The interlobular artery, afferent arteriole, efferent arteriole, and glomerulus are shown in Figure 33.6, with the afferent and efferent arterioles measuring approximately 15 to 20 µm in diameter. The podocyte surrounding a glomerular capillary is depicted in Figure 33.7, with the podocyte cell body containing a large nucleus and interdigitating foot processes. The filtration barrier of a glomerular capillary is presented in Figure 33.7, consisting of the endothelium, basement membrane, and foot processes of the podocytes. The filtration slit diaphragm is highlighted, bridging the floor of the filtration slits. The outer surface of glomerular capillaries is shown in Figure 33.8A, with podocyte processes extending towards the capillaries and forming filtration slits. The inner surface of a glomerular capillary is displayed in Figure 33.8B, showing fenestrations in the endothelial cells. Figure 33.9 illustrates the mesangium, which is the area between glomerular capillaries containing mesangial cells. The basement membrane is described as a porous matrix of negatively charged proteins, functioning as a charge-selective filter for plasma proteins.
Fig. 33.6 Scanning electron micrograph of the interlobular artery, afferent arteriole (af), efferent arteriole (ef), and glomerulus. The white bars on the afferent and efferent arterioles indicate that they are about 15 to 20 µm in diameter. (From Kimura K et al. Am J Physiol 1990;259:F936.) • Fig. 33.7 A, Electron micrograph of a podocyte surrounding a glomerular capillary. The cell body of the podocyte contains a large nucleus with three indentations. Cell processes of the podocyte form the interdigitating foot processes (FP). The arrows in the cytoplasm of the podocyte indicate the well-developed Golgi apparatus, and the asterisks indicate Bowman’s space. C, capillary lumen; GBM, glomerular basement membrane. B, Electron micrograph of the filtration barrier of a glomerular capillary. The filtration barrier is composed of three layers: the endothelium, basement membrane, and foot processes of the podocytes. Note the filtration slit diaphragm bridging the floor of the filtration slits (arrows). CL, capillary lumen. (From Kriz W, Kaissling B: Structural organization of the mammalian kidney. In: Alpern RJ, Moe OW, Caplan M [eds]: Seldin and Giebisch’s The Kidney, 5th ed. London, Elsevier, 2013.) (i.e., contains 700-Å holes, where 1 Å = 10−10 m) and freely permeable to water, small solutes (e.g., Na+ , urea, glucose), and most proteins but is not permeable to red blood cells, white blood cells, or platelets. Because endothelial cells express negatively charged glycoproteins on their surface, they minimize the filtration into Bowman’s space of albumin, the most abundant plasma protein, and most • Fig. 33.8 A, Scanning electron micrograph showing the outer surface of glomerular capillaries. This is the view that would be seen from Bowman’s space. Processes (P) of podocytes run from the cell body (CB) toward the capillaries, where they ultimately split into foot processes. Interdigitation of the foot processes creates the filtration slits. B, Scanning electron micrograph of the inner surface (blood side) of a glomerular capillary. This view would be seen from the lumen of the capillary. The fenestrations of the endothelial cells are seen as small 700-Å holes. (From Kriz W, Kaissling B: Structural organization of the mammalian kidney. In: Alpern RJ, Moe OW, Caplan M [eds]: Seldin and Giebisch’s The Kidney, 5th ed. London, Elsevier, 2013.) other plasma proteins. In addition to their role as a barrier to filtration, the endothelial cells synthesize a number of vasoactive substances (e.g., nitric oxide [NO], a vasodilator, and endothelin 1 [ET-1], a vasoconstrictor) that are important in controlling renal plasma flow (RPF). The basement • Fig. 33.9 Electron micrograph of the mesangium, the area between the glomerular capillaries containing mesangial cells. C, glomerular capillaries; cGBM, capillary glomerular basement membrane surrounded by foot processes of podocytes (PO) and endothelial cells; M, mesangial cell that gives rise to several processes, some marked by stars; mGBM, mesangial glomerular basement membrane surrounded by foot processes of podocytes and mesangial cells; US, urinary space. Note the extensive extracellular matrix surrounded by mesangial cells (triangles) (×4100). (From Kriz W, Kaissling B: Structural organization of the mammalian kidney. In: Alpern RJ, Moe OW, Caplan M [eds]: Seldin and Giebisch’s The Kidney, 5th ed. London, Elsevier, 2013.) membrane, which is a porous matrix of negatively charged proteins (type IV collagen, laminin, the proteoglycans agrin and perlecan, and fibronectin), is an important filtration barrier to plasma proteins. The basement membrane is thought to function primarily as a charge-selective filter in which the ability of proteins to cross the filter is based on charge.