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Urine Formation - can only reabsorb a limited...

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Urine Formation Tubular Reabsorption and Secretion Filtration removes wastes from the blood, but many useful solutes are also removed. Tubular reabsorption recovers these substances from the tubular filtrate into the interstitial fluid and thence into the peritubular capillaries surrounding the tubules. Some substances will actively be transported out of the blood (into the filtrate) by a process known as tubular secretion. Proximal Convoluted Tubules and Peritubular Capillaries The PCTs reabsorb 65% of the filtrate into the peritubular capillaries. Unsurprisingly, the PCTs are very long, have prominent microvilli and abundant mitochondria to provide energy for active transport. Reabsorption of solutes can occur through the epithelial cells of the PCT or between the epithelial cells of the PCT. Because most reabsorbed solutes are water soluble, they require proteins to be transported from the filtrate into the blood and the proteins can become saturated . If excess solutes, for instance glucose, are removed from the blood, then the glucose transporter
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Unformatted text preview: can only reabsorb a limited amount (because they can become saturated) and the remainder will be excreted in the urine -- a hallmark of diabetes. As solutes are reabsorbed, water is also absorbed by osmosis and carries other solutes with it paracellularly in a process known as solvent drag. The unusually high osmotic pressure in the peritubular capillaries around the PCT and low BP also favors reabsorption of water into the blood. This "obligatory" reabsorption of water is necessary to reabsorb solutes and does not change significantly in response to environmental changes. The major mechanism for transport of water soluble solutes from the tubular fluid into the PCT epithelial cells is by secondary active transport. From the cells to the blood it is often facilitated diffusion. However, many other solutes move by solvent drag or transcytosis. Examine the figure below for details....
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