Neurohumoral regulation of blood flow to bones and marrow.

We used labeled microspheres to measure bone and marrow blood flow under control conditions and during several interventions. In dogs, blood flow to compact cortical bone (femoral diaphysis) was 2 +/- 0.1 ml.min-1. 100 g-1, and in hematopoietic marrow (femur), flow was 24 +/- 5 ml.min-1. 100 g-1. Hematopoietic cancellous bones (sternum, ilium) received substantial blood flow. We estimate that as much as 11% of cardiac output is directed to the skeleton. Bone and marrow vessels were responsive to several stimuli. During exercise, vascular resistance in bone increased two- to fourfold, in contrast to profound vasodilation in adjacent skeletal muscle. Hemmorrhage also increased bone vascular resistance. Bone and marrow vessels responded to humoral stimuli: norepinephrine increased vascular resistance and adenosine decreased resistance. Bone vessels also responded to neural stimuli: stimulation of carotid baroreceptors decreased vascular resistance, and activation of sympathetic pathways by baroreceptor deafferentation in cats increased vascular resistance. These studies indicate that bone and marrow vessels respond actively to physiological stresses and to several humoral and neurogenic stimuli