so the mechanisms of your information our filtration reabsorption and secretion let's look at them in more detail starting of course with filtration it happens in the renal corpuscle remember that's a part of the nephron and the renal corpuscle is really made of two structures the glomerulus and the glomerular capsule the glomerulus is a capillary bed so this is where blood is arriving into this capillary bed and things are exiting the blood waste products like urea but also good stuff water and sugar and salts and so on all of that stuff good and bad is captured by this capsule that surrounds the glomerulus that's called the glomerular capsule or often Bowman's capsule now the stuff in that capsule is not blood anymore but it's not urine yet because it's a mix of bad stuff and good stuff so until we finish modifying that liquid we call it filtrate starts this blood make filtrate and eventually that will become urine let's a look a little at the renal corpuscle I remember first we should say we know that it's made of a glomerulus that's this capillary bed here and a capsule all the parts of the capsule are listed here we can look a little more closely first at this capillary bed by looking at well where does the blood come from originally and it comes through the afferent arteriole remember a is for arrives here's the blood arriving at the capillary bed and then the blood will move through these capillaries and these capillaries are modified with fenestrations so that endothelial layer the simple squamous epithelial lining that makes these tiny capillaries has fenestrations has pores allowing stuff to exit the blood we can't really see the capillaries in this glomerulus except for a tiny peek through the filtration slit the little red slit here is where we're peeking in at the capillaries and the reason we can't really see them is because of photo sites these cells here with these wavy little feet our podocytes remember pode and ped both mean foot like a podiatrist is a foot doctor so Poteau sites wrap over the capillaries except for these little slits left behind and they are also helping keep in big stuff and let out small stuff you don't want blood in your urine you don't want to be losing white blood cells or platelets you want to be losing only small stuff after the filtration process finishes the blood leaves through the efferent arterial remember that E is typically for exits now it may seem like we're done but remember filtration is just the first step in the process of creating urine so this afferent arteriole will actually see it again later but for now let's look back at this capsule we talked about these podocytes and how they're part of the glomerular capsule in fact what they are is the visceral epithelium because this capsule is a double layered sac not unlike the pericardium the pleural membranes and the peritoneum that we talked about previously these Sarris membranes double layered fluid in between the only differences in those three P's the fluid is meant to provide a friction-free environment so the heart and the lungs and the stomach and intestines can squish around in this case this double erred membrane is here to allow us to capture this filtrate in this space so we just call that space Bowman's space or the capsular space and we call this outermost layer the parietal epithelium or you could call it the parietal wall of the glomerular capsule if you're rusty on your saris membrane so you can always go back and watch the video where I walk through that process one thing we should note before we move on is the difference in size of the afferent and efferent arterioles there's a little bit subtle but here's this fairly wide diameter lumen of the afferent arteriole and here is a much smaller diameter lumen in the efferent so this is creating a pressure differential you've got a lot of blood coming in and it has to leave through a smaller hole so you either have to let the blood back up and back up and back up or you got to keep it moving by putting it under high pressure and high pressure is what's causing these waters salts sugars amino acids electrolytes vitamins drugs and metabolic wastes like urea to all pour out of the capillary bed called the glomerulus it wasn't the best filtration job but it is the end of step one now we just have to reabsorb like 99% of that stuff including the water salts sugar and other stuff like plasma proteins from the blood lots of stuff so how are we going to do this reabsorption well it's going to happen in the renal tubules but the renal tubules as a lot of different parts for parts that we'll study starting with the proximal convoluted tubules ear remember proximal is a term we used back when we studied skeletal Anatomy and so we said like the proximal end of the humerus is more towards the shoulder and the distal end is farther away so in this case we have a proximal convoluted tubules ear because it's very close to the renal corpuscle if we follow this down and up and down and around and there's a loop and now it comes back up and down we'll reach the DCT the distal convoluted tubules in between those two is a loop of Henle also known as the nephron loop and I'm even gonna switch colors to drive this point home the collecting duct is going to be involved in reabsorption but it's not actually part of the renal tubules it's its own thing we'll talk about that too later on so really three parts to the renal tubules and one thing called the collecting duct are gonna help us reabsorb 99% of what was lost out of the blood let's look a little bit closer first at the proximal convoluted tubules so if we could open that tube up and cut it into a cross-section it would look something like this nice big fat simple cuboidal cells nucleus in the middle and micro villi remember whenever you see micro villi you should be thinking absorption is going on here or more specifically reabsorption remember sugar salt as water you brought it in when you ate or drank it then it was absorbed out of your intestines into your blood now we're going to reabsorb it because you lost it so we have to reabsorb it and really the PCT that's your primary reabsorption Center especially for these organic nutrients glucose water salts lots of the important stuff as we move down from the PCT will reach the loop of Henle or the nephron loop this loop first comes down and that's called the descending limb and then it will travel back up which is the a sending limb and really it's just supporting the PCT it's going to help out with a lot of stuff especially in regards to water and salt now there are some interesting modifications to the loop of Henle that allow both water and salt to be reabsorbed but I think we'll save those for physiology and we'll move on to the DCT the distal convoluted tubules no micro villi so it's not very good at absorption or in this case reabsorption really what mostly happens in the DCT is secretion and secretion is of the last step in our urine formation process we're not there yet what about that collecting duct the collecting duct the nephron because there are branches off of it going out to a bunch of nephrons so one collecting duct takes infiltrate from a whole bunch of different nephrons and then it will transport that