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Renal anatomy and overview of nephron function
Nephrology Nursing Journal, April, 2003 by Christine Chmielewski
Macroscopic Anatomy
The renal parenchyma consists of two distinct regions, the cortex and medulla (see Figures 3 and 4). The cortex has a granular appearance because of the structures contained in this layer, namely, the glomeruli, proximal and distal tubules, cortical collectingtubules, and adjacent peritubular capillaries. The medulla contains triangular wedges that have a striped appearance. These wedges are the renal pyramids formed by the long loops of Henle, medullary collecting tubules, and vasa recta. The tapered end of the pyramid, the papilla, directs urine toward the minor and major calyces. Urine then enters the hollow, funnel-shaped renal pelvis, which has avolume of 5 ml to 10 ml, before flowing into the ureters.
Microscopic Anatomy
The nephron is the basic functional unit of the kidney. There are approximately 1 million nephrons in each kidney. There are two types of nephrons, cortical and juxtamedullary, named according to the location of their glomeruli within the renal parenchyma (see Figures 3 and 4). The cortical nephrons, which comprise about85% of the total nephrons, are subdivided into superficial and midcortical nephrons. The superficial cortical nephrons have their glomeruli in the outer cortex and have short loops of Henle. The midcortical nephrons, as their name suggests, have their glomeruli in the midcortex region. Their loops vary in length and may be either short (contained within the cortex) or long (extending partiallyinto the outer medulla). The cortical nephrons perform excretory and regulatory functions. The juxtamedullary nephrons make up the remaining 15% of the nephrons. Their glomeruli are located deep in the cortex near the corticomedullary border. They have long loops of Henle that descend into the medulla often to the tips of the pyramids. These nephrons play an important role in the concentration anddilution of urine by generating a steep interstitial fluid osmotic gradient between the cortex and deep medulla. The vasa recta are responsible for maintaining this gradient.
Glomerulus. This specialized capillary bed is a network of interconnected loops surrounded by Bowman's capsule. As described above, the glomerular capillaries have unique characteristics that contribute to its filteringcapabilities. The porosity of the endothelial layer increases capillary permeability, the meshlike structure of the basement membrane provides a barrier to large molecules, and the portal structure allows maintenance of an intracapillary pressure that favors filtration.
The glomerular membrane has three layers: (a) endothelial, (b) basement membrane, and (c) epithelial. The endothelium lines thecapillary lumen and contains many pores, or fenestrae, that favor the filtration of fluid and small solutes. The glomerular basement membrane (GBM) is a matrix of collagen and similar proteins as well as glycosaminoglycans that provides a size and charge barrier to the movement of large particles out of the capillary lumen. The visceral epithelial cells of Bowman's capsule, or the podocytes, havecytoplasmic foot processes that extend over the basement membrane. Spaces between these foot processes are called slit-pores and allow the filtrate into Bowman's space. Mesangial cells are located between the capillary loops of the glomerulus and form a support network within the tuft. Some of these cells have phagocytic properties.
The glomerular membrane allows filtration of fluid and smallmolecules. Large molecules are prevented from entering the filtrate in two ways. First the size of the spaces in the glomerular epithelium and basement membrane limits the passage of these larger molecules and cells such as the white and red blood cells and albumin. Second, the podocytes and, to some extent, the GBM have a net negative charge that repels large negatively charged molecules, particularly...