Spatial distribution of red blood cells in individual skeletal muscle capillaries during extreme hemodilution

The effect of extreme hemodilution on single capillary red blood cell (RBC) distribution and microcirculatory hemodynamic parameters was studied in the resting rabbit tenuissimus muscle. Systematic hematocrit was progressively reduced to 26 +/- 3% of control by isovolemic hemodilution with a 6% dextran solution (70,000 MW). Heart rate and mean arterial pressure were monitored and noted to be constant throughout the procedure to ensure isovolemic exchange. Hemodilution induced an increase in the median spatial distance between RBCs within a capillary segment and a broadening of the range. Comparison of histograms constructed from normalized RBC spacing data relative to the median at each dilution level showed no statistical deviation from control, suggesting that spatial RBC distribution during capillary transit is independent of hematocrit reduction. Thus progressive hemodilution, while reducing capillary hematocrit and increasing the median spacing, did not alter the relative spatial distribution of RBCs within a vessel. The hemodynamic parameters analyzed were capillary hematocrit (HCTc), RBC flux (RBCf), and RBC velocity (VRBC). The reduction of the systemic hematocrit (HCTs) was not followed by a proportional fall in HCTc, implying a Hctc regulation. RBCf was maintained by an overall increase in VRBC which reached 101 +/- 61% of the control level at an average HCTs of 11 +/- 1.4%. The relative pattern of RBC spacing remained fixed, despite the change in cell number and modification of velocity. During extreme hemodilution, the potential supply of oxygen to the tissue remained constant and the pattern of RBC delivery pass the tissue was unchanged.