Blood flow through the vessels at an uneven speed. It flow much faster in large arteries and much slower in capillaries. Although an individual capillary is much narrower, but the capillary beds have an enormous number of such capillaries. So that the total diameter of these vessels is much greater than the arteries. For this reason the blood flows slowly in the capillaries, permitting the exchange of materials between the blood and interstitial fluids. As blood leaves the capillary bed and passes to the venules and veins, it speeds up again due to the reduction in total cross sectional area.
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