now we're going to turn our attention to the urinary system the urinary system has a variety of functions but we're going to start by focusing on how it allows for the concentration of dissolved particles in the blood to remain constant now we measure the concentration of dissolved particles with the osmolarity what's osmolarity well it's quite simply the number of particles per liter now you're probably familiar with molarity which would be the number of molecules that we might mix into solution osmolarity is distinct in an important way that is the term particles is essentially how many molecules are produced in solution by some chemical so the number of particles you can think of as the number of the molecules that you dump into the solution times the number of particles per molecule in solution not all molecules stay together but instead they dissociate so for example a molecule that doesn't dissociate is glucose so so one mole of glucose produces one osmole in solution and that's because it produces one particle per glucose molecule glucose doesn't fall apart when you mix it in water sodium chloride does and so we get two osmoles from every mole of nacl because we get two particles per molecule and that's because nacl dissolves into sodium and chloride ions when they're mixed into water so blood has a very consistent osmolarity it's around 300 milliosmoles and milliosmoles would be the units and we're gonna milla makes it easier because then you get numbers like 300 so milliosmoles will be our focus here in order to achieve that we have to articulate later on a control system with 300 ml osmols as a set point or you can start to envision that control problem this occurs despite the perturbations that we ourselves create you know imagine if you drink a large glass of water that's going to go into your gi tract and if you have diarrhea then maybe you'll sort of end up pooping it out but most of the time we absorb most of that water it enters right into the bloodstream and then potentially dilutes all of the nutrients the red blood cells all of the good stuff that circulates in the blood what the kidneys will do to compensate for that is to get rid of more water so that the osmolarity remains relatively constant to achieve that we need a more intelligent exchange organ than what we've considered so far not only is it a challenge if you drink a lot of water but it's a challenge if you haven't had a drink of water in a long time so we need a capacity to modulate the concentration of particles in the urine in order to maintain a constant osmolarity in the blood and we've all seen our own urine sometimes it's really dark sometimes it's really clear that indicates the osmolarity so the really dark urine would have a high osmolarity and the really clear urine the low osmolarity and here we see the quantitative metric of that familiar notion it's huge all right and it spans the osmolarity that we see in the blood okay the urine can be either more dilute or more concentrated than the blood we can contrast this with the lungs the idea behind the lungs working is basically let's bring the air in from the outside of the body parcel it into really small packages the alveoli so that chemical diffusion can work to allow the blood to reach equilibrium with that air that we inspired so that the partial pressure of dissolved gases in the blood as it leaves the lungs is equal to the inspired air that's a pretty simple exchange organ just get the two sides really close to each other and then let chemical diffusion do all the work here we've we've got something much more complicated because the blood is going to have a that's inside the body is going to have a different osmolarity than what we're going to end up peeing outside the body that is the osmolarity of the urine so how does it do this well the kidneys do this through a number of steps and here we see schematically those steps so first of all it takes blood and blood has red blood cells it has small molecules it's got large molecules large proteins primarily it's a mix of a bunch of things and we want to get rid of water and the waste products that are in the blood and the waste products are among the small molecules but the small molecules also include nutrients so we don't want to get rid of those we don't want our urinary system to be getting rid of glucose fatty acids good stuff to fuel the production of atp by the cells in the body so we first have a step of filtration that allows both small molecules and water to create a solution called the filtrate which ultimately will become the urine but then there's a little bit more to it than that we want to reabsorb the nutrients we want to secrete or deposit further toxins into the urine and ultimately we want this concentration this solution to be tuned so that the osmolarity of the blood remains relatively constant that is making this stuff down here more or less concentrated by either subtracting or adding water and then ultimately the solution the filtrate that we get rid of we say that it's excreted not to be confused with secretion which is the active pumping of small molecules into the filtrate excretion is essentially what you urinate now this mini lecture is focused on the first step filtration and we're just going to pose a simple question how does filtration work in the kidneys and to understand that we're going to return to a simple thought experiment where we take a u-shaped glass tube and we put a membrane down on the bottom this is a semi-permeable membrane and this particular membrane allows water to traverse the membrane now osmosis in this case is going to favor the movement of water towards a higher concentration of dissolved particles so now we know we're familiar with the term osmolarity we're going to move