Wang W, Michel C C
Department of Physiology and Biophysics, Imperial College Medical School at St. Mary's, London, UK.
Microcirculation. 1997 Sep;4(3):381-90. doi: 10.3109/10739689709146802.
To investigate effects of anastomoses between the descending vasa recta (DVR) and the ascending vasa recta (AVR) on the distribution of small solutes in the interstitial fluid of the renal medulla.
Countercurrent capillary loops, surrounded by a secretory epithelium, were used to model microvessels in the renal medulla. Anastomoses between the DVR and AVR were modeled as a decrease in cross-sectional area of the vessels and a decrease in flow velocity from the base to the tip of the capillary loop. When experimental data were used to evaluate parameters of the model, it was seen that diffusive transport of solute in the axial direction of the capillary was negligible, and the equations could be greatly simplified.
General formulae of the solute concentration distribution were derived for different degrees of shunting between the two limbs of the capillary loop. Analytical solutions for the steady-state solute distribution were obtained when the sizes of capillaries and flows in them were assumed to decrease linearly with the distance from the base to the tip of the capillary loop. When the effects of reduction in the size of the limbs were compared with the effects of reduction of flow velocities on solute distribution, it was found that, in the presence of anastomoses, change in flow velocity increases the axial gradient of the solute concentration more than change in the cross-sectional area. The combined effects of a decrease in flow velocity and cross-sectional area can easily double the axial gradient of the solute concentration for a modest degree of anastomotic shunting.
In this study, we separated effects of anastomotic flow between the DVR and AVR from other factors affecting the complicated countercurrent solute exchanges in the renal medulla. Results from the model show that anastomoses increase the solute concentration in the medullary interstitium and also the axial gradient of the solute concentration there.
研究直小血管降支（DVR）与直小血管升支（AVR）之间的吻合对肾髓质间质中小溶质分布的影响。
用被分泌上皮包围的逆流毛细血管袢来模拟肾髓质中的微血管。将DVR与AVR之间的吻合模拟为血管横截面积减小以及从毛细血管袢底部到顶端流速降低。当用实验数据评估模型参数时，发现溶质在毛细血管轴向的扩散运输可忽略不计，方程可大大简化。
推导了毛细血管袢两分支不同分流程度下溶质浓度分布的通用公式。当假设毛细血管大小及其内的血流随距毛细血管袢底部到顶端的距离线性减小时，获得了稳态溶质分布的解析解。当比较分支尺寸减小和流速降低对溶质分布的影响时，发现在存在吻合的情况下，流速变化比横截面积变化更能增加溶质浓度的轴向梯度。对于适度的吻合分流，流速和横截面积减小的联合作用可轻易使溶质浓度的轴向梯度加倍。
在本研究中，我们将DVR与AVR之间吻合流的影响与影响肾髓质复杂逆流溶质交换的其他因素分开。模型结果表明，吻合增加了髓质间质中的溶质浓度以及那里溶质浓度的轴向梯度。
