the kidney is my favorite organ in the whole body it's a Marvel of Engineering in this video we're going to go through how it works what's going to be tested on the MCAT and hopefully by the end of the video you'll begin to appreciate this awesome organ as much as I do the kidney has many functions in the body and the six major ones that you need to know for teste are as follows number one the kidney filters out the blood and removes waste or extra substances number two it regulates blood pressure and blood volume to make sure that we have a steady homeostatic balance of fluids in our body number three it regulates electrolyte balance so the salt water balance in our body number four it regulates the acid base balance or pH balance in our body our body needs to maintain a pH of 7.4 and the phosphate buffer system related to the kidney as well as excretion of excess hydrogen ions allows us to maintain that balance number five is rethrow pootin production so the kidney actually prod produces arthro poitin which is a precursor for stimulating production of red blood cells in our bone marrow and number six is it's involved in the activation of vitamin D which is essential for us absorbing calcium into our bloodstream so go ahead and screenshot this list you're going to need to know it for testing now let's move on to the structure of the kidney and how that informs its function so this is the gross anatomy or the large scale anatomy of the kidney this is one of our two kidneys that are in our bodies it's in our lower abdomen and you can see here that we have our urer here this is where urine flows into the bladder it's not shown here but the blood vessels feeding into the kidney also come through this central region here which is known as the renal pelvis now this Center Point is where all of the filtrate all of the waist products get sent down into the uror into the bladder and on the edges here this is known as the cortex all right the cortical region is where we do our filtration down in the center here this is known as the medular region this is where we start to concentrate our filtrate as it turns into urine so we're going to zoom in here to a functional unit of the kidney known as the nefron and the nefron is kind of like a vertical slice into the kidney there's thousands and thousands of nephrons in the kidney we're going to zoom into one of them to look at its function this is the nefron which is our functional unit of the kidney we're going to walk through each part of the Nephron include and include its functions and its connection to other organ system there are four major processes that happen in the nephron we have filtration secretion reabsorption and excretion each has a very specific definition and location so as we go through the functional unit of the Nephron I'm going to be calling out where each of these processes take place starting with filtration so filtration occurs at this beginning part of the nefron where the blood is coming in and ready to get filtered so it's filtration and it's happening in the glomerulus is the structure g m glus so the glomerulus is the site of filtration and this is where blood is coming in and due to forces known as Starling forces that's some Physics overlap here in our biology topic these Starling forces are a combination of oncotic and hydrostatic pressures that allow for filtration to happen passively that's pretty much all you need to know about Starling forces for the bio biochemistry section of the MCAT and that Happ happens at the Glarus the Glarus is surrounded by a structure known as the Bowman's capsule all right Bowman's capsule and the filtrate goes through the Bowman's capsule and into the rest of the nefron all right so this is our Bowman's capsule here and this is where the filtrate is entering as soon as it gets filtered the fluid and the materials in the nefron are known as filtrate filtrate is on its way out of the body all right so everything that's inside this nefron structure is known as filtrate and it's on its way out all right so it's no longer part of the body and so as we hit the proximal convoluted tubule we want to double check that everything that's in the filtrate is stuff we actually want to remove so often times certain things get filtered that we didn't mean for to get filtered right it's not a perfect process so things then get reabsorbed so reabsorption is where it's returning to the body from the filtrate so it's leaving the filtrate going back into the body getting reabsorbed by the body and so this happens at the proximal convoluted tubule that's this part here and also to some extent at the distal distal further away right convoluted tubule so that's what these two structures are called and this is where reabsorption of nutrients that we didn't mean to filter out happen all right so things that get reabsorbed all the time are amino acids right we usually need those we don't like to filter those out and if we do by accident we're going to reabsorb them also glucose right sugars get 100% reabsorbed all right because we definitely do not want to be excreting glucose we need to use it for fuel right some disease States when we have way too high blood glucose like in diabetes we will end up overloading this reabsorption system and we'll end up having glucose in our urine and that's why um glucose in the urine or sweet smelling urine is one of the first Hallmarks of diabetes unregulated diabetes because we just have too much sugar in the bloodstream for our nefron to reabsorb effectively right and that causes some problems so we reabsorb glucose we reabsorb amino acids the other major thing that happens at the proximal conut tubule is with a pump that we may be familiar with from our nervous system and I'm going to kind of take this little box