1. Introduction to the physiology of the kidney. 2. The glomerulus filters the blood. 3. The structure of the glomerulus allows efficient, selective filtration. 4. Glomerular filtration rate is determined by the mean net filtration pressure, permeability of the filtration barrier, and area available for filtration. 5. The filtration barrier is selectively permeable. 6. Glomerular filtration rate is regulated by systemic and intrinsic factors. 7. Glomerular filtration rate is measured by determining the plasma clearance rate of a substance. The glomerular tuft is composed of a network of capillaries (Figure 41-2). In mammals, renal arterial blood flows to the afferent arteriole, which divides into numerous glomerular capillaries. The capillaries anastomose to form the efferent arteriole, which conducts the filtered blood away from the glomerulus (Figure 41-3). Avian kidneys contain both mammalian-type and reptilian-type nephrons; in glomeruli of reptilian-type nephrons, the capillaries have few branches. The structure of the glomerular capillaries is important in determining the rate and selectivity of glomerular filtration. The wall of the capillary consists of three layers: the capillary endothelium, the basement membrane, and the visceral epithelium (Figure 41-4). The capillary endothelium is a single layer of very thin cells that faces the blood in the capillary lumen. Endothelial fenestrae (“windows”) are transcellular pores that conduct water and noncellular components in the blood to the second layer of the glomerular capillary wall, the glomerular basement membrane (GBM). The GBM is acellular and composed of various glycoproteins, primarily laminins, type IV collagens, nidogens, and the heparin sulfate proteoglycans, agrin in mature animals and perlecan in developing glomeruli. Compared to other basement membranes, the GBM is thicker and contains distinct glycoprotein isoforms. The GBM has three layers, created during development by the fusion of the basement membranes of the endothelial and epithelial cell layers. The three layers are named according to their density and relative position. As shown in Figure 41-4, the lamina densa (dense layer) is relatively dark because it is relatively resistant to the passage of electrons when viewed with a transmission electron microscope. The lamina densa is composed of tightly packed glycoprotein fibrils. It is sandwiched between the lamina rara interna (inside thin layer) on the endothelial side of the GBM and the lamina rara externa (outside thin layer) on the epithelial side of the GBM. The laminae rarae are composed of a loose network of glycoprotein fibrils. The third compartment of the glomerular capillary wall is the visceral epithelium, which is a layer of intricate, interlocking cells called podocytes. Numerous long, narrow extensions, the primary and secondary foot processes, interdigitate with foot processes from other podocytes and wrap around the individual capillaries (Figure 41-5). The epithelial slit diaphragm spans between adjacent foot processes (see Figure 41-4). The transmembrane protein, nephrin, is a critical component of this structure; the extracellular domain of nephrin molecules extending from adjacent foot processes interacts to form the slit diaphragm.
Glomerular Filtration
The Structure of the Glomerulus Allows Efficient, Selective Filtration
Glomerular Filtration Rate Is Determined by the Mean Net Filtration Pressure, Permeability of the Filtration Barrier, and Area Available for Filtration
You may also need
Glomerular Filtration
Only gold members can continue reading. Log In or Register a > to continue