The circulatory system can be viewed as a network of cylindrical tubes. The velocity of blood flow varies inversely with the total cross-sectional area of the blood vessels. As the total cross-sectional area of the vessels increases, the velocity of flow decreases. The slow flow in capillaries is because of the high total cross-sectional area of capillaries and not because capillaries are narrow. The collective cross-sectional area of all the capillaries in the human body is about a 1000 fold greater than the aorta. The slow flow in capillaries favours gas exchange and nutrient supply to cells.