Medullary osmotic gradient: countercurrent multiplier, urea recycling & vasa recta countercurrent exchange

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▪ The primary cause of the medullary osmotic gradient is the active transport of solutes. • In the ascending limb of the loop, active transport of Na+ ions drives passive reabsorption of Cl- ions. • Addition of these ions to the interstitial fluid of the medulla increases its osmolarity. • Squamous epithelial cells of the descending limb of the loop are permeable to water but impermeable to most solutes. • Water leaves the filtrate in the descending limb of the loop, but the solutes cannot enter, thus increasing the filtrate osmolarity. • Due to water movement, new filtrate entering the descending limb becomes more and more concentrated as it flows to the bottom of the loop. • The cuboidal epithelial cells of the ascending limb provide for active reabsorption of Na+ and Cl- ions, but are impermeable to water. • Due to active reabsorption of solutes along the ascending limb, the filtrate being concentrated in the loop bottom becomes more and more diluted towards the distal convoluted tubule. • The limbs of the loop are close enough that each influences the processes occurring in the other. • Water moves out of the descending limb and produces the more "salty" filtrate toward the loop bottom. • In the ascending limb, the solutes pumped out of the concentrated filtrate increase the medullary osmotic gradient. • More solutes leaving the ascending limb cause more water to leave the descending limb and vice versa. • These processes multiply each other until the dynamic equilibrium is achieved between osmolarity of fluids in the different limbs of the loop of Henle and the surrounding medullar space. • This mechanism that constantly establishes the osmolarity gradient thoughout the renal medulla is called the countercurrent multiplier. • The effect of urea recycling greatly increases the medullary osmotic gradient values to their final amounts. ▪ When filtrate enters the medullary part of the collecting duct, most water has been reabsorbed, leaving urea relatively concentrated. • Collecting duct cells are highly permeable to urea, so urea diffuses into the medulla, thus increasing the interstitial osmolarity. • The rest of the nephron tubules are poorly permeable to urea, therefore, the urea is recycled back to the collecting duct in the medulla. • Along with sodium chloride, urea provides a great deal of the solute load in the medulla, producing much of the medullary osmotic gradient. • An ordinary capillary carrying blood from the cortex, and through the medulla, would remove the solutes necessary to generate the medullary osmotic gradient. • The shape of the vasa recta follows the limbs of the loop, providing a mechanism to maintain the gradient. • Blood enters the medulla of the kidney with normal osmolarity. • As blood moves into the medulla, highly permeable vasa recta capillaries exchange solutes with interstitial fluid. • Blood osmolarity increases. • As the blood moves out of the medulla, up to the cortex, it loses solutes. • Blood osmolarity decreases nearly to the normal value. • The small increase of osmolarity in the blood leaving the vasa recta: • Is the result of the blood colloid osmolarity. • Indicates that some water is lost from the body. • Tissues are provided with nutrients and oxygen, but solutes that maintain the medullary osmotic gradient are not transported away from the nephron.

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