Relative hematocrit in human skin capillaries and its relation to capillary blood flow velocity

Abstract

Blood flow velocity (CBV) and relative hematocrit (Hct) in human nailfold capillaries were measured by noninvasive video-densitometric cross-correlation technique. Through a microscope a video recording was made of the flow of blood cells in the skin capillaries. Two small videophotometric windows were positioned along a monitored capillary. The velocity in the capillary was measured by continuously computing the delay-time to maximum cross-correlation between identical optical signals generated at the two windows by blood cells and plasma gaps. The relative hematocrit was deduced from continuous records of the video-densitometric output of one window. Relative calibration was accomplished by assigning zero hematocrit to the output of the window in the absence of red cells, and 100% to the maximum output recorded during a period of 5 min. More or less periodic fluctuations of CBV and relative Hct were found in most capillaries studied. The frequency of the fluctuations was in the order of 5 to 10 cycles per minute. The changes of flow were almost invariably accompanied by changes in hematocrit. In most instances changes in CBV preceded changes in Hct by one to several seconds. The normal vasomotion appears to cause a continuous change in the proportion between red cells and plasma until the diameter of the arteriole is so narrow that erythrocytes are mechanically prevented from passing into the capillary. Prior to complete closure of the arteriole there is sometimes a flow of plasma such that the capillary is empty of blood cells when flow stops. Adjacent capillaries seem to have the same major vasomotion pattern but individual variations are often seen.