everyone and welcome to miss estc biology in this video it's an updated version of my nefron video going through all the information you need to know about how the nefron filters the blood so let's get into it so what we'll cover in this video is the structure of the nefron function of the Ral capsule proximal convoluted tubal Loop of Henley distal convoluted tubal and the collecting ducts we'll go through how ultra filtration occurs and where it happens and what selective reabsorption is and again where it happens so starting off then just with an overview of the kidney you don't actually need to know the structures in detail all you need to know is that the filtering and osmo regulation which will be in a later video occur in the nephrons the nephrons are found within the medulla which is just here so the nephrons then what they actually are is these long tubules which are surrounded by capillaries and you have about 1 million nephrons in each of your kidneys so the structure of the nefron just zooming in on one here you have as we said the capillaries surrounding them and leading into the nephron you have an afferent arterial which then branches into lots and lots of smaller capillaries and that is what we call the Glamis so that there is lots of capillaries the Glamis now those lie inside of this capsule which is called the renal capsule sometimes called the Bowman's capsule or on this picture here the Glam capsule but the AQA spec calls it the renal capsule so that's what I'll be using after that it then leads into the proximal convoluted tubal which we can see winding around here into the loop of Henley then up to the distal conut tubule and the collecting duct so we'll go through what happens at each of these positions in the nephron to create urine so that's the overall function of the Nephron is to create urine um and that is because you're filtering the blood to remove waste so any excess water Ura is going to be removed useful substances will be selectively reabsorbed back into the blood so the urine will only contain water excess water dissolved salt or mineral ions Ura and any other small substances that um can be filtered out so it could be hormones or excess vitamins in a healthy person you should never find proteins blood cells or glucose and this is actually from gcsc as well the knowledge of knowing why you would never find a protein or blood cell in urine and why you'd never find glucose so just as a reminder the proteins and blood cells are both both too big to be filtered out so they'll always remain in the blood glucose does get filtered out but all of the glucose is reabsorbed by active transport in the selective reabsorption stage which occurs in the proximal convoluted tubule or PCT for short so an over view first of all of all of this filtering and reabsorption step one is in the Glamis you have this ultra filtration of water and small molecules due to high pressure and that will force out the small molecules and water into the renal capsule then you'll have your filtrate which is called the Glamis filtrate passing into the proximal convoluted tubule and at this stage 85% of that filtrate gets reabsorbed back into the blood the loop of Henley is the next stage and at this point the sodium ion gradient is maintain contained and that is to enable water to be reabsorbed by osmosis into the blood and then the final step we've got our distal convoluted tubal and collecting duct where further water is removed by osmosis or diffuses out by osmosis and is reabsorbed back into the blood and any of the remaining liquid in the collecting duct goes on to form urine so let's go through each of these stages then in detail starting with the ultra filtration so at this point we have blood entering through the afferent arterial and the arterial splits into lots and lots of smaller capillaries and as we said at the start that is what the Glamis is because you've gone from a wider Lumen or a larger space into lots of smaller narrower capillaries you end up creating this hydrostatic pressure so that high pressure forces out small molecules on water and it has to be small because it has to be small enough to be able to fit through the tiny gaps in the cells in the epithelium of the capillaries that forms the Glamis filtrate so we call it Glamis filtrate because it's formed from the glis staying in the blood you'll have large proteins and blood cells because they're too big to fit out of the gaps so they will then pass out of the eperen arterial and continue to circulate around the blood in the body so just to show you where that is happening we' got zoomed in here we see our afferent arterial splitting to make the Glamis and then leaving we have the efferent arterial so the ultra filtration will be occurring here and the Glamis filtrates will be going into this renal capsule so just here so we're going to go through in a bit more detail here how that filtration happens so we've talked about how the high hydrostatic pressure is generated and now we're going to zoom in on the capillaries so this is a capillary within that glaris and the capillaries have just a single layer of cells making up their endothelium so that's what these structures here are representing really really zoomed in looking at those cells making up the endothelium and there are tiny gaps between those cells that is your first place where the filtration happens so a bit like a Sie anything that's small enough to fit through those gaps will pass through then you have a basement membrane which again acts as a filter and then finally on the outside you have poto sites which are these cells that wrap around the capillary and there are tiny gaps between those as well which adds another filtration layer so here are the poter sites we can see on the outside of the capillaries wrapping around those and looking at it from a different angle those are the poly sites and there's tiny gaps again so you've essentially got three filtrations we've got the gaps between the endothelium of the capillary basement membrane and then the gaps between the podocytes so that's the ultra filtration so that Glamis filtrate is now going to flow and pass into the proximal convoluted tubule and this is where selective reabsorption happens so 85% of that filtrate that's just been created gets reabsorbed back into the blood um at this stage in the proximal convoluted tubal so we're going to go through how that happens before we do that just going to point out some adaptations of the cells lining the proximal comut tubule and that's what we're looking at here so this bit is the Lumen so that is the space in the middle that the filtrate passes through this bit proximal convoluted tubal cells so those are the epithelial cells we then got a slight gap which we call the interstitial space which is the gap between the proximal convoluted tubule and the capillaries that surround it and then here we have our capillary so the two key adaptations are the proximal convoluted tubal cells have all of these micro Villi and that creates a really large surface area to maximize the reabsorption of glucose there's also lots of mitochondria within these cells and that's because energy is needed for active transport at this stage so those are our adaptations what we need to look at next then is how selective reabsorption happens so step one the concentration of sodium ions in the proximal convoluted tubal which I've just abbreviated PCT um is low within that