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PROGRESS IN ATOMIC MEDICINE
phosphorus (P**} or radiochromium (Cr°’).°* In the normal person, the
white blood cells and platelets constitute less than 1 per cent of the.
blood volume and are generally not considered. The measurement of
total red cell volume with labelled cells is independent of the hematocrit
of the blood sample. There are no theoretic objections to the measurement of total red cell volume with labelled red cells. Fifteen minutes is
usually allowed for mixing at which time it is difficult to demonstrate
further mixing by studying peripheral blood. A considerably longer
period, 30 to 40 minutes, is actually required for mixing in the normal
person.” In some patients. particularly those with splenomegaly,” a
large component of mixing continues for 45 minutes.
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The measurement of plasma volume is more difficult. Opinions differ
as to what actually constitutes the plasma volume and howit should be
measured. Historically, the oldest method is based upon the dilution of
the volume of a dye. the most satisfactory of which is the Evans blue
dye (T-1824). This dye is bound to albumin” and in essence the volume
of distribution of the albumin dye complex is measured. Analysis of the
early disappearance of the dye from plasma shows a rapid phase lasting
about 10 to 15 minutes followed by several slower rates of disappearance.
Generally, the most rapid of the slower phases has been extrapolated
to the time of administration and the plasma volume calculated. Com.
parable values for plasma volume are obtained with I'™ labelled albumin.
However. whenlabelled fibrinogen is used in the dog,’ or gamma globulin
in man. the measured plasma volumeis significantly smaller. It appears
likely. at least in part. that some of this early phase attributed to mixing
within the plasma volume is actually due to disappearance of the dyeprotein complex from the plasma. For this reason. the plasma volume
as measured by T-1824 or I’? albumin, is probably overestimated.
Mineral Components
Lae
Exchangeable Potassium: There are two short-lived isotopes of potassium. K* tts, = 12 hours) and K*’ (ti, = 22 hours). Radiopotassium*
does not appear to be widely used. In man. the bulk of potassium is
intracellular. Radioactive potassium equilibrates within approximately
24 to 40 hours with virtually all of the total body potassium.'**** The
requirements for a satisfactory measurement of total body potassium
are met. However. the short half-life of K*? makes it necessary to have
at hand either a production facility or to receive frequent shipments.