foreign chapter 26 lecture 4 is renal physiology and production of urine reabsorption and secretion normally the filtration rate is so high that the volume of filtrate in the capsular space in just half an hour is greater than your total volume of plasma and the job of the Nephron then is to reabsorb 99 of that the vast vast vast majority of filtrate is and solutes are reabsorbed in proximal convoluted tubule remember this is the part of the Nephron where we have the microvilli that forms a fresh border that increases surface area and helps to increase absorption rates solutes can be reabsorbed through both active and passive processes and remember active requires ATP or the energy from the gradient of another ion and passive means it goes down as gradient water is going to follow the solutes through osmosis and that's the movement of water across the semi-permeable membrane the small proteins that have escaped through that filtration membrane are returned to the blood through the process of pinocytosis another important function of the Nephron is secretion and this helps us with maintain our blood pH because of secretion of hydrogen ions so it prevents the blood from becoming acidic it also helps to eliminate certain substances like the ammonium ion creatinine and potassium substances can be reabsorbed through cells or between cells if they go between cells that's called paracellular if they go through it is trans cellular and either way they're going to go um into the interstitial fluid before they go into the capillary the peritubular capillary so about half of reabsorbed material moves between the cells by diffusion and about the other half is going to be reabsorbed through the cells and this is showing um diffusion into the apical side of the cell and um or no I'm sorry active transport through the apical side of the cell followed by a sodium potassium pump that's pumping the sodium out into the interstitial fluid here's the peritubular capillary and it diffuses across with the paracellular it's just diffusion all the way the solar reabsorption drives water reabsorption which is why I keep saying waterfall of salt where salt goes water follows the mechanisms that accomplish is sodium reabsorption not only filter sodium but also electrolytes nutrients in water the apical membrane is going to be the part of the cell membrane that faces the hollow interior or lumen the basal lateral is going to be the part of the membrane that faces the capillary and both of these membranes are going to have different types of transport proteins reabsorption of sodium is very important because of fluid balance and so there's a number of different Transport Systems that are exist that to reabsorb the sodium we have sodium potassium hpase pumps that pump the sodium from the tubule cell through the basol lateral membrane only and then water is going to be reabsorbed by osmosis there are two types of water reabsorption one is obligatory and that's where water is obliged to follow the solutes so where salt goes water follows and the other is facultative and that happens in the collecting duct because of the antidiuretic hormone so when you're dehydrated anti-diuretic hormone is released from the posterior pituitary it's also known as vasopressin this is going to bind to The receptors of the principal cells in the collecting duct when it does it's going to cause them to synthesize and insert aquaporin proteins and these proteins are going to allow water to leave the collecting duct and go back into the bloodstream and that's facultative water reabsorption I think of facultative I think like faculty and principal principal cells faculty and principles active and passive transport processes remember that active requires energy so primary active transport is going to use the energy directly from that terminal Bond of ATP and it's going to involve some type of ATP Ace that's going to break the bond secondary active transport is going to use energy stored in ions electrochemical gradient to drive another substance across the membrane so there's energy in the gradient you can think of it as a gradient sort of as a hill and if you're going from high to low like you do in diffusion it's kind of like riding your bike down a hill you don't have to put any energy into it yet you generate energy I'm going down the hill if you go against the gradient you have to Pedal your bike up the hill you're going from low to high now in the example I gave of diffusion where you're riding your bike down the hill and you don't have to pedal and you're creating energy you can now use that energy to drive something else against the gradient and that's what secