Longitudinal Oxygen Tension Gradients in Small Cortical Microvessels in the Rat Brain on Development of Acute Anemia

Oxygen microelectrodes were used to study longitudinal oxygen tension gradients (pO 2 ) in small arterioles (with lumen diameters in control conditions of 5–20 μm) and capillaries in the rat cerebral cortex during stepwise decreases in the blood hemoglobin concentration ([Hb]) from an initial 14.4 ± 0.3 g/liter to 10.1 ± 0.2 (step 1), 7.0 ± 0.2 (step 2), and 3.7 ± 0.2 (step 3) g/liter. All data are presented as mean ± error of the mean. Oxygen tension was measured at two points in arteriole segments, separated from each other by distances of ΔL = 265 ± 34 μm, n = 30. The mean diameter of arteriole lumens in controls was 10.7 ± 0.5 μm, n = 71. The magnitude of the longitudinal pO 2 gradient (ΔpO 2 /ΔL) for the study arterioles was (mmHg/μm): 0.03 ± 0.01, n = 15 (controls), 0.06 ± 0.01, n = 16 (step 1), 0.07 ± 0.01, n = 14 (step 2), and 0.1 ± 0.01, n = 30 (step 3). In capillaries, ΔpO 2 /ΔL (with a segment length ΔL = 201 ± 45 μm, n = 18) was (mmHg/μm): 0.07 ± 0.01, n = 17 (controls), 0.09 ± 0.02, n = 16 (step 1), 0.08 ± 0.01, n = 15 (step 2), and 0.1 ± 0.02, n = 18 (step 3). The more than threefold decrease in oxygen content in arterial blood did not lead to significant changes ( p > 0.05) in ΔpO 2 /ΔL in capillaries. Longitudinal hemoglobin oxygenation gradients (ΔSO 2 /ΔL) in capillaries (calculated using the animals' systemic blood oxyhemoglobin dissociation curve) was some three times greater than in arterioles, which provides evidence for the leading role of capillaries in supplying oxygen to the cerebral cortex in these conditions. These data include the first measurements of longitudinal pO 2 gradients in arterioles and capillaries in the cerebral cortex on development of acute anemia.