Diffusive shunting of gases and other molecules in the renal vasculature: physiological and evolutionary significance

Abstract

Countercurrent systems have evolved in a variety of biological systems that allow transfer of heat, gases, and solutes. For example, in the renal medulla, the countercurrent arrangement of vascular and tubular elements facilitates the trapping of urea and other solutes in the inner medulla, which in turn enables the formation of concentrated urine. Arteries and veins in the cortex are also arranged in a countercurrent fashion, as are descending and ascending vasa recta in the medulla. For countercurrent diffusion to occur, barriers to diffusion must be small. This appears to be characteristic of larger vessels in the renal cortex. There must also be gradients in the concentration of molecules between afferent and efferent vessels, with the transport of molecules possible in either direction. Such gradients exist for oxygen in both the cortex and medulla, but there is little evidence that large gradients exist for other molecules such as carbon dioxide, nitric oxide, superoxide, hydrogen sulfide, and ammonia. There is some experimental evidence for arterial-to-venous (AV) oxygen shunting. Mathematical models also provide evidence for oxygen shunting in both the cortex and medulla. However, the quantitative significance of AV oxygen shunting remains a matter of controversy. Thus, whereas the countercurrent arrangement of vasa recta in the medulla appears to have evolved as a consequence of the evolution of Henle's loop, the evolutionary significance of the intimate countercurrent arrangement of blood vessels in the renal cortex remains an enigma.

Renal oxygenation: From data to insight

“…For example, blood flow (and thus DO 2 ) to the renal medulla is at least partly regulated independently from the blood flow to the bulk of the renal cortex . Furthermore, the potential for counter‐current shunting of oxygen and carbon dioxide, between arteries and veins in the renal cortex and descending and ascending vasa recta in the renal medulla, introduces uncertainty about the quantity of oxygen delivered to the microcirculation (see Diagram Model B in Figure ) . So we rapidly get to the stage where a diagram model of renal oxygenation becomes so complex it loses its value.…”

Micro‐computed tomographic analysis of the radial geometry of intrarenal artery‐vein pairs in rats and rabbits: Comparison with light microscopy

“…One such case of transport analysis is the diffusive arterial‐to‐venous (AV) shunting of oxygen in the renal vasculature. Throughout the mammalian kidney, veins are often seen to partially wrap a nearby artery (geometric wrapping) . This structural arrangement could potentially enable AV shunting of various gases .…”

“…There is experimental evidence that diffusive AV oxygen shunting in the renal vasculature may be significant . However, the quantitative significance of this phenomenon remains a matter of controversy . Currently, it is not possible to accurately measure AV oxygen shunting experimentally.…”

“…4,5 This structural arrangement could potentially enable AV shunting of various gases. 5 The amount of shunting that can occur is extremely sensitive to the distance between each artery-vein pair 4,6 and to the degree of geometric wrapping. 4 Furthermore, estimation of the rate of blood flow, and thus the time available for shunting to occur, 4,7 is also strongly dependent on accurate measurement of the cross-sectional areas of vessels.…”

“…[10][11][12] However, the quantitative significance of this phenomenon remains a matter of controversy. 5,8,9,13,14 Currently, it is not possible to accurately measure AV oxygen shunting experimentally. For this reason, we rely on mathematical modelling to investigate this phenomenon by simulating diffusion and convection of oxygen within the renal cortex.…”