Question

why does speed of blood in arteries differ from speed of blood in capillaries

Answer 1

The cross sectional area is much different between the two.

Answer 2

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.

Answer 3

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.

Answer 4

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

Answer 5

this is not any simpler

Answer 6

is sectional area the same as surface area

Answer 7

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.

Answer 8

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).

Answer 9

i'm going to type out whole question

Answer 10

*forgot the first paragraph
Take, for example, a cube with dimensions 3 in x 3 in x 3 in.

Answer 11

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?

Answer 12

And as already explained, the difference in velocity is due to the difference in cross sectional area between the arteries and capillaries.

Answer 13

cross sectional area in arteries narrower

and in capillaries they are thinner

Answer 14

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?

Answer 15

artery bigger than capillary

Answer 16

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?

Answer 17

are you familiar with the book "biology for life" by MBV Roberts

Answer 18

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.

Answer 19

i don't understand isn't the answer supposed to be related to how blood moves around the body

Answer 20

Yes, that is exactly what we are working toward explaining.

Answer 21

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

Answer 22

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?

Answer 23

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

Answer 24

yeah

Answer 25

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.

Answer 26

go on

Answer 27

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?

Answer 28

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.

Answer 29

is the gorge supposed to represent the cross sectional area

Answer 30

The gorge stands for the capillary with a small cross sectional area.

Answer 31

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.

Answer 32

It's clear?

Answer 33

yeah thanks

Answer 34

Cool! You really hard on that one!

Answer 35

sorry

Answer 36

No, don't be. It's good to work hard and at the end of it understand something new!