and zoom in here and this is known as the sodium pottassium atpa transporter or antiporter because it's opposite antiport all right and this antiporter is a channel so I want you to visualize it as a Channel or a pump right where this is our filtrate this is our nefron and here is our body right back into the body and what we do is we pump again against the gradient we pump three sodiums back into the body so this is getting reabsorbed I'm just going to write ra for reabsorbed and then in exchange because it's an antiporter so opposite direction two potassiums are going to get what's known as secreted secreted means into the filtrate all right so secretion is we're going into the filtrate which is ending up outside the body reabsorption back into the body so we can see here that we're pumping out two pumping out of the body two potassiums going to get EX secreted into the filtrate and then in exchange we're going to pump three sodiums back into the body now this is a conceptual check that I want you to think about here and this connects to our nervous system so we basically have a system to make sure that we're reabsorbing sodium which means that we have a lot of extracellular sodium ions chilling in the body this makes sense if you think about how a neuron works right it's contact connection neurons require sodium influx so when an action potential happens in a neuron a neural signal sodium floods into cells and in order for it to do that we need to have a lot of sodium outside of the cells right in order to allow for that concentration gradient so we have a lot of systems in our nephron built to make sure that we have plenty of sodium extracellularly in our body to allow for that influx to happen with our neurons on the same side of the coin we also want very low levels of Po potassium outside the cell because potassium leaves the cell in the repolarization part of the action potential so we want to keep our pottassium levels low extracellularly and our sodium levels High which is why we have these pumps all right so that's making that connection it's also why high sodium diets are dangerous because we have all of these systems for keeping our sodium levels High we don't have a lot of systems for getting rid of excess sodium right so this is one of our issues with having hypertension related to high sodium diets and why is that because we can see here that we have a net movement of plus one solute right plus one salt going back into the body you know what I'm going to do is I'm going to rewrite that as purple since that's going back into the body plus one solute back into the body all right what that means is that we are driving an oncotic pressure back into the body and water one of our secret rules is water always follows solute right so if solute is moving back into the body water will as well so water will follow that solute and get reabsorbed back into the body so H2O and sodium all right now let's say we really want water to get back into the body our blood pressure drops and we need to have more water reabsorption we can do something with our hormonal system I'm going to write this in blue all right so our hormonal system has a hormone called aldosterone and aldosterone is produced by the adrenal glands all right via the hypothalamus pituitary axis it's a good concept check to go back and check this and aldosterone's function is it increases the production of the sodium pottassium pumps on the distal convoluted tubule and the proximal convoluted tubule PCT and DCT all right DCT so that means that we have more of these pumps happening basically aldosterone it's a steroid hormone goes all the way into the cell and upregulates transcription factor that upregulates the production of these sodium pottassium pumps and what that ends up doing is it has an increase in sodium reabsorption which then reabsorbs water into the body because water is following the sodium cool and so that's how aldosterone as its end result increases blood volume and blood pressure is by increasing the sodium pottassium pumps on the distal and proximal computed tubules so far so good right so that's our connection to the endocrine system the endocrine system and the renal system are very closely tied together it's great to learn them at the same time all right so now our fil tray we've made our adjustments uh by the way Ura also gets secreted here all right um yuria is a waste product it's a nitrogen-based waste product that will get secreted here in the proximal convoluted tubal additional waste products as well then we go down into this funky looking thing which is known as the loop of Henley all right so this is known as the loop of Henley I'll write that down here and the loop of Henley is a complicated system so we're going to talk about it in conjunction with talking about this part of the nefron which is known as the collecting duct because in order to understand why the loop of Henley exists we need to understand what the collecting duct is trying to accomplish so in the collecting duct if we remember back to our big kidney structure our cortex is on the outside edge and our medula is in the middle and these guys are kind of like vertical slices so the cortex is up here and the medular region is down here all right and what we want to do medula and what we want to to do is our point of our collecting duct is we want water to get reabsorbed we want to get rid of water from our filtrate and back into the body because we want to concentrate our urine we don't want to be peeing out tons of water with our waste because then we'd be just drinking water and peeing all day long right it's not a very efficient system but it's not easy to pump water out if we were to just use like ATP pumps that would CA so much energy for our body it'd be really