How Many Capillaries? The aorta has an inside diameter of approximately 2.1 cm, compared to that of a capillary, which is about 1.0 × 10^?5 m (10µm). In addition, the average speed of flow is approximately 1.0 m/s in the aorta and 1.0 cm/s in a capillary. Assuming that all the blood that flows through the aorta also flows through the capillaries, how many capillaries does the circulatory system have?