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The effect of hemorrhage on blood flow to marrow and osseous tissue was determined in conscious rabbits by injecting radioactive microspheres (15 micron diam) at three time intervals: pretreatment control; 15 min after nonfatal hemorrhage (20 ml/kg body wt), and 16 h posthemorrhage. Cardiac output (CO), blood pressure, heart rate, resistance, arterial PCO2, PO2, and pH, and hematocrit were measured at each time interval. Tissues analyzed included heart, spleen, kidney, and femur with marrow and osseous tissue separated. At 15 min posthemorrhage, CO decreased to all tissues measured except the heart, where it increased. Sixteen hours later, CO returned to normal, and blood flow increased significantly to heart and marrow, while osseous tissue flow returned to control levels. The results show no immediate shunting of blood from bone to marrow; however, after 16 h there is a selective increase in marrow blood flow. This response is only detected by separating marrow and osseous tissue and it may reflect a preparatory mechanism for increased erythropoietic activity.

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