Organization of nephron function.

Recent studies of mammalian nephron segments have revealed an unexpected diversity of renal transport functions. Most substances are transported by several segments, and the transport mechanisms differ from segment to segment. In this paper we review some of these findings in order to fit them into an integrated picture of kidney function. The main question we ask is what is the advantage of spatial separation of the various transporters along the nephron. We propose that spatial separation of transport functions allows independent and efficient control of the excretion of substances whose renal handling is interdependent. The organization of sodium and water handling along the nephron is considered in the greatest detail. Sodium and water play central roles in controlling the renal excretion of many other substances. Yet the excretion rates of sodium and water themselves are independently controlled. The intricate anatomical structure of the kidney not only provides spatial separation of transport processes along the nephron but also couples the function of different segments by juxtaposing them in specialized regions within the medulla and cortex. These anatomical arrangements provide a framework for integrating the complex array of renal functions.