Diffusive oxygen shunting between vessels in the preglomerular renal vasculature: anatomic observations and computational modeling

Abstract

RG. Diffusive oxygen shunting between vessels in the preglomerular renal vasculature: anatomic observations and computational modeling. Am J Physiol To understand how geometric factors affect arterial-to-venous (AV) oxygen shunting, a mathematical model of diffusive oxygen transport in the renal cortex was developed. Preglomerular vascular geometry was investigated using light microscopy (providing vein shape, AV separation, and capillary density near arteries) and published micro-computed tomography (CT) data (providing vessel size and AV separation; Nordsletten DA, Blackett S, Bentley MD, Ritman EL, Smith NP. IUPS Physiome Project. http://www. physiome.org.nz/publications/nordsletten_blackett_ritman_bentley_ smith_2005/folder_contents). A "U-shaped" relationship was observed between the arterial radius and the distance between the arterial and venous lumens. Veins were found to partially wrap around the artery more consistently for larger rather than smaller arteries. Intrarenal arteries were surrounded by an area of fibrous tissue, lacking capillaries, the thickness of which increased from ϳ5 m for the smallest arteries (Ͻ16-m diameter) to ϳ20 m for the largest arteries (Ͼ200-m diameter). Capillary density was greater near smaller arteries than larger arteries. No capillaries were observed between wrapped AV vessel pairs. The computational model comprised a single AV pair in cross section. Geometric parameters critical in renal oxygen transport were altered according to variations observed by CT and light microscopy. Lumen separation and wrapping of the vein around the artery were found to be the critical geometric factors determining the amount of oxygen shunted between AV pairs. AV oxygen shunting increases both as lumen separation decreases and as the degree of wrapping increases. The model also predicts that capillaries not only deliver oxygen, but can also remove oxygen from the cortical parenchyma close to an AV pair. Thus the presence of oxygen sinks (capillaries or tubules) near arteries would reduce the effectiveness of AV oxygen shunting. Collectively, these data suggest that AV oxygen shunting would be favored in larger vessels common to the cortical and medullary circulations (i.e., arcuate and proximal interlobular arteries) rather than the smaller vessels specific to the cortical circulation (distal interlobular arteries and afferent arterioles). hypoxia; kidney circulation; oxygen tension; renal oxygenation FIFTY YEARS AGO Levy et al. (17,18) provided evidence that oxygen is transported more rapidly than erythrocytes through the renal circulation, suggesting the existence of diffusional oxygen shunting between intrarenal arteries and veins. Shunting reduces the delivery of oxygen to renal tissue (26,27). In further support of the existence of arterial-to-venous (AV) oxygen shunting, Welch et al. (29) observed that the PO 2 of renal venous blood exceeds that of blood in the efferent arterioles of the outer cortex. In common with other countercurrent systems in biology and chemical engineering, it...

Renal oxygenation: preglomerular vasculature is an unlikely contributor to renal oxygen shunting

“…The permeability of oxygen in tissue is defined such that K ϭ ␣TDT, where DT is the diffusion coefficient of oxygen in tissue. The treatment of the capillary source/sink term is adapted from Gardiner et al (11): Mc ϭ M(PO2 Ϫ PO2,c)/␤c, where PO2,c is the capillary PO2. Hence, when PO2,c Ͻ PO2, capillaries act as a sink, and when PO2,c Ͼ PO2, capillaries supply oxygen to the tissue.…”

“…However, the model did not take into account transport processes in the tissue but rather employed fluxes out of the arterioles and into the venules based on weighting factors to fit experimental data. In a later study (11), the same group inferred structural information on the preglomerular vasculature from histology and used it for the formulation of a two-dimensional computational model of oxygen transport in cross-sections of arteriole-venule pairs of various configurations. This model predicted that considerable shunting can occur only when capillaries in the tissue are taken into account as sources or sinks for oxygen or when a vein wraps around the paired artery.…”

The role of hypoxia for the development of diabetic nephropathy : Temporal relationship and involvement of endothelin receptor signaling

“…This has been suggested to be the result of an intrarenal shunting mechanism. The shunting of oxygen is proposed to be the effect of close alignment of arterial and venous vessels within the kidney, resulting in an arterial-to-venous diffusion of oxygen [2][3][4]. This shunting mechanism may also contribute to the heterogeneous distribution of oxygen within the kidney, with a highly oxygenated kidney cortex (50-55 mmHg) and a poorly oxygenized inner medulla (15)(16)(17)(18)(19)(20).…”

Blood Flow, Oxygenation, and Oxidative Stress in the Post-stenotic Kidney

“…In the renal cortex, some arteries and veins are arranged in an intimate counter-current fashion not seen in other vascular beds ( Fig. 9.3 ) [ 5 ]. This arrangement favors arterial-to-venous Smaller arteries and veins, many of which will be specifi c to the cortical circulation, appear to be less intimately associated (With permission from Gardiner et al [ 5 ]) oxygen shunting, which acts to limit delivery of oxygen to renal tissue [ 5 , 6 ].…”