Extraction Ratio and Bone Clearance of Sr 85 as a Measure of Effective Bone Blood Flow

When a nondiffusible plasma dye (T-1824) and Sr85 were injected into the nutrient artery of the tibia in 10 dogs, 90% of the T-1824 and 21% of the Sr85 were recovered from the femoral vein in the next five minutes. From these data, the extraction ratio for removal of Sr85 was calculated to be 0.764 ± 0.066 (SD). The difference from unity may be accounted for in part by blood flow through bone marrow. It was concluded that initial (0 to 5 minute) Sr85 clearance should give a useful measure of effective bone blood flow. Values obtained for femur, tibia, humerus, and talus in 10 dogs and 80 rabbits were all remarkably similar. The effective bone blood flow in all cases was approximately 10 ml/100 g/min or, for the entire skeleton, approximately 5 to 7% of the resting cardiac output. The rate of blood flow far exceeds the metabolic needs of bone. This may be related to its important homeostatic function in mineral metabolism.