Edwards A, Silldforff E P, Pallone T L
Department of Chemical Engineering, Tufts University, USA.
Front Biosci. 2000 Jun 1;5:E36-52. doi: 10.2741/edwards.
Blood flow to the renal medulla is supplied through descending vasa recta (DVR), which are derived from the efferent arterioles of juxtamedullary glomeruli. In addition to their role as conduits for blood flow, it is accepted that the vasa recta are countercurrent exchangers. That process, however, involves events which are more complicated than paracellular diffusive exchange of NaCl and urea. Urea transport in DVR is accommodated through the combined expression of endothelial and erythrocyte facilitated carriers while transport of water involves solute driven efflux across water channels. Unlike DVR, which have a continuous endothelium, ascending vasa recta (AVR) are fenestrated with a very high hydraulic conductivity. Transport of water in AVR is probably governed by transmural hydraulic and oncotic pressure gradients. The parallel arrangement of DVR in outer medullary vascular bundles coupled with their capacity for vasomotion implies a role for regulation of the regional distribution of blood flow within the medulla The importance of the latter process in the urinary concentrating mechanism and the exchange of nutrients and O2 is poorly defined. The large number of hormones and autacoids that influence DVR vasomotion, however, suggests that DVR have evolved to optimize the functions of the renal medulla.
肾髓质的血流通过直小动脉降支（DVR）供应，直小动脉降支源自近髓肾单位的出球小动脉。除了作为血流的通道外，直小动脉被认为是逆流交换器。然而，这个过程涉及的事件比氯化钠和尿素的细胞旁扩散交换更为复杂。直小动脉降支中的尿素转运通过内皮细胞和红细胞易化载体的联合表达来实现，而水的转运则涉及溶质驱动的跨水通道流出。与具有连续内皮的直小动脉降支不同，直小动脉升支（AVR）有窗孔，水力传导率非常高。直小动脉升支中的水转运可能受跨壁水力和胶体渗透压梯度的控制。直小动脉降支在外髓质血管束中的平行排列及其血管运动能力意味着其在调节髓质内血流的区域分布中发挥作用。后一过程在尿液浓缩机制以及营养物质和氧气交换中的重要性尚不清楚。然而，大量影响直小动脉降支血管运动的激素和自分泌物质表明，直小动脉降支已经进化以优化肾髓质的功能。
