Kidney, Normal Histology
The kidneys are paired, bean-shaped organs located in the retroperitoneum.
Kidneys filter blood in a three-step process: (1) the nephrons filter blood through the capillary network in the glomerulus (glomerular filtration); (2) the filtrate is collected in the renal tubules (tubular reabsorption); and (3) additional solutes and wastes are secreted into the kidney tubules (tubular secretion).
Juxtaglomerular cells are specialized cells located within the afferent arterioles of the kidney which produce renin to control blood pressure.
Size and Weight
Adult kidneys average 150 g and are approximately 12 cm long, 6 cm wide, and 3 cm thick.
Each kidney consists of an outer cortex and an inner medulla. The medulla has roughly 12 pyramids, with their bases at the corticomedullary junction. A medullary pyramid and its overlying cortex constitute a renal lobe.
The nephron is the functional unit of the kidney and includes the glomerulus and its tubule, the latter terminating at a common collecting system. The glomerulus is a specialized network of capillaries covered by epithelial cells named podocytes and supported by modified smooth muscle cells called mesangial cells (Fig. 1). As it enters the glomerulus, the afferent arteriole branches into capillaries, which form the convoluted glomerular tuft and eventually coalesce into the efferent arteriole that exits the glomerulus. The Bowman space lies between the podocytes and the epithelial cells that line the Bowman capsule. The major segments of the tubule that arise from each glomerulus are the proximal tubule, loop of Henle, and distal tubule, which empties into the collecting duct. At the origin of the proximal tubule from the glomerulus, the flat epithelium of the Bowman capsule abruptly transforms into tall columnar cells of the proximal tubule, which have numerous tall microvilli forming a brush border. The initial segment is very tortuous and is called the proximal convoluted tubule. As it descends into the medulla, the proximal tubule straightens into the thick descending limb of the loop of Henle. Further into the medulla, the thick descending limb thins into the thin limb of the loop of Henle, which eventually loops back toward the cortex. Approaching the cortex, the thin limb becomes the thick ascending limb. This abuts the glomerulus from which it arose, contributes to that glomerulus’ juxtaglomerular apparatus, and then becomes the distal convoluted tubule. Several distal tubules unite to form a collecting duct, which ultimately empties into the ducts of Bellini, which discharge urine through the papillae into the calyces. The juxtaglomerular apparatus is at the hilus of the glomerulus and consists of (I) macula densa, a region of the thick ascending limb of the loop of Henle with closely packed nuclei; (II) extraglomerular mesangial cells, between the macula densa and the hilar arterioles; and (III) terminal afferent arteriole and proximal efferent arteriole. The wall of the afferent arteriole contains characteristic granular cells involved in the synthesis and secretion of renin and angiotensin.