>>Dr. Ketchum: Okay, then we have bicarbonate reabsorption. Okay, so same thing is going
to hold true here. We’re still in the proximal convoluted tubule, so that means you still
have a sodium-potassium pump on the basolateral membrane. So we know that we have low sodium
in the cell and we have high potassium in the cytoplasm of the cell. Keep in mind that
the plasma is over here, correct? And we want to reabsorb bicarbonate. We want to get bicarbonate
into the plasma, but we also want to secrete hydrogen ions. Let’s continue with sodium,
since we already know sodium's at a low concentration inside of the cell. If we look at the concentration
of sodium in the lumen of the proximal convoluted tubule, we know we have a high concentration
of sodium moving toward its low concentration. And then we know that hydrogen ions, right,
based on the previous slide, they’re at a low concentration and they’re moving toward
a high concentration. So number two here, then, is going to be sodium hydrogen counter-transport
across the apical membrane.
We also know—okay, let’s continue with hydrogen ions. We just said that we have a low hydrogen ion
concentration inside of the cell. So if hydrogen's going from low toward a higher concentration in
the lumen and this is using some sort of carrier protein, we know that can’t be facilitated
diffusion, correct, because that gradient is going low to high. So the only type of
transport that number three here could be then is a type of primary active transport.
If you go clear back to your chapter four notes, there is a table or a slide that gives
you examples of different types of primary active transport, and this is listed as one
of those. We’ve now secreted hydrogen ions.
So once those have been secreted, okay, then they can combine with bicarbonate. So we
filtered bicarbonate. That will allow us to form carbonic acid, H2CO3. And then here we
have a brush border enzyme, okay, carbonic anhydrase, that’s going to break down carbonic
acid into water and carbon dioxide. So carbon dioxide then can use simple diffusion to get
into the cytoplasm of the cell. Then it can combine with your water. Okay, we know water
can move into the cell as well, correct? Then that’s going to form, once again, using
carbonic anhydrase we’re going to form carbonic acid, which readily dissociates into your
hydrogen ions and your bicarbonate. So we’ve already talked about what happens to these
hydrogen ions; now let’s focus on these bicarbonate ions.
So we know that bicarbonate then is moving from a high concentration toward
a low concentration and we’ve already said that sodium is low inside the cell and it’s
moving from low to high. So this type of transport here, that we’ll label number four across
the basolateral membrane. So number four is sodium bicarbonate co-transport.
Then we also have bicarbonate being reabsorbed by using a chloride transporter. So again,
we have a high concentration bicarbonate. Chloride ions, then, must be moving from a
low concentration toward a high concentration inside the cytoplasm of the cell. So that
means step five here is called your chloride bicarbonate counter-transport.
There’s two more ways that you can get carbon dioxide inside of that cell. One of those,
if you remember, is via cellular respiration. So CO2, remember, is a product of
cellular respiration. So that gives us a second mechanism by which we can get carbon dioxide inside
of our cells. The other way is to think about where this is. This is the plasma; it’s
a peritubular capillary. When you reabsorb, you reabsorb into the venous end of the capillary.
So if this is the venous part of the capillary, it’s going to have a relatively high partial
pressure of carbon dioxide. Inside the cell we’re going to have a low partial pressure
of carbon dioxide. So that means that carbon dioxide can follow its pressure gradient
and move into the cell via simple diffusion.