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Renal Tubule Renal tubule consists of : 1-proximal convoluted tubule(PCT). 2-Loop of Henle(LH). 3-Distal convoluted tubule(DCT). Renal corpuscle, PCT,DCT lie in the renal cortex. Loop of henle extends in to the renal medulla makes a hairpin turn and then return to the renal cortex. In a nephron, the loop of Henle connects PCT and DCT. the first portion of loop of henle dips in to the renal medulla and called descending limb of loop of Henle ,then bends in a U shaped and returns to the renal cortex as ascending limb of loop of Henle. The DCT of several nephrons empty in to a single collecting duct. Proximal convoluted tubule: is lined by cuboidal epithelium with numerous apical microvilli which give a brush border appearance. The basolateral membrane of the cell is highly folded and this area contains plenty of mitochondria. 65% of water and 100% of some solutes like glucose and amino acids in the filtrate are absorbed in the PCT. Organic acids, bases like drugs and drug metabolites are secreted in the PCT. Distal convoluted tubule and collecting duct: they are lined by cuboidal epithelium with few microvilli. Two types of cells are present in DCT and collecting duct: 1- Principal cells: which respond to ADH and aldosterone. 2- Intercalated cells: Which secrete hydrogen ions. 1 Juxtaglomerular apparatus: In each nephron, the ascending limb of loop of Henle makes contact with the afferent arteriole of its own renal corpuscle. The cells of the renal tubule in this region are tall and crowded together and that part is known as macula densa. These cells monitor the Na+ and Cl˗ concentration of fluid in the tubular lumen. The wall of afferent arteriole contains modified smooth muscle fibers of tunica media, called Juxtaglomerular cells. the granules in the Juxtaglomerular cells store renin. Between the afferent and efferent arterioles in this area are present Lacis cells, Juxtaglomerular cells, Lacis cells and macula densa together constitute Juxtaglomerular apparatus. 2 Function of Juxtaglomerular apparatus: JGA helps to regulate arteriole blood pressure, Na+ conservation, K+ excretion and rate of formation of glomerular filtrate by the kidney. Glomerular filtration: Glomerular filtration occurs across the glomerular membrane. The Mechanism is similar to tissue fluid formation. Since the pressure in the glomerular capillary is very high, water and solutes which are ˂ 4nm are forced through the glomerular membrane in to the capsular space. This fluid is called glomerular filtrate which is an ultra filtrate of plasma. About 180 L of filtrate enter the capsular space each day. Out of this 178179L return to circulation by tubular reabsorption. Only 1-2L are excreted as urine. 3