cell that is low because sodium ions are actively transported out of the PC into the bloodstream so that is why we have all these mitochondria to provide energy for the active transport of sodium ions out of the proximal convoluted tubal cells into the blood the impact that has is the cell here has um a very very low concentration of sodium ions compared to the Glamis filtrate which is going to be flowing through the Lumin so that then means that the sodium ion can move into the proximal convoluted tubu by diffusion going down their concentration gradient now the protein that the sodium irons diffuse through is a co-transporter protein and that particular protein both sodium ions and glucose attached to so when that sodium ion attaches so does glucose and therefore that is how glucose gets from the Glamis filtrate into the proximal convoluted tubu so the final step is you'll now have a large concentration of glucose within your proximal convoluted tubal cell and because you've got that high concentration compared to in the Bloods you have a concentration gradient so the glucose can diffuse from the PCT cell into the bloodstream and that is how all of the glucose is reabsorbed that was initially filtered out now one thing I just want to emphasize is reabsorbed if you just say absorb that is incorrect because absorb means it's the first time it was taken in this is reabsorption because it was already in the blood then it was filtered out but then we take it back into the blood so whenever you are talking about the kidneys you will only ever be using the term reabsorb so next then the filter would have passed through the PCT and now it's leading into the loop of Henley and at this stage the function of the loop of Henley is to maintain a sodium ion gradient so that's what we're going to have a look at but first of all just to see the structure in more detail so the loop of Henley we describe as being made up of two limbs so we have an ascending limb and a descending Limb and this is named after the direction that the filate is moving in so this side here is the ascending limb because at that point the filate is moving up it's ascending up through the loop of Henley and you can see on the diagram that it has really really thick walls and because of that it is impermeable to water so no water can actually move out of the loop of Henley in the ascending limb which is what these arrows are representing soons are going to be actively transported out but we'll come on to that the descending limb is where the filtrate is moving down the loop of penley and the walls are much thinner and therefore they are permeable to water and this is where the water is going to move out by osmosis to be reabsorbed into the blood so to go through the stages then of what's happening at the Loop of hening step one there are mitochondria within the walls of the ascending limb and that's to provide energy for the active transport of sodium ions so sodium ions are actively transported out of the filtrate into what we call the interstial space which is the space between the nefron and the capillaries in doing this there's an accumulation of sodium ions in this interstitial space in the medulla and it creates a really low water potential now the numbers that you can see here in the loop of Henley and in the interstitial space that is representing water concentration so we're having um a lower water concentration here we've got more sodium ions moving in so we have a more concentrated solution or a lower water potential because there's a lower water potential that means the water in the descending limb of the loop of Henley will move out bi osmosis into the interstitial space and then it'll be reabsorbed into the blood so that's how the water is reabsorbed into the blood F things to point out on the loop of Henley is step forward down here and it's at the very very base of the ascending limb because there's now very dilute um solution or very low concentration of sodium ions at the base some of the sodium ions will move out by diffusion so the next step in our nefron we've gone through the renal capsule proximal convoluted tubule the loop of Henley next is the distal convolute tubu and the collecting ducts next then is the distill convoluted tubal and the collecting duct and one thing just to point out although I've used PCT and dctt at different points in this video in the exam you do have to write out those full terms so proximal convoluted tual and now the distal convoluted tual so due to all the sodium ions that have been actively transported out of the ascending Loop of Henley by the time the fil trate gets to the top so at the distill computed tubal you actually have a very dilute filtrates that is remaining inside the tubule especially in comparison to the water potential of the medulla so as that filtrate moves into the distal convoluted tubu and then the collecting ducts you have that concentrated solution or very negative water potential and that is what causes even more water to move out by osmosis from the distal convoluted tubule and the collecting duct and whatever filtrate remains in the collecting duct goes on to form the urine so that is the whole process of ultra filtration and reabsorption and how urin is created an example of an application question that comes up to do with the loop of Henley is the one we've got here so in the past students have been asked to suggest how the length of the loop of Henley will differ for a desert animal compared to a human and then you'd be asked to explain why so he said the loop of Henley the function is to Main main the sodium iron concentration gradients so that more water can be reabsorbed so if they're in the desert they're going to need more water to be reabsorbed and therefore they'll have a longer Loop of Henley so that more sodium ions can be um actively transported out and therefore more water will be reabsorbed so if you've got this longer Loop of Henley there's a larger surface area for sodium ions to be actively transported out so you'll then have have even more sodium ions lowering the water potential more water will move out by osmosis and that gets re absorbed into the blood and as a result they get more water going back into the blood and very very concentrated urine which for a desert animal That's essential for survival because their environment there's very little water so whatever water they do get from their food or from their water it's essential that that gets reabsorbed back into the blood rather than lost and wasted in the urine so just to summarize then the nefron is made up of the renal capsule the PCT Loop of Henley DCT and collecting ducts and they're surrounded by capillaries the Glam filtrate is created in the renal capsule glucose and water are reabsorbed back into the blood by the PCT the sodium iron gradient is maintained in the loop of Henley and that's to enable water reabsorption and then further reabsorption happens in theep DCT and the collecting ducts so that is it for filtering re and reabsorption in the nefron make sure to watch the next video on osmo regulation so you can see the negative feedback and homeostasis of how the permeability of the distal convoluted tubule and the collecting duck changes um depending on how much water you have in your blood if you want to have a go at some practice questions to test knowledge head over to miss estri and if you hav't 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