The kidney, which receives approximately 20% of the cardiac output, has one of the richest and most diversified endothelial populations found within any organ. Such diversity may be explained on at least two counts. First, phenotypic diversity reflects the capacity of the endothelium to contribute to differential transport capabilities across the various segments of the nephron. Second, the endothelium must withstand unparalleled environmental extremes in oxygenation and osmolality (Figure 138.1; for color reproduction, see Color Plate 138.1). For example, endothelial cells (ECs) in the outer cortex are exposed to a normal osmolality and oxygen (O2) tension, whereas those in the inner medullary region are exposed to an osmolality of up to 1,200 mOSM and O2 content as low as 20 mm Hg.

From a mechanistic standpoint, phenotypic diversity of the renal endothelium must arise from site-specific epigenetic modification of ECs and/or reversible effects of the tissue/blood microenvironment. To date very little is known about the different endothelial populations within the kidney, let alone the molecular basis for their heterogeneity. Thus, the goals of this chapter are to delineate current knowledge concerning ECs in the kidney, stimulate research related to understanding the differing functions of renal ECs under physiological conditions, and explore the role of phenotypic diversity in disease processes.

STRUCTURE AND FUNCTION

The unique structure of the vascular tree within the kidney is shown in Figure 138.1. Anatomically, the kidney is divided into four zones. The cortex occupies the outermost aspect of the kidney. As one moves toward the renal pelvis from the cortex, one encounters the outer medullary region, consisting of outer and inner stripes, and finally the inner medullary region.