AJP - Heart and Circulatory Physiology, Vol 250, Issue 6 908-H922, Copyright © 1986 by American Physiological Society
Microvascular hemodynamics during systemic hemodilution and hemoconcentration
H. H. Lipowsky and J. C. Firrell
Measurements of intravascular pressure, red blood cell (RBC) velocity, and
microvessel hematocrit (Hctmicro) were made in arterioles and venules of
the cat mesenteric microvasculature during systemic hemodilution (cell-free
plasma) and hemoconcentration (packed cells). For a range of systemic
hematocrits (Hctsys) from 5 to 67%, changes in volumetric flux of red cells
(QRBC) were derived from the product of microvessel bulk flow and Hctmicro.
During hemodilution, a heterogeneous response of changes in QRBC was found
with larger distributing arterioles (43-54 microns) exhibiting a monotonic
fall, whereas increases in QRBC above control were found in smaller
arterioles that were indicative of a potential enhancement of oxygen
delivery. Although the dilution response of all arterioles and venules
averaged for all calibers of vessels demonstrated a decline in QRBC,
alterations of Hctmicro suggested a lessening of the disparity between
Hctsys and Hctmicro, which was indicative of a more efficient utilization
of the remaining circulating RBC volume. In response to hemoconcentration,
a decrease in QRBC also occurred, which, in concert with the dilution data,
suggested that QRBC was maximized for a range of 28 less than Hctsys less
than 46%. From measurements of the arteriovenous pressure drop across
mesenteric modules, regional resistance was found to exhibit a relative
plateau as Hctsys was increased above its control value. This behavior was
attributed to a decrease in vascular hindrance of the principal resistance
vessels and an invariance of blood viscosity at the capillary level due to
RBC redistribution and the attendant viscous behavior of blood.
