Page 17, Turbulence & Rheology, Dr. D. Penney
Circulatory Design (continued..)
What is the best size for the terminal blood vessels, i.e the capillaries? The underlying assumption is that the diffusion distance from the vessel to the tissue will be no more than 10 um. If the capillaries are large, the flow resistance will be low; however, their volume as a percent of tissue volume will be high (as much as 70%). This would be disadvantageous, especially in an essential, metabolically active tissue such as heart muscle. This means that 70% of the tissue volume is blood and only 30% is the parent tissue.
If on the other hand, the capillaries are tiny, the vascular volume will be less than 15% of the tissue volume. Flow resistance, however, will rise sharply as vessel caliber decreases. A compromise must be struck, and that compromise would be vessels approximately 12 um in diameter in a situation involving a Newtonian fluid. Because blood is non-Newtonian, flow resistance is less in small vessels and consequently the capillaries can be yet a bit smaller. Design, whether by engineers or through evolution, is always a compromise between opposing forces.
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