Whole body structural vascular adaptation to prolonged hypoxia in chick embryos

We studied the role of hypoxia in the development of the blood vascular system using functional measurements of whole body and hindlimb structural vascular resistance in the chick embryo. The method is based on a newly developed whole body perfusion technique in which the maximally dilated blood vasculature of 14- to 15-day chick embryos is perfused through the extraembryonic blood vessels. Embryos were grown in 12% oxygen (Po2 65 mmHg, n = 18) or 16% oxygen (Po2 96 mmHg, n = 19) for the last 7 days of incubation and were compared with weight-matched (n = 17) and age-matched (n = 18) normoxic control groups (Po2 134 mmHg). Pressure-flow curves were generated for all embryos by increasing and decreasing the aortic pressure along 1-mmHg steps over a pressure range of 0-6 mmHg. Venous pressure was held at 0 mmHg by allowing the perfusate to flow freely from severed extraembryonic veins. The hydraulic resistance of the maximally dilated vascular bed, called the "structural vascular resistance," was decreased in a dose-related manner in the hypoxic groups to greater than 50% of control in the whole body and hindlimbs of the 12% oxygen group. The vessels of the 12% oxygen group were able to carry two and three times as much flow to the whole body and hindlimb tissues, respectively, as compared with the weight-matched normoxic control group. Therefore, the results support the hypothesis that prolonged exposure to hypoxia causes the blood vascular system to adapt its structure to allow greater amounts of blood to flow to the tissues at any given perfusion pressure gradient.