or osmosis is going to move water towards the higher osmolarity and these large dots are meant to represent proteins which are typical of the blood so we essentially have a model for blood on the right which is water with some proteins mixed in it so colored osmotic pressure works towards the right now that term it's no accident that that term includes the word pressure because this movement of water across the membrane creates a mechanical pressure force per unit area and that's going to drive the water column on the right to rise and resist gravity and from a difference in the height of these two columns we could calculate what that osmotic pressure is because it's essentially the difference between the pressures on the two sides and pressure is generated here we can think of the pressure at the level of the membrane as the force generated by the weight of the water that's acting on it so as this column raises up then we have that extra weight that is not balanced on the left hand side that weight of the water that's rising up and that pressure on both sides is proportional to the depth of the membrane so on this side we have less depth and on this side we have more depth okay so that's a hydrostatic pressure at the membrane and the difference of the hydrostatic pressure is important because it's reflecting in this case the the balance against the collet osmotic pressure now we can also exert a pressure on one side so if we do that on the right then we're going to add a little plunger here and if we push against that osmotic pressure and actually drive water so that on the left hand side it's the same height as the right hand side and you could create a pressure by putting weights on this plunger and the pressure generated would be equal to the force or the number of the amount of weight that we have to apply on it divided by the cross-sectional area of the plunger and that's an additional hydrostatic pressure so we have this sort of gravitational hydrostatic pressure now we're exerting hydrostatic pressure and in this case we've perfectly balanced the colloidic pressure and we know that because the water heights are now at the same level and this additional hydrostatic pressure is what we needed to exert in order to balance the um the colloid osmotic pressure now if we went further and were to push this plunger down more that would drive water towards the left and since this is a model of blood then that would be equivalent to blood pressure driving water through a membrane into some space outside of the bloodstream and that process is known as filtration so filtration occurs if the net hydrostatic pressure that is net because it refers to hydrostatic pressure on the left and right hand sides of the membrane so if the net hydrostatic pressure is greater than the net colloid osmotic pressure so in this case we're focused on hydrostatic pressure as as being generated greater in the on the blood side of the membrane if that is greater than the colloid osmotic pressure which acts in the opposite direction then you create filtration this is not always how it works if the the hydrostatic pressure is actually less than the net colored osmotic pressure which does happen then the circulatory system is going to take on water and so that's known as absorption so if it's if hydrostatics less than colored osmotic then you you have absorption now filtration and absorption is not an abstract abstract concept this happens in the capillaries all the time so here we have a schematic capillary with blood flowing towards the right and we've already considered blood pressure right we talked about how the blood pressure is really high in the left ventricle it's maintained in the major arteries but then drops as the blood flows through the capillaries of the body and we know because of poussey's law that that's a very strong effect on peripheral resistance we're taking a viscous fluid and forcing it through a small vessel and that viscosity of the blood sacs the energy in the blood and that's what the pressure reflects it's energy per unit volume of blood so what this also means in a mechanical sense is that the blood on the capillaries closer to the arteries are going to have a higher hydrostatic pressure right they're going to be closer to this side of the curve than on the downstream end where we have a lower pressure so let's consider that in our schematic the hydrostatic pressure is really high on the upstream end and we can measure that and by the way we're comparing it to an interstitial fluid so just imagine this capillary surrounded by a fluid which doesn't have any hydrostatic pressure in this case on the upstream end because of that strong arterial pressure in the major arteries here we've got a higher hydrostatic pressure on that end then on the downstream end where we've got 15 millimeters of mercury compared to the 32 on the upstream end and we're going to show that schematically by the length of these green arrows all right so that's going to tend to favor filtration expelling water from the capillary collard osmotic pressure resists this mechanical force instead we've got a higher concentration of proteins in the interior in the blood and we've got nothing in that interstitial fluid so this difference in osmolarity creates a pressure to call it osmotic pressure and it's the same everywhere because even though we might be losing a little bit of water it's negligible it doesn't have a very strong effect on concentration the concentration is effectively the same all along the length of the capillary so we've got that collationic pressure tending to favor water entering into the blood everywhere in equal magnitude now if we consider the relative magnitude of these two pressures then what we see is that on the upstream end