Final answer:
Capillaries have a low blood flow rate due to their large total cross-sectional area, allowing for effective exchange without clotting.
Explanation:
The rate of blood flow in capillaries is low due to the significant increase in the total cross-sectional area at the capillaries compared to arteries. This decreased flow rate is necessary to allow sufficient time for effective exchange to occur while preventing clotting. Capillaries play a crucial role in facilitating nutrient and gas exchange between blood and tissues.
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