On an average, capillaries are about 1 mm long and 8 mm in diameter. Although each capillary is very narrow, there are so many of them that the capillaries have the greatest total cross sectional area of any other type of vessel. Consequently, the blood decreases in velocity as it passes through the capillary beds, allowing more time for it to exchange materials with the surrounding extracellular fluid. Blood also loses most of its pressure in passing through the vast capillary networks and so is under very low pressure when it enters the veins.