1. Fetal sheep at 120 days gestation were fitted with upper and lower body arterial and venous catheters in addition to a flow sensor and occluder placed around the aorta below the renal arteries. 2. After 7 days of recovery, the occluder was partially inflated to reduce aortic blood flow to 70% of control. Blood flow reduction was maintained at this level for the remainder of the experiment. 3. Blood samples were taken after 60 min of blood flow reduction and again after 3 or more days of blood flow reduction. 4. There was no change in upper body arterial or venous blood pressure. Lower body arterial blood pressure decreased, as expected. Arterial PO2 decreased while packed cell volume and haemoglobin concentration increased. There was no change in plasma erythropoietin concentrations or plasma renin activity. 5. While both red cell mass and haemoglobin mass increased during the period of the study, the rate of increase was no different from the rate of blood volume increase.
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