Distal Convoluted Tubule (DCT): Tubular Transport

by Carlo Raj, MD

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    00:01 Continuing our discussion of the nephron where at the level of the loop and the distal convoluted tubule. Put these together and as we go through this, it is important that you pay attention here.

    00:11 Once again the physiology is important so that you can then make sense of the pathology.

    00:16 Otherwise things become really confusing and you start memorizing. There is no end to it.

    00:23 Overview: We are done with the PCT in its entirety. We are moving down through the descending limb, through the loop and as you move through the loop, tell me about the urine tonicity? Increased. Hypertonicity. As we move into the area of C in ascending limb of thick ascending limb and you have a symport there called your sodium- potassium-2 chloride and then, later on, we get into the portion where it says D as in delta. There will be distal tubule and there we shall take a look at our sodium chloride channel. Let us take a look at our loop. Now the loop is well as we moved down the descending limb, it is impermeable to water and thus, you have medullary hypertonicity. What kind of segment would you then call this? The concentrating segment. Good and makes the urine as you can imagine where? In the medullary loop of Henley as being hypertonic. Impermeable to water. What is my next step? You are going to move up the ascending limb and as you move up the ascending limb, what then happens? While you are focused, once again we have organized this cell plenty. Let us do it here once more.

    01:34 We have your urine on your left side, the lumen. Then you have the epithelial cell or the renal epithelial cell and then on the right side, you have interstitium in your blood that which is being reabsorbed or then move through your symports .

    01:49 So therefore what are you permeable to here? Take a look at C as in Charlie. Here in the thick ascending limb, you are permeable here to the solute. Correct. If you are permeable to solute, what are you impermeable to? Water. Okay. Another important concept. Sodium travels with whom? Chloride so that it maintains electroneutrality. You know that from basics. Next, along with sodium, wherever sodium goes with the solute what is its pressure called? What is its method of diffusion? It is called osmosis. Right. What does osmosis mean? It means diffusion of water from one compartment to another or you have increased solute. Are you clear? Interesting.

    02:38 So here in the thick ascending limb, you are ripping the solute away from the water. Where is the water in this picture? In the lumen. You are separating and ripping the solute away from the water. What do you call this water now? Free water. It has been freed.

    02:57 Freedom, liberty. Do as it wants. Prior to this, what kind of water did you have? Clinically we call this obligated water. Obligated water is when you have sodium bound with water.

    03:13 Here when you rip the solute apart and you are reabsorbing it, then the water that's left in your thick ascending limb is called free. Thus what do we call the distal convoluted tubule, the diluting segment? What is the urine osmolarity? Hypotonic. You listen to what I am saying. Hypotonic. What was it in the descending limb? It was hypertonic because it was permeable to water. What is this mechanism in which you have exchange of water and solute in this area between the descending limb and the ascending limb called? It is called the countercurrent mechanism. Remember this. Right. The countercurrent mechanism. Now we don't have enough time to go through all of that, but at least you know that the counter current mechanism is being established here so that you can have proper tonicity information of the urine. If you do not do all of this meaning the counter current mechanisms within the loop, you cannot properly concentrate the urine, thus you cannot properly dilute the urine. That is important for you to understand. Once again if this counter current mechanism is going to be pathologically affected, magic word, pathologically affected, you cannot properly concentrate this urine nor can you properly dilute this urine. Why is that important? You shall see. Now the important physiologic aspects of this that we have talked about already. Take a look at sodium-potassium- 2 chloride. You see it. That's your symport.

    04:49 What is the name of the drug that knocks it out quickly? Good. Loop of furosemide Very good. Next if you bring in that sodium- potassium into the cell, you increase the concentration. Where do you begin all off this process? Like everything else with the sodium-potassium pump. There is potassium that is accumulating inside my cell and this potassium is going to be forced to do what? Back leak, which then facilitates what? Take a look at the picture please. You see the magnesium and calcium that is being paracellularly reabsorbed.

    05:27 So if this entire mechanism of the loop where it destroys the sodium-potassium-2 chloride mechanism, you lose everything.

    05:36 Now the thick, continuing, actively reabsorbs here sodium-potassium-2 chloride, indirectly.

    05:42 What does it mean to you? Back leak will reabsorb magnesium, calcium. Why? As the potassium comes back out into the lumen, back leak, you are increasing the positivity. Right? Transmembrane.

    05:56 Increase in the positivity. So therefore you are literally going to what would likes do? Charges. Would you likes to do? Repel. So there it goes. It is repelling the magnesium, calcium between the cells. Think of it such. You will be fine. Now it is important that you pay attention to the potassium channel. You want to know the name of this potassium channel.

    06:17 Think of it as being the ROMK. At least know outer medullary potassium channel the receptor for.

    06:23 Receptor for the outer meduallary potassium, ROMK. Know this, because at some point in time when they refer to this, they now know that this is the back leak channel responsible for reabsorption of magnesium, calcium. You need to know that detail. Yes you do. Trust me. New questions all the time. Evolution of exams. Evolition of what we know so that you have the proper information and equipped properly so that any question that comes at you, you are not fooled and you know the level of detail that is required of you. Move on. Impermeable to water. We talked about, where am I? The thick ascending limb. What we call this? The beginning of the diluting segment. What does the diluting mean? There it is. Hypotonic. So down here.

    About the Lecture

    The lecture Distal Convoluted Tubule (DCT): Tubular Transport by Carlo Raj, MD is from the course Diseases of the Nephron.

    Author of lecture Distal Convoluted Tubule (DCT): Tubular Transport

     Carlo Raj, MD

    Carlo Raj, MD

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