The juxtaglomerular (JG) apparatus. This apparatus brings into close apposition the afferent (A) and efferent (E) arterioles with the macula densa (MD), a specialized region of the thick ascending limb (TAL). The extraglomerular mesangium (EM), or lacis "Goormaghtigh apparatus (cells)," forms at the interface of these components. MD cells express the Na-K-2Cl (sodium-potassium-chloride) cotransporter (NKCC2) at the apical membrane [10,11]. By way of the action of this transporter, MD cells sense the sodium chloride concentration of luminal fluid. By way of mechanisms that are unclear, this message is communicated to JG cells located in and near the arterioles (especially the afferent arteriole). These JG cells increase renin secretion when the NaCl concentration in the lumen is low [12]. Cells in the afferent arteriole also sense vascular pressure directly, by way of the mechanisms discussed in Figure 2-9. Both the vascular and tubular components are innervated by sympathetic nerves (N). B—Bowman's space, G—glomerular capillary; IM—intraglomerular mesangium. (From Barajas [13]; with permission.)

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