Now, what does change within the blood vessels is the A the cross sectional area and B the velocity of that fluid. Now, it turns out that if we take the sum of all the cross sectional areas of all the individual capillaries it will be much greater than the cross sectional area of the arteries and of the veins. So that cross sectional area, the total cross sectional area for the capillaries is the greatest. Now, as we increase the A to keep the Q the same, the V must decrease. And that's exactly why within the capillaries the velocity of blood flow is the lowest because the total cross sectional area of the capillaries is basically the greatest. So we see that within the aorta and the large and smaller arteries we have a high pressure.