What is the most influential factor regulating blood flow according to Poiseuill

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First consider the simple hypothetical case of a single spherical cell suspended in a large (>times the cell volume), well-stirred volume of aqueous medium in equilibrium with room air and containing other nutrients. Oxygen availability is often a limiting factor for cell survival, and it is generally supplied to a cell by passive diffusion. As oxygen molecules diffuse into the cell, they are consumed, so that there is a progressive fall in oxygen concentration from the surface of the cell to the lowest concentration which occurs at the center of the cell. For a spherical cell with a typicaldiffusion coefficient for oxygen (˜10-5 cm2/s) and an oxygen consumption of resting skeletal muscle (˜10-2 ml O2 cm-3 min-1), the critical size (radius) which is just adequately supplied with oxygen from the surrounding medium is about 1 mm. Thus, we find that diffusion puts an upper limit on the size of cells in regard to their need for oxygen. Although diffusion is an efficient transport process over short distances (<100 µm) as seen by the average time required for a molecule to diffuse a distance x (t ˜ x2/2D), how can a much larger multicellular organism, such as the human body containing about 100 × 1012 cells, be adequately supplied with oxygen? For mammals, the bathing medium for cells is water and total body water is about 60% of body weight. For a 70-kg person, total body water is distributed among three compartments with the following approximate volumes: intracellular ˜23 l (33% of body weight); interstitial ˜16 l (22.5% of body weight); and circulating plasma ˜3 l (4.5% of body weight). Cells are bathed in interstitial fluid (ISF), but interstitial fluid volume is only a little more than half the intracellular fluid volume. Thus, ISF cannot be considered a large reservoir of fluid, and its composition is directly influenced by cellular metabolism.

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