Why Does The Exchange Of Materials Between The Body Cells And Blood Take Place In The Capillaries And Not In The Arteries And Veins?

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Arteries and Veins have a large diameter to allow large volumes of blood through. While the wall is by no means impenetrable, the majority of the blood is in the middle of the vein or artery, and without contact with the cells outside, the nutrients do not diffuse.

In capillaries the diameter is much, much smaller, allowing only one blood cells at a time through in many areas. Because of this, every blood cell that passes through the capillaries is much more likely to make contact with the cells outside, at which point it can give up it's oxygen and take out some CO2. Similarly, since the diameter is smaller, much more of the plasma is in constant contact with the cells surrounding the capillaries, and nutrients can get into cells more easily.

Some good images here:

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