hard to do so instead we want to create some sort of passive system where water just wants to get reabsorbed it wants to leave the filtrate and the way we do this is we create a concentration gradient where in the interstitium of the kidney right of the Nephron around this nefron unit we increase in concentration so up here it's about 300 milliosmoles right it's just a measure of how much solute to water there is how concentrated is down here here it's about 1,200 milliosmoles all right so it's four times concentrated and so what happens is as we increase in concentration 600 900 right the filtrate will have a lower concentration this would be like 500 right and it'd be lower than the outside remember water follows solute that's our secret rule so water is going to want to go where there's more solute when there's where there's more concentration and so as the concentration increases down through the medular region water is going to leave the nefron be reabsorbed by the body to balance out that higher concentration but how did that concentration gradient get started in the first place that's the loop of Henley so what the loop of Henley does is it has selective permeability so on the ascending Loop so this is like going up going up we have sodium pumps so sodium is getting pumped out of the ascending Loop but water cannot leave so it's impermeable to water but it can pump out sodium on the descending loop it's impermeable to salts but water can leave so one of the cool thing is as we pump out sodium we're concentrating this medular region right we're pumping it out as we go but in order to keep this pumping we need to make sure there's enough concentration inside the nefron so what we do is we pump water here or sorry we don't Pump It water leaves to balance out this sodium concentration that's right here so as the filtrate is moving down the outside's getting more and more concentrated so water's leaving which means inside the filtrate we get more concentrated because water is leaving and then that allows us to pump more sodium out and it all stays balanced now this is pretty complex and I don't have a cool animation here uh to show you you but I just want you for the mcap purposes to take away a key thing is that the loop of Henley is what's causing this concentration gradient that's its function and it's doing this in a system called the countercurrent multiplier system which is just this idea that we're pumping sodium water follows pump more sodium water follows right multiplier system so if it's a little complex to understand that's totally fine I just want you to know that the loop of Henley's function is to concentrate the area around the nefron to allow for water to passively reabsorb in the collecting duct all right that is the key to the loop of Henley that's all you really need to know for teste and then just content wise the ascending Loop pumps sodium the descending Loop allows water to move with its gradient passively all right so they'll sometimes ask some cell biology pumps here the final piece here with the collecting duct is what actually allows the water to move so again again the distal conv tual we just do our final adjustments of reabsorption and secretion and then down into the collecting duct we go and then we have these cool channels called aquaporins all right aquaporin aqua water pouring into the body right so aqua water getting back and reabsorbed and these are passive channels all right they're they're just uh facilitated diffusion channels where we just if they're open and there's more concentrated solutes outside of the nefron water will move through those aquaporin these aquaporin are opened by a molecule by a hormone called ADH anti-diuretic hormone you may have also learned it as vasopressin all right vasopressin or ADH opens aquaporin all right and if those aquaporin are open that allows increased water reabsorption directly right reabsorption r a and that's going to then have the final result of increasing blood volume and blood pressure now what we can notice here is that ADH and aldosterone have the same end results right they both increase blood pressure and blood volume but very very different mechanisms aldosterone is a steroid hormone it goes directly into the cells of the nefron to adjust the transcription of these pumps and then indirectly results in an increase of water reabsorption ADH vasopressin is a peptide hormone a peptide hormone produced by the posterior pituitary and peptide hormones can't cross membranes they're too big and polar so instead they bind and in this case they bind to the aquaporin resulting in a confirmational change to open them up uh so peptide hormones tend to be much faster acting so ADH is going to be our fast acting change to increase our blood pressure whereas aldosterone is going to be our more slower chronic acting uh adjustment to blood pressure that's why most drugs that uh keep our blood pressures low uh go after aldosterone right in the aldosterone production process versus the ADH or vasopressin which is more of the acute response to low blood pressure so again cool connections to hormones including steroid versus peptide hormones that's why we have like a quick switch and then a longer term switch for our kidneys here finally once it gets through the collecting duct it is now going into the eror and we are now excreting so excretion is once we've gotten through all the processes we're no longer reabsorbing we're no longer secreting we are purely excreting out of the body via the uror and the bladder and that was kidney Basics if you found this video helpful please share it with your Premed Community remember the MCAT is really hard for everyone and we could use all the help we can get hope you enjoyed it and as always happy [Music] studying