the net hydrostatic pressure of 32 millimeters of mercury is greater than the net osmotic pressure and so we would expect water to leave the capillary on the upstream and whereas on the downstream end we would predict absorption because the colored osmotic pressure at 25 millimeters mercury is greater than the hydrostatic pressure at 15. this actually happens in the capillary so here we have a schematic of the capillaries on the upstream end the hydrostatic pressure drives water from the capillaries so we get water flow and then that's filtration and then there's absorption that occurs on the venous side because of the collagen pressure and we have a means for avoiding any building up of water that's provided by the lymphatic system the lymphatic system we'll see plays a role in fat absorption or fat transport when you absorb it in your small intestine the lymphatic system plays these kind of supporting roles from time to time in this course here we see it helping out dealing with an imbalance of water so we don't have a building up of water say around your ankles most of the time instead the water is transported into the lymphatic system ultimately it is then redeposited into the circulatory system all right so we've got filtration and absorption and i told you at the outset that the kidney does filtration and it does it in a similar way from the filtration that we see in the systemic capillaries on the left here it has these bundles of capillaries in the kidneys and they create filtration that is driven by the hydrostatic pressure the kidneys are located really close to the heart and so the it has a major artery the renal artery which carries a high hydrostatic pressure by the time we get to the level of these capillaries the hydrostatic pressure is down to a level of about 55 millimeters of mercury that's going to tend to drive filtration and we have a small but um non-zero hydrostatic pressure in this space here this structure which i'll tell you what it is in a moment serves to capture the filtrate capture the water and the small molecules that emerges from the capillaries and this space hydro hydro has a hydrostatic pressure of 15 millimeters of mercury so the net hydrostatic pressure is the difference between the two 40 million millimeters of mercury which tends to generate a filtrate okay that's the direction of filtration which we see by this green arrow filtration is of course resisted by the colloid osmotic pressure and the the filtrate uh in this space around the capillaries we're gonna round it down to having well there are no large proteins and so it has um a zero colloid osmotic pressure whereas the blood of course with all of those large proteins has a non-zero or 30 millimeters of mercury called osmotic pressure the difference between the two is therefore 30 and we can see that 30 is less than the net hydrostatic pressure so filtration is favored and that produces a filtrate all right so this is a small structure inside of the kidney let's let's give it some anatomical terms and see where it fits in that larger structure so the space that gathers up the the filtrate is known as bowman's capsule okay so bowman's capsule you can think of as is this uh vestibule for capturing the filtrate this bundle of capillaries is called the glomerulus and the capillaries in the glomerulus are actually modified a little bit in order to favor filtration uh so if you take i take a cross-section through one of these capillaries its walls are composed of these endothelial cells just like a regular capillary but what we have here are these pores openings that allow for water and small molecules to escape these are really delicate structures so they're reinforced by accessory structures called podocytes which reinforce that they still get damaged you know so if you have hypertension then you can blow out the capillaries in the glomerulus cause kidney failure which is not good obviously um okay so where does this all fit in these are microscopic structures they are part of um a slightly larger structure called a nephron now the nephron we can see the glomerulus and the bowman's capsule there the nephron is packed into the kidney the kidney you can think of as being a factory of nephrons i guess uh so if we look at the kidney then we can see those uh tiny structures uh highlighted here there's two nephrons and there the nephrons do vary in their anatomy we're gonna just consider how your average nephron works but at any rate you can see that it radiates out and the filtrate ultimately gets gathered up and then deposited along this vessel here which corresponds to that greater length there and so the stuff that emerges here is what ultimately becomes the urine which gets deposited into the urinary bladder through a vessel called the ureter which we see there now all of these filaments that radiate outwards from the kidney you can think of as just being lots and lots of nephrons and lots of capillaries okay this is a highly vascularized system it's not just the glomerulus but there are capillaries all around the nephron as well now the position of the nephron um a position sort of towards the center and away from the center matters and so we have the similar terms as used for the brain the cortex which is more superficial and the medulla which is more it towards the interior of the kidney cortex and medulla will matter later on when we're talking about other functions of the kidney all right but this lecture was focused on filtration and we've established how that works that was our initial question we know now that it's driven by hydrostatic pressure and resisted by colored osmotic pressure and is facilitated by the anatomical features of the glomerulus and the bowman's capsule which are the first structures of the nephron