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184 WALTER R. STAHL

B. Artificial Kidney

The artificial kidney is very dissimilar in appearance from a real kidney. As described byKolff (1961) or Salisbury et dl. (1959),itusually consists of a long plastic tubę immersed in a bath of rinse fluid, with motion of tubę or fluid induced by some means. Little progress has been madę in creating models of the highly selective renal tubular action, whicb is based on molecular membranę phenomena. Nonetheless, the following numerical criteria apply to the natural and artificial Systems: clearance (a flow ratio), dialysance (ratio of flows or clearances), con-centrations and concentration ratios, relative transit times, and membranę constants. Ali of these parameters are essentially size-independent for comparable systems and may be expressed in dimensionless form when desired. Someren et al. (1963) compare a number of artificial kidneys by criteria of the above type. The design of artificial kidneys for children or laboratory animals clearly reąuires the use of scaling factors and invariant dimensional constants. For example, blood flow per unit of exchange area in the artificial kidney is invariant of size and allows rational choice of such a device for a child.

An interesting demonstration of the basie physical similarity of natural and artificial organs is seen in a report of Filatov et al. (1962), who attempted to use lungs from dead animals as part of an artificial kidney system. The lung membranę was surrounded by fluid on both sides and operated in a pulsatile “batch-processing” manner. Clearance and several other criteria apply to this model.

The design of artificial placentas (Lawn and McCance, 1962) involves use of mechanisms from both heart-lung machines and artificial kidneys. Deduction of suitable performance criteria for these artifacts will allow rational scaling for animal or human size.

It must be emphasized that numerous problems other than those of physical scaling are involved in construction of practical artificial organs, such as problems pertaining to development of nontoxic and nonclotting plastics, provision of power, dissipation of heat, and smootb operational control. The various criteria given in Table I cover most of these matters to the extent that a model of the real organ is being madę, but in many cases it is morę practical to design a completely new kind of physical system which is a qualitative Simulator and not quantitative model in any sense. However, the fact that an “artificial heart” is called by this term in itself suggests certain physical analogies with the real organ in