[Rapid determination of the sedimentation rate of blood free from the influence of gravity (author's transl)]

The measurement of blood density and its meaning

Density is defined as mass per unit volume. The classical technique to measure the density of fluids consists of a determination of mass and volume. Blood density is proportional to hematocrit or, more exactly, to the total protein concentration of blood; only to a minor extent is blood density influenced by other plasma solutes. Since the introduction of the mechanical oscillator technique for the continuous recording of fluid density a sizeable amount of experience has accumulated. This review summarizes recent work performed with this technique. It appears that the scientific interest in a variable like blood density depends on the availability of a suitable and simple method. Until the oscillator technique was available the measurement of density was too complicated or too inaccurate for routine laboratory use. A further new technique permits us to determine fluid densities by measuring sound velocity transmission. The density dilution method can be used for the determination of distribution volumes, of flow through organs, and of the cardiac output. The influence of temperature and of certain artifacts like acceleration forces in the density measuring device have to be considered any may be used for additional diagnostic purposes like determination of erythrocyte sedimentation velocity. The new technique opens a reasonable simple way to study fluid shift between interstitial space and capillaries. The arterio-venous density gradient in an organ depends on the lymph production. The injection of a hypertonic solution leads to an osmotic fluid shift from the extravascular space towards the blood. This fluid shift can be recognized by a reduction of the blood density. A simple model for the description of this reaction is presented.

Quick measurement of hematocrit and erythrocyte sedimentation-rate by means of a density tracking method

An instrument to track the sedimentation properties of red blood cells quickly and automatically and to evaluate the hematocrit as well as the plasma density of a blood sample in the same operation is described. Erythrocyte-sedimentation basically is due to the density difference between the red blood cells and the plasma and the new method makes use of the mechanical-oscillator-technique, where the resonant frequency of the bending-type oscillations of a U-shaped glass tube is related to the density of the fluid with which the tube is filled. The oscillating U-tube is part of a sedimentation-tube of special dimensions and particular spatial orientation. The tube is tilted at 60 degrees, the tip of the U-tube being raised above the horizontal plane. Filling the sedimentation-tube with anticoagulated blood first permits recording of the density of the homogeneous suspension. Then, after a few minutes, the onset of erythrocyte-sedimentation produces a decrease in recorded density. The final value is related to the density of the supernatant plasma-fluid. Hematocrit, blood and plasma density of the undiluted blood sample as well as the sedimentation-rate of the anticoagulated sample are automatically calculated from the recorded sedimentation-curve. The particular arrangement of the tilted sedimentation-tube facilitates a determination of the maximum sedimentation-rate within 15 min at most.

[Investigations concerning postural influences on blood and blood plasma (author's transl)]

Body position exerts considerable influence on transcapillary balance of protein-poor fluid, i.e., on the whole-body Starling equilibrium. Movement into upright position causes hemoconcentration whereas supine position makes hemodilution. This means that the alteration of blood and plasma volume influences, e.g., plasma protein concentration, hematocrit, and blood hemoglobin content. The application of the mechanical oscillator technique for high-precision density measurements on capillary blood and plasma, especially to quantify the time-course of alterations caused by posture, is discussed in this paper. Tilting into upright position (70 degrees) 45 min after recumbency led to an average increase of 6.6% after 10, and of 11.1% of plasma volume after 30 min in 12 test persons. In 13 test persons, tilting back into supine position (0 degrees) after 30-60 min of standing (70 degrees) caused a mean increase of plasma volume amounting 6.5% after 10, and of 10.5% after 30 min. Postural blood density variations showed shapes similar to that of plasma density, indicating blood volume alterations ranging between 5 and 10%. It is emphasized that, as a consequence for clinical practice, the body positioning before and during blood sampling must be taken into consideration especially in case of precise controls of the course of hematological variables, and in case of statistical comparisons among several test groups.

[Application of the mechanical oscillator technique for the measurement of blood density and hematocrit (author's transl)]

Density measurements were performed with an accuracy of 0.01 g/l by means of a density meter based on the "mechanical oscillator technique", on human whole blood, on plasma, and on erythrocyte concentrates. A highly significant correlation between blood density and hematocrit was found. If plasma and red cell densities are known, hematocrit can be estimated more precisely than using the centrifugation method. This fact is important if small variations of the hematocrit are to be determined. Such variations can be produced, e.g., by alteration of the transcapillary filtration equilibrium. Furthermore, the statistical distribution of plasma and erythrocyte densities allows to determine the range of variation of blood densities for certain hematocrit values and vice versa.