filtrate from the nephrons to the renal papilla the thing about the collecting duct is it responds to ADH remember the endocrine system we learned the mnemonic device Oh a and then flag top oh and a are the first letters of the hormones made by the hypothalamus stored in the posterior pituitary I'm not interested in oxytocin the cuddle hormone right now but ADH ADH is a messenger that tells the kidneys to conserve water and if you're like me and you've got a bunch of ABH all the time because you don't drink enough water the way that ADH delivers its conserve water message is really all about aquaporins and like the name implies these are pores that allow water to pass through now let's think about this if you're a little bit dehydrated what you don't want is all your water stuck in this collecting duct because it's gonna go to the renal papilla and then the renal pelvis and the ureter and the bladder and it will leave your body so anytime you have ADH delivering a message that says conserve water you don't want the water to stay in the collecting duct what you do want is these little pores to open and the water and come out and you're rescuing the water you want a whole bunch of those pores in fact so ADH conserves water by ensuring that water leaves the collecting duct through the aquaporins but those pores will close up if you are well hydrated then you can pee out a bunch of water no problem or if you block the release of ADH from diuretics like coffee and alcohol then the ADH won't ensure the water exits and you'll just be peeing out a lot more than you need to one thing about reabsorption before we move on we're saying that we rescue this good stuff water and sugars and salts but if we reabsorb it where does it go well it came from the blood so it's going back into the blood more specifically right here is the afferent arteriole it's left the glomerulus but we said we would see it again so what it does is it comes out and it starts curving its way around all up and down and through the renal tubules and we can name these blood vessels these capillaries as they hang out around the renal tubules we can call most of them peritubular capillaries meaning capillaries that are around the tube down by the loop of Henle we have some very straight vessels Vasa recta which do the same thing they are there to facilitate reabsorption they are there to pick up the water and the sugar and the salts and the other good things plasma proteins and such as they leave the renal tubules they need to go back into the blood because remember anything you leave in that renal tubules will eventually go away in urine okay now that we have reabsorbed the good stuff we need to remember the other problem with our filtration process was that even though we let out a lot of stuff with the waste we still didn't get all the waste out we still have urea and uric acid and other nitrogenous wastes in the blood so the good news is these blood vessels are still hanging out right next to the renal tubules to reabsorb the good stuff so it's the perfect time for them just to pump out the bad stuff especially in the distal convoluted tubules here the blood will actively push things out of the blood and into the renal tubules stuff you don't want is ammonia based byproducts like urea some drugs acids like hydrogen ions other solutes that you're trying to get rid of will move into the DCT and join the filtrate and soon they will leave the body and then you've made urine because once the filtrate drips from the renal papilla into the minor calyx it's not going to be modified any further now we can call it urine so remember we started with this dirty blood we filtered it to make filtrate but 99% of that filtrate shouldn't be in there and we still need to push more waste product into the filtrate once we've modified the filtrate through the reabsorption and secretion processes then we can call it urine and now the blood is clean and can exit the kidney one more thing though the kidneys do a lot more than just filter blood and I'm not talking about reabsorption and secretion I'm talking about things like affecting the red blood cell count and blood pressure and most of these extraordinary kidney functions are taking place at something called the JGA the juxtaglomerular apparatus also known as the juxtaglomerular complex this region is between the distal convoluted tubules and the afferent and efferent arterioles so we saw this picture before except I didn't really talk about the JGA but the other thing I want you to do is you may need to go back to some of the earlier images of the nephron and realize that this here is the DCT and it does on every nephron it comes back up and it's nice right up next to the afferent and efferent arterioles if we go back to like this slide it's not doing that because this image is not intended to teach that lesson so they kind of spread stuff out to make things more clear but if you go back to the earlier slides of the nephron you will see that indeed yes the distal convoluted tubules swings back up next to the renal corpuscle because check this out if you do that now you can see the blood when it's arriving at the capillary bed and the blood when it's leaving and you can see the filtrate pretty soon before it becomes urine and you can basically compare those three you can then regulate these processes you can regulate the filtration rate and blood pressure so how do we do that we won't get into much detail we'll just look at three kinds of cells and basically they all chip in juxtaglomerular cells are smooth muscle cells right here they're along the afferent arteriole and they're monitoring blood pressure remember we said that afferent arterial is wide in the Ephraim is narrow so that makes pressure which causes the filtration process to occur if you want to filter better then you can increase the pressure and more stuff will exit the blood more good stuff too but also more bad stuff or the waste products so these juxtaglomerular cells are important because they're going to monitor that blood pressure and decide if anything needs to be done about it those smooth muscle cells can shrink the diameter of the lumen of that arteriole macula densa is the dense spot in other words this is a cluster of cells that are kind of tall and kind of packed in tight densely and they are mana D monitoring the filtrate in the distal convoluted tubules of how the filtration process is working and then in between these cells is a cluster of other cells called mesangial cells contributing to blood pressure and the filtration rate although these are the least well understood cells of the GGA and their function and how they've executed is still being researched so this is where you're headed you need to have that base adda me down understanding the names of these tubes and their basic functions because from a physiological standpoint what you really need to get into as things like osmotic pressure and permeability of the tube you'll at different locations and the action of different hormones and how they affect the filtration and reabsorption and secretion so we won't get into anything else in this video lecture but if you're headed towards physio make sure you go in with all the knowledge that you'll need