18. The velocity of blood flow is the lowest in capillaries because (A) the capillary walls are not thin enough to allow oxygen to exchange with the cells. (B) the total cross-sectional area of the capillaries is greater than the total cross-sectional area of the arteries or any other part of the circulatory system. (C) the capillaries are far from the heart, and blood flow slows as distance from the heart increases.
(D) the diastolic blood pressure is too low to deliver blood to the capillaries at a high flow rale.
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