The Disappearance of K ⁴² from the Nonuniformly Mixed Circulation Pool in Dogs

Abstract

The disappearance of injected labeled substances which disappear rapidly from the circulation is discussed theoretically and tested in experiments with injected K 42 in the dog. Comparing the inflow and outflow from a capillary bed, using semilogarithmic plots cf label concentrations, events upstream are repeated downstream but displaced in time by the amount of the mean circulation time between the points of observation. Downward displacement occurs owing to the intervening loss of material, and smearing of the time relations is produced by the dispersing action of variable path lengths in the capillary labyrinth. For the circulation as a whole, the time relations consist of the sum of a periodic compcnentand an aperiodic component. The periodic component is governed by the mixing relations in a virtual circulation whose architecture approximates that of the real circulation. Extrapolating the aperiodic component to zero time \ ields the volume of dilution of the injected material in the virtual circulation, providing (hat injection and sampling points are close together. When the points are separated, correction must be made for time delay and loss of label between. The initial slope is governed not only by (he mean rate of exchange of injected substance with the tissues but also bv the mean circulation time.

Manganese Metabolism in Man: Rapid Exchange of MN56 with Tissue as Demonstrated by Blood Clearance and Liver Uptake1

“…It is well represented analytically where a (t) is radioactivity in standardized units (Figure 4). The first exponential component is comparable to the aperiodic time relation of Sheppard, Overman, Wilde and Sangren describing the rapid blood clearance of indicators (30). The last component is identical with the exponential (antilogarithmic) form of the least squares regression line previously calculated.…”

“…This conclusion was based on the virtual lack of change in the "plateau" reading of Figure 1 when the counter was moved in a circle with the liver as center (Curves D, E, and F of Figure 1). (29,30,32). Figure 2 indicates that much of the radiomanganese initially in the blood is cleared within a minute of virtual mixing, with very little remaining by 45 minutes.…”

Regional Distribution of Myocardial Blood Flow in the Dog as Determined by Rb 86

“…The initial distribution of tracer is controlled by circulatory fae- tors and by individual exchange rates. 4 It is likely that the exchange rates for the various regions of the myocardium are quite similar, although at present data are not available on this point. One variable which may affect the relative exchange rates for the two ventricles is the relative rate of blood flow in each region.…”

“…4 Since rubidium behaves similarly in many respects to potassium, 7 at equilibrium its distribution should resemble that of potassium. The potassium concentration of the myocardium is fairly uniform in the right and left ventricles, and is significantly lower in the atria.…”

The Measurement of Cerebral Blood Flow by External Isotope Counting*

“…The correction for the redistribution of Rb86 to the noncerebral cephalic structures represents approximately 10 per cent of the total cephalic Rb86 uptake, and therefore would only slightly alter the final calculations if it were underestimated. The pulmonary indicator losses for K42 and Rb86 have been variously reported between 1.9 and 2.7 per cent of the injected dose in rats, dogs, and rabbits (6,9,14), and therefore represent negligible error.…”