Titratable Acids

00:01 Now you also have a few other mechanisms to get hydrogen ions out of this apical membrane in the proximal tubule.

00:12 So, we talked about the sodium hydrogen ion exchanger and that was very important for eventually binding into bicarbonate.

00:21 You also have a secondary hydrogen ion transporter, which is a hydrogen ion pump or ATPase.

00:29 So, this HATPase will also push hydrogen ions across the apical membrane.

00:36 No, why is this going to be an important process? Well, remember that a certain portion of the acid that you took in by a non-volatile acids, you are going to have to excrete the hydrogen ion and make a new bicarb.

00:53 So, with the Titratable acids is one of the ways that occurs.

00:57 So, let’s take phosphate as our example.

01:00 So, phosphate will be coming down the renal tubule, and you noticed phosphate has a negative charge on it.

01:08 So, this is HPO4, 2-.

01:14 You will kick out hydrogen ion from the hydrogen ion exchanger or form the V type hydrogen ion pump.

01:22 It will bind then to phosphate. Once it’s bound as phosphate, you can urinate it out.

01:32 So, this is how you can remove a hydrogen ion, is you push it across the apical membrane, have it bind to something and then, have it be removed from the system.

01:45 You don’t have it bind to bicarb, why? Because you want to reabsorb bicarb.

01:50 But phosphate, you can get rid of some of your phosphate.

01:53 But you have to reabsorb your bicarb so you don’t urinate out your bicarb, you bind it rather to one of these Titratable acids.

02:03 The hydrogen ions that then will be left will be in a bound form.

02:10 You may ask, how low can you get your pH of your urine? They say about 4,4 is about the lowest pH of your urine.

02:20 but that could be anywhere from 4,4 up to whatever is in the blood, maybe 7,4 or so forth.

02:27 So, that’s kind of your range of pH of your urine.

02:32 You noticed that phosphate has a pK of around 6,8.

02:36 This becomes important because it’s kind a maximally bind hydrogen ions at it’s pK which is 6,8.

02:45 So, you can utilize this when your pH is very close to 7.

02:51 Some of the other Titratable acids such as urate and creatinine, these have a lower pK.

02:58 Therefore, the pH of the urine has to be lower Before you’re going to be able to utilize these two great extent.

03:06 You also have lactate and pyruvate but those pH’s are actually, or pK’s are below the normal pH That you can obtain in the urine.

03:15 So, these are less important. So, they have very minor roles because of the lower pK values.

03:21 In or with Titratable acids, you can, or make new bicarb in the proximal tubule, the distal convoluted tubule and the collecting duct.

03:33 You make about 40 percent of your new bicarb using titratable acids, phosphate, urate, and creatinine.

03:43 Which one are you going to use? Depends upon the pH of the urine and it’s pK value.

03:50 So, if we look at something like a pH range here for something like phosphate has a wide pH range, so you can utilize that throughout many different pH’s of the urine.

04:02 Something like creatinine, is a little bit tighter. And so, you can’t always use creatinine to bind those hydrogen ions to be able to urinate them out.