00:01 Bicarb reabsorption occurs in the nephron. 00:05 About 80 percent of it is pulled off in the proximal tubule, about 10 percent in the thick ascending limb, about 6 percent in the distal convoluted tubule and about 4 percent in the collecting duct. 00:19 And I like to highlight here that you really absorb all of your bicarb. 00:23 you will not urinate any of it out in normal conditions. 00:27 In the proximal tubule, but let’s now take this into a little bit more detail. 00:33 How are you reabsorbing the bicarb? It’s actually more complex than you think because bicarb is a charged molecule. 00:44 If we don’t have a specific transporter, that’s going to be able to move it across the apical membrane. 00:52 So, we have a little and genius way of doing this. 00:55 We utilize a transporter called the NHE3, sodium hydrogen ionic exchanger. To kick out a hydrogen ion. 01:08 It kicks out this hydrogen ion combines to form bicarbonic acid. 01:14 Carbonic anhydrase then converts it into water and carbon dioxide. 01:22 So, now you have water, which can be in the renal tubule and carbon dioxide, which now can freely move across the membrane. 01:32 It moves into the cytosol, you have a different carbonic anhydrase, which is the enzyme that helps convert this back to bicarbonate acid. 01:42 So, you have two carbonic anhydrases. 01:45 Carbonic anhydrase 4, which is on the apical membrane. Carbonic anhydrase 2, which is in the cytosol. 01:53 This complex process all it did was allow you to get a molecule across the apical membrane. 02:01 Now, once you have formed carbonic acid it once again can disassociate into a hydrogen ion which then can be used to be kicked back out through the sodium hydrogen exchanger. 02:14 and the bicarb can be reabsorbed across the basolateral membrane. 02:19 So, that’s how you reabsorb 80 percent of your bicarb through this mechanism. 02:25 Across the basolateral membrane, which is the opposite side of the apical membrane, where the sodium hydrogen ion exchanger occurs. 02:33 There is two different ways we move bicarbonate out of the cell. 02:38 The first way to do that is by a co-transporter that involves sodium. 02:47 The sodium co-transporter then allows for both bicarb and sodium to travel across the basolateral membrane following the sodium gradient to move out bicarb. 03:01 The second mechanism that you would have to move bicarb out of the basolateral membrane is through an exchanger. 03:09 This exchanger uses chloride, and chloride moves into the cell and bicarb moves out of the cell. 03:17 This is an important process to think about because you need to now get the bicarb, which was in the cytosol out across the basolateral membrane. 03:27 So, either co-transported with bicarb and sodium or exchanged for chloride.
The lecture Renal reabsorption of HCO3 – Acid Base Balance by Thad Wilson, PhD is from the course Acid-Base Balance.
Which segment of the nephron reabsorbs the greatest quantity of bicarbonate filtered from the blood?
Which isoform of carbonic anhydrase is located on the apical membrane in the proximal convoluted tubule?
Which of the below transporters is located on the apical membrane of the proximal tubule cell?
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