Renal reabsorption of HCO3 – Acid Base Balance

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.