "biology for life" by MBV Roberts
blues
  • blues
Please answer my question. No, I'm not familiar with that particular book but I'd bet a lot that I know everything that's inside it.
anonymous
  • anonymous
i don't understand isn't the answer supposed to be related to how blood moves around the body
blues
  • blues
Yes, that is exactly what we are working toward explaining.
anonymous
  • anonymous
is there a way to explain it without using cross sectional area because i don't understand it and i don't want to write it down if i don't understand it
blues
  • blues
No, there isn't. Cross sectional area is easy. Imagine a piece of plumbing pipe. You cut a slice off the end of the pipe and you get a circle. You find the area of the circle. That is cross sectional area. Clear?
blues
  • blues
Now imagine that the pipe is a blood vessel. The artery is bigger and thicker than a capillary so it has a bigger cross sectional area. The capillary has a much smaller cross sectional area. Is that clear? ; D
anonymous
  • anonymous
yeah
blues
  • blues
Cool. The next big thing to understand is that the blood pressure is the same in the arteries and the capillaries. That is, if took an equivilent volume of space enclosed by a big artery and a little capillary it would contain the same amount of blood. That is actually as simple a statement as it seems.
anonymous
  • anonymous
go on
blues
  • blues
Now put it all together: what happens as blood flows from arteries with big cross sectional area (i.e., thick) through smaller and smaller vessels until it's in the little tiny capillaries? Well, you have the same total volume of blood trying to fit through a smaller space. So it all fits through, it has to move faster and faster. Does that make sense?
blues
  • blues
You can think about it as a river, if that analogy makes more sense. Think about a great big river in a flat place with a lot of room to spread out. It will flow really slowly, right? What happens if the same river flows through a gorge? It flows faster and faster.
anonymous
  • anonymous
is the gorge supposed to represent the cross sectional area
blues
  • blues
The gorge stands for the capillary with a small cross sectional area.
blues
  • blues
Or you can imagine yourself a blood cell which flowing from an artery into a capillary. The capillary is smaller than the artery (smaller cross sectional area), so there is less room in the capillary. You have all the pressure from the blood cells behind you in the artery pushing you into it, but there is less space in it. So you push harder on the cells already in the capillary and they move faster.
blues
  • blues
It's clear?
anonymous
  • anonymous
yeah thanks
blues
  • blues
Cool! You really hard on that one!
anonymous
  • anonymous
sorry
blues
  • blues
No, don't be. It's good to work hard and at the end of it understand something new!

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