Leukocyte plugging in vivo in skeletal muscle arteriolar trees

To provide quantitative data on leukocyte plugging and to estimate its effects on blood flow resistance in the microcirculation, in vivo observations of leukocyte plugging of capillaries were made in all branches of 27 terminal arteriolar trees in spinotrapezius muscles of anesthetized Sprague-Dawley rats. The durations of 1,257 observed plugs had a lognormal distribution with a median of 0.12 s. Of 596 branches, 211 experienced at least one plug; this subpopulation had a median plugging frequency of 0.016/s and a median plugging fraction (plugging frequency times average duration) of 0.28%. A new variable, the network occlusion fraction, was defined to quantify plugging in a whole arteriolar tree. Applying in vivo data to a model relating plugging to resistance increases resulted in a lognormal distribution of resistance increases with a median of 1.6%. Each of the resistance increases that were greater than 3% was due to one or two individual leukocytes that plugged a capillary for greater than 30 s. The results suggest that, under physiological conditions, leukocyte plugging has little effect on blood flow resistance.