Regional venous outflow, blood volume, and sympathetic nerve activity during severe hypoxia

We examined the dynamic changes in venous outflow from the splanchnic, coronary, and remaining other vascular beds and changes in systemic blood volume (SBV) in response to severe hypoxia (PO2 = 17 mmHg) in dogs using cardiopulmonary bypass and a reservoir. Splanchnic venous outflow, which also includes renal outflow in this study, decreased by 40%, and coronary venous outflow increased by 400% at 3.5 min after initiating severe hypoxia. Severe hypoxia caused a marked decrease in SBV of 23 +/- 1 and 9 +/- 2 ml/kg in spleen-intact and splenectomized dogs, respectively. The decrease in SBV was attenuated by 60 (P less than 0.01) and 83% (P less than 0.01) after the carotid and aortic chemoreceptor denervation (which was accompanied by baroreceptor denervation) and after hexamethonium infusion (10 mg/kg), respectively. Sympathetic efferent nerve activity revealed a tremendous augmentation, which began to rise at a PO2 of 40 mmHg before chemoreceptor denervation and at a PO2 of 22 mmHg after denervation. These results show that severe hypoxia causes a marked decrease in SBV, 60% of which is caused by active splenic contraction, and suggest that the sympathetic efferent nerve activity, which is augmented by the stimulation of chemoreceptors as well as the central nervous system, contributes greatly to those hypoxic changes.