Transport & Excretion Figs and Salt and Water Balance

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Transport & Excretion Figs and Salt and Water Balance

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 38 Current Lecture Reabsoprtion of Na + , Glucose, HCO3- , etc. Protons that are extruded across the apical membrane combine with bicarbonate ions in the tubular fluid (yellow). These bicarbonate ions are filtered bicarbonate ions – they came out in the filtrate along with everything else. They create carbonic acid, which dissociates to CO2 and water. CO2 is very permeable, so it easily permeates through the apical membrane, basolateral membrane, and between adjacent cells. There is always CO2 coming out of the tubular fluid. In the epithelial cell, some of the CO2 produced in the tubular fluid enters the epithelial cell, where it combines with water. And there are high levels of carbonic anhydrase ions inside the epithelial cell, and that forms carbonic acid, which dissociates to form protons and bicarbonate. Its these protons, formed by the dissociation of carbonic acid, that provide the protons to be extruded across the apical membrane which is necessary for it to have sodium come in. The bicarbonate leaves the cell – in the basolateral membrane there is a sodium bicarbonate symport (one Na for 3 bicarbonate per cycle of the symporter). This symporter is driven by the bicarbonate gradient. The bicarbonate in the tubular fluid disappears as it combines with a proton, and another bicarbonate appears in the epithelium (H+ + HCO3). Almost all of the filtered bicarbonate gets reabsorbed by becoming H2CO3 and then H2O and CO2, which combines with water inside the epithelium to form HCO3, which is extruded from the epithelium with the Na. This is not only a mechanism for the reabsorption for sodium, but it is also one of the main mechanisms for reabsorption of bicarbonate. Because this is a 3 bicarbonate/1 sodium symport, this is an electrogenic transporter and makes the inside of the cell more negative. So far, we have movement of sodium and bicarbonate to the right (reabsorption). All the movement of these solutes will cause water to follow by osmosis. And things that are not transported will build up in concentration because water is moving to the right. There is a buildup in concentration of Cl-, for example, you can see the conc of 140mM in the tubular fluid but only 105 in the ECF. So the concentration of chloride is because water has left, and Cl is left behind. And that creates a gradient for Chloride to move across the epithelium. About 2/3 of the chloride goes through the paracellular route, going between the cells. And one third goes through chloride ion channels in both membranes. So the chloride movement is driven by the concentration gradient for chloride, which is created by the osmotic movement of water. Just like chloride got concentrated by the movement of water out of the lumen, many other solutes have been affected as well. Urea gets concentrated in the tubular fluid and in the proximal tubule and the distal tubule and the upper part of the collecting duct, urea is not permeable ...

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