Animals and people that use a closed circulatory system under positive pressure from the heart face a problem.  How to supply every single cell of the body with oxygen and remove metabolic waste?  The reason this is problematic is that as the size of a blood vessel decreases, its resistance to the flow of fluid within it increases.  If blood were simply water, the heart could not generate the pressure needed to push it through vessels the size of capillaries.  This is just a physical fact.  The miracle is that in blood, this does not happen because the flow characteristics of blood are changed because it contains 45% by volume of cells.  We are not even aware that the design of our circulatory system faced this problem because of the solution that was put in place by the Creator from the very beginning.  Creation Matters Vol. 20 No. 2, p.4.  March-April 2015.  References: Denny, M and A McFadzean, 2011 Engineering Animals: How Life Works.  Cambridge, MA: Harvard University Press.