Abstract
Introduction and Aims: Diffusion of oxygen from arteries to veins in the kidney (AV oxygen shunting) acts to limit oxygen delivery to renal tissue. Thus, it may contribute to acute kidney injury, particularly if it limits oxygen delivery to the renal medulla. The factors critical in determining the quantity of AV oxygen shunting within the renal circulation include (i) the distance between the arterial and venous lumen, and (ii) the degree to which the vein wraps around the wall of the artery. Therefore, we quantified how these factors change along the course of the renal circulation and how they differ between the various anatomical regions of the kidney.
Methods: The renal vasculature of male Sprague Dawley rats (n = 6) was perfusion fixed and filled with Microfil®. All arteries were identified in an individual section from each kidney. The shortest arterial/arteriolar diameter, the shortest distance to the nearest vein (diffusion distance), and the degree to which the closest vein wrapped theartery was measured for all 1628 arteries identified in the 6 sections.
Results: The diffusion distance between arteries and veins increased progressively with decreasing arterial diameter. It was also considerably less for vessels in the corticomedullary border (64.2 ± 8.4 μm) and inner-cortex (66.0 ± 6.0 μm) than in the mid-cortex (115.6 ± 6.1 μm) or outer-cortex (139.7 ± 9.5 μm). The proportion of the arterial wall surrounded by the vein (wrapping) reduced progressively as arterial diameter decreased. It was considerable greater for vessels in the corticomedullary border (15.0 ± 1.4 %) and inner-cortex (12.5 ± 1.0 %) than in the mid-cortex (6.0 ± 0.6%) or outer-cortex (3.5 ± 0.5 %). When arteries were partitioned according to whether or not they were wrapped to any extent by a vein, diffusion distance varied little with arterial diameter, but was considerably less for wrapped arteries (11.6 ± 0.4 μm) than non-wrapped arteries (145.2 ± 3.9 μm).
Conclusions: Renal artery-vein pairs appear to be characterized either by the presence of both a short diffusion distance and wrapping of the vein around the artery, or a longer diffusion distance and no wrapping. The spatial relationships that promote AV oxygen shunting (short diffusion distance and wrapping) are more prominent in the larger vessels than the smaller vessels and in vessels in the corticomedullary border and inner cortex than the mid cortex or outer cortex. These observations challenge the conventional notion that most AV oxygen shunting occurs in the smaller cortical vessels (e.g. interlobular arteries) after the divergence of the cortical and medullary circulations. Thus, AV oxygen shunting may limit oxygen delivery to the renal medulla as well as the renal cortex and so render the medulla susceptible to hypoxia.