[Music] the job of the kidneys is to filter the blood basically we get dirty blood entering and clean blood leaving and all of the waste products that are taken from that dirty blood exit as urine words like dirty blood and clean blood on particularly scientific but it is a really convenient way to think about it if we look at the cross-section of the kidney the outermost part is known as the cortex the next part is the medulla next is the pelvis and the artery that supplies it is known as the renal artery renal is just an adjective which describes things to do with the kidney it makes sense then that the blood vessel that leaves the kidney is known as the renal vein and of the tube which exits the the kidney with all of the waste products is known as the ureter and the ureter connects to the bladder and then from the bladder materials can exit the body inside of the kidneys we've got lots and lots and lots of these tiny tiny little structures which are known as nephrons inside of each kidney there might be about a million of these things so they're really really small let's look closer at the first step of the nephron here and now notice that blood which is entering which is coming from a branch of the renal artery it's coming through a blood vessel which is relatively wide compared to the blood vessel which is leaving this area and that blood will eventually connect up to the renal vein in between all of this is a capillary network known as the glomerulus and what happens is because the blood vessel that enters is wider it kind of gets a little bit constricted as it's trying to leave after the glomerulus so the result is we get really high pressure inside of the glomerulus the area surrounding this is known as the Bowman's capsule or the renal capsule and the high pressure forces material to move from the glomerulus into the Bowman's capsule once the materials in the Bowman's capsule irk and then continue on through the tube you'll but let's have a little bit of a closer look at how Cheerios move across from the glomerulus and into the Bowman's capsule notice that there's lots of tiny little spaces between them these spaces are really small meaning that red blood cells can't get through they're too large similarly other large structures like proteins are mostly too big to get through smaller structures however little molecules like urea they are small enough to get through so urea will transfer into the Bowman's capsule now you might be tempted to think that this is the job done we've got rid of the toxic material that's gone into Bowman's capsule the rest of the blood can continue on as normal and reenter circulation and the job's done unfortunately this isn't the end of the story because small things can fit through other small materials which are quite useful will also fit through and enter the nephron into the Bowman's capsule things like salts glucose water these things are all going to enter now this part is known as ultra filtration the movement of materials from the glomerulus into the Bowman's capsule so in order to get the useful materials back we know there's got to be a next step so we've got these materials now moving through the tubular let's investigate what's going to happen in a section of this here we've got the blood capillary represented in red and the nephron represented in green and we can see in the nephron or lots of materials some of them are useful some of them are not so useful these materials can move from the nephron into the blood capillary via active transport notice that some of the materials have moved some of them have not in the blood capillary we will have glucose salts and water being reabsorbed from the nephron the materials that are left behind obviously we've got urea because one of the main purposes here is to get rid of urea but what we will also have is excess salts and excess water your body needs salts and it needs water but it needs to have these things present in the correct amount so only the excess materials are left in the nephron to eventually leave the body notice here that there is absolutely no glucose left in the nephron your body works really hard to get glucose into its system and it's not going to lose any of it through the urine it's not going to get rid of any of this stuff glucose may be present in urine but it only happens if there is some sort of underlying medical condition that causes this now this whole process of taking back the materials that are needed there's known as selective reabsorption don't make the common mistake of calling it absorption reabsorption it's very important that you get it right now that you know all the process has to go on let's have a quick look at all of the labels that need to go under the nephron you already know the glomerulus and the Bowman's capsule often known as the renal capsule and next the two tube areas are known as the proximal convoluted tubules and the distal convoluted tubule and they're just the parts that come before and after this loop that's in the middle and this loop is known as the loop of Henle the loop has two parts to it there is the descending limb and the ascending limb the descending limb being the part where the material falls down and the ascending limb where it rises up next is the collecting duct and this is where all of the material from the two Buhl is going to enter from there it will go down until it eventually exits the kidney through the ureter and it can then exit the body I'm sure by now you've come to realize that the kidney function is actually quite complicated so let's try and represent it in a slightly different format and see if we can clarify all of the different steps so dirty blood comes in and then via the glomerulus the materials get separated we get filtrate which contains the waste material but unfortunately also contains lots of useful material and that goes into the tube you'll this gets separated from the blood but it's incomplete blood right now because there's still lots of useful material like glucose and amino acids and things like that which are absent from the blood vessel through a process of selective reabsorption material that is useful will be transferred black back into the blood and we've now got clean blood which is complete it's got all of the useful things and this leaves behind in the tube you'll only the waste material and that's the urea as well as salts that are in excess of requirements water that's in excess of requirements this material is then going to leave the body it's going to go to the bladder the material is now known as urine and from there it can exit the body and go into the toilet obviously now the blood which is useful will go out of the kidneys via the renal vein and it will go back into the circulation where it can be used like normal obviously because this is quite a difficult topic this is a place where people can quite easily lose marks and exams so let's look at a possible question the diagram shows a single nephron in the kidney describe the functions of the areas labeled x and y on the diagram and you should recognize that X is the glomerulus and Y is the tubular so the formal question let's see what you might say a tot X the glomerulus ultra filtration takes place always named the processes that's one of those kind of obvious points that people sometimes miss small molecules move from the blood into the renal capsule so it's the direction of the movement and what isn't moving they are forced into the capsule under high pressure that concept of high pressure is quite important to how this process carries on I thought why selective reabsorption of useful molecules notice selective reabsorption you lose the marks if you just say absorption give an example of the molecule that could be transferred one of them would be glucose but this loss you could mention and these materials are reabsorbed into the blood capillary so that's saying that the useful stuff is then taken back to where it needs to be [Music]