 In this video we will try to understand the renal handling of glucose and basically try to understand this particular graph which is given in many books. So for this we should know the fundamentals of handling of glucose by the nephron. So here is a schematic diagram of nephron. So suppose if the plasma concentration of glucose is 100 milligram per 100 ml which is the physiological concentration of glucose and say suppose GFR is 125 ml per minute then because this glucose is not bound to plasma proteins and is freely filtered at the level of the glomerulus we can easily determine how much is the filtered load of glucose. So basically in 100 ml there is 100 milligram of glucose so in 125 ml how much glucose will be there it will be 125 milligram and since we have to say how much is the filtered load per minute so this amount is the filtered load per minute. It is also known as tubular load that is the amount of glucose which the tubules have to handle. A simple formula to determine this filtered load or tubular load of glucose is we take the plasma concentration of the substance and multiply it with the glomerular filtration rate but remember to use this formula the substance should be freely filtered at the level of the glucose that means it should not be bound to the plasma proteins because plasma proteins are held in the blood they are not filtered. Okay so now what does the tubule do with the filtered load see here this part is the tubule and this is the interstitial space and this is the capillary. Now this tubule is lined by tubular epithelial cells and what happens is that the filtered glucose is reabsorbed by this tubular epithelial cells and enters into the interstitial space finally enters into the capillary and this process completely happens in the proximal convoluted tubule. So to understand how this is occurring let's magnify this particular portion shown here. So this side is the tubular fluid so the filtered glucose will be present here green color shows the epithelial cell this is the interstitial space and here is the capillary. Now the glucose has to pass from tubular fluid towards the interstitial space then to the capillary. See the part of the epithelial cell which faces a tubular fluid is known as apical membrane while the rest of the part of the membrane of the epithelial cell is known as basolateral membrane. So these membranes have different transporters which help in the absorption of the glucose. The apical membrane has sodium glucose transporter especially the type 2 transporter SGLT-2 while the basolateral membrane has Glute-2 transporter. Now this SGLT transporter transports glucose by secondary active transport into the cell and from there glucose passes by facilitated diffusion via these Glute-2 transporter and then once it enters into the interstitial space it directly enters into the capillary by passive diffusion. Okay so fundamentally the transport of glucose is an active process. See if in the transport process anywhere energy is required which in this case is required by this SGLT transporter then it is an active process. So any active process of transport has a maximum rate of transport which is known as saturation kinetics that means after a certain limit that rate of transport cannot increase further. So for glucose this maximum rate of transport is 375 milligram per minute. This maximum rate of transport by the nephrons is known as tubular maximum of glucose or DMG. That means the epithelial cells cannot transport or reabsorb glucose faster than this rate. Now sometime back we said that tubular load is equal to plasma concentration of the substance into GFR. So that means if we assume that GFR is fairly constant if plasma concentration of the substance increases then the tubular load also increases. So it's kind of a linear relationship between the plasma concentration of the substance and its tubular load. This we can represent in a graph form also. So here if x-axis represents a plasma glucose concentration in milligram per hundred ml and the y-axis represents the glucose the filtered load in milligram per minute then we can draw this straight line. So this represents that as plasma concentration is increasing the tubular load of glucose is increasing. Now you see as the tubular load increases the rate of transport also increases. So say suppose the plasma concentration is 100 milligram per 100 ml then tubular load is 125 milligram per minute then the rate of transport will be 125 milligram per minute because as filtration is increasing the transporters will work more and there will be a reabsorption of all the filtered load. Now say suppose the plasma concentration increases to 200 milligram per 100 ml. So then the tubular load by the same formula it will become 250 milligram per minute. So then again the reabsorption rate will increase and all the glucose will be reabsorbed. See it has not yet reached the maximum limit of reabsorption. Now say suppose if the plasma concentration becomes 300 milligram per 100 ml then you see the tubular load will be 375 milligram per minute. So yes this will be again reabsorbed but now if the plasma concentration increases a little bit more now the transporters have reached their maximum limit they cannot reabsorb the glucose faster than this value 375 milligram per minute. So what will happen that the glucose will start appearing in the tubular fluid and it will appear in urine. So let's draw this in the graph which we have plotted. So here the rate of transport will increase linearly as the filtered load is increasing till the time it reaches to 375 milligram per minute and then it will become constant. So after that whatever glucose is getting filtered it will start appearing in urine. So at this level glucose should start appearing in urine but in actuality doesn't happen. What happens is in actuality we see that glucose starts appearing in urine at much lesser value at about the plasma concentration of 200 milligram per 100 ml the glucose starts appearing in urine. Then it is not matching with what we have discussed till now. Well this is what is known as display. See what happens is that this value of 375 milligram per minute is the maximum rate of transport of all the nephrons. So it's kind of an average of the rate of transport of different nephrons. So there may be some nephrons which have the rate of transport of say suppose 300 milligram per minute while other may be having 275 milligram per minute or 250 milligram per minute while there may be others say suppose 400 or 450 milligram per minute. So if we average all of this it will come to 375 milligram per minute but the nephron whose maximum rate of transport is 250 milligram per minute what will happen that at plasma concentration of 200 milligram per 100 ml the glucose will be present in the tubular fluid beyond proximal tribute isn't it because it will not be reabsorbed completely. So when the contents of the tubular fluid of this particular nephron mixes with the content of tubular fluid of other nephrons this glucose will appear in urine isn't it. So fundamentally there is a deviation from what we have expected. This is because of different nephron having different rate of transport what is known as nephron heterogeneity. So this graph actually which we drew as a perfect sharp bend it's actually like this it's a curve so this is known as spleen this is the expected thing but this is the actual thing which is happening. So if we extend it below to see that at what plasma concentration the maximum rate of transport of this particular nephron is reached we see that it is at around 200 milligram per 100 ml. So at this plasma glucose concentration glucose will start appearing in urine. So the plasma concentration at which glucose starts appearing in urine is known as renal threshold for glucose. So that's how the glucose is handled by the nephrons. Well thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel Physiology Open.