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URINARY SYSTEM II – The tubes Roger Bick . Key Words: Proximal tubule, thin limbs, thick ascending limb, JGA, distal convoluted tubule THE TUBULAR SYSTEM The following diagram is taken from your text to facilitate study with your syllabus. 1. Proximal Tubule (PT) Structure: The epithelial cells lining this tubule are simple cuboidal to columnar and are hallmarked by the presence of a dense microvillous border, or brush border , that greatly increases the luminal surface area for absorption of filtrate. This specialization is found only in the PT, and not the other portions of the nephron. The cytoplasm of these cells is abundant and acidophilic. PT cells display abundant endocytic vesicles for pinocytosis of macromolecules such as proteins which have filtered, either normally or abnormally. These cells also have an elaborate development of the lateral surfaces (interdigitations with neighboring cells), giving them a somewhat stellate appearance three-dimensionally. The lateral processes contain abundant mitochondria for energy for transport; the lateral plasma membranes provide surface for Na-K-ATPase and other enzymes. The tight junctions at the cell
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apices are shallow and leaky, providing a large paracellular pathway for filtrate to move from the lumen to the interstitium through the network of intercellular channels. The proximal tubule has two parts: The convoluted part (proximal convoluted tubule), begins at the urinary pole and for a variable distance is highly coiled. It lies entirely in the cortex. It then straightens out and extends as a straight piece from the cortex into the medulla as the straight part or thick descending limb. Function - The PT reabsorbs about 85% of the filtrate isosmotically via active and passive transport processes. It is thus the major site of reabsorption of glucose, amino acids, electrolytes, water, and due to its pinocytotic activity, proteins. 2. Thin Limbs a. Structure The tall columnar-cuboidal cells of the thick descending limb abruptly change to the simple squamous lining cells of the thin descending limb . These cells have scanty cytoplasm with few mitochondria. All nephrons have thin descending limbs that descend variable distances in the medulla. In many nephrons (long-looped nephrons), thin limbs go all the way down to the tip of the papilla, turn, and ascend back up to the border of the inner and outer medulla where the thick ascending limb (TAL) begins. In other nephrons (short-looped nephrons), the thin descending limb curves at that same border and feeds directly into the TAL. In these nephrons, therefore, there is no thin ascending limb at all. b. Function - Studies in lower animals indicate that the thin limbs participate in urine concentration. Juxtamedullary nephrons (long-looped nephrons) are of prime importance in establishing the gradient of hypertonicity in the medullary interstitium.
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