In the early evolution of living organisms the first forms to emerge were unicellular species which absorbed nutrients via the cell wall, and removed waste-products in the same way. This metabolic configuration is still present within many of todays most basic forms of plant life, such as protozoa. This transport relies upon the random motion of particles causing a constant smoothing effect upon concentration variations termed diffusion. If a high concentration of metabolic byproducts resides within a cell relative to the concentration outside, then there will be a flux of particles randomly moving out of the cell which exceeds the flux of particles randomly moving into the cell, producing a net movement of the metabolite out of the cell.
With the advent of larger collections of cells to form a multicellular organism it became more difficult to depend upon diffusive transport for metabolic inter-change. The time taken by a single particle to cross a distance via diffusion alone increases as a function of the square of distance [Levick]. Were the human body to depend upon diffusion for nutrient transport, the subject would have the right
to vote by the time nutrients reached the foot from the heart, as illustrated in Table 1.1.
The necessity of a more efficient transport mechanism favoured the development of a convective metabolic configuration. The convective configuration relies upon moving a carrier fluid through the system which nutrients and metabolites can be dissolved or held in suspension. This movement by virtue of being within a moving fluid is termed convection, and is the dominant form of metabolic transport in the human body. This complex system of vessels, pumps, and reservoirs is discussed within this chapter, with an emphasis upon those aspects which will have particular bearing upon the fluid dynamics of the system.
The human body consists of three convective systems; namely the cardiovascular system, the lymphatic system, and the respiratory system. The latter two systems are not the primary concern of the present study, but their existence and continued functionality is an absolute prerequisite for the proper operation of the former system.
J. R. Levick, An Introduction to Cardiovascular Physiology, Arnold, 2000.