Abstract
Cameron and Moorhouse 1 have proposed using the cerebrospinal fluid calcium as a measure of the diffusible calcium of the blood plasma, arguing that the choroid plexus as a living, colloid impermeable membrane gives us a more perfect distribution of the diffusible constituents of the blood than can be obtained in any in vitro manner. This implies both that the cerebrospinal fluid is formed by a process of diffusion rather than of active secretion and also that the fluid is continuously in diffusion equilibrium with the constituents of the plasma. Whether the cerebrospinal fluid is formed by an active process of secretion or is simply a dialysate of the blood plasma is still a mooted question.
Histological data and evidence from the mechanics of circulation, favoring the dialysis theory have recently been reviewed by Fremont-Smith. 2 It has been pointed out by Updegraff, Greenberg and Clark 3 and by Fremont-Smith, 2 that the published data of various authors for the amounts of uric acid, urea, glucose, magnesium and inorganic phosphate present in plasma and spinal fluid respectively are difficult to reconcile with the diffusion theory.
Even if it is granted that the spinal fluid is a dialysate of the blood plasma, there still remains the important point of whether the spinal fluid is continuously in equilibrium with and gives a true picture of the changing states of the constituents of the blood plasma, or if it is only rarely in actual diffusion equilibrium with the blood plasma. If there is diffusion equilibrium, the distribution of the ionic constituents between plasma and spinal fluid is expected to conform to the relations governing a Donnan membrane equilibrium. Hamilton 4 arrived at the conclusion: “ …. an equilibrium of the Donnan type may, at least partly, govern the distribution of electrolytes between serum and spinal fluid, but it seems probable that the equilibrium is modified by unknown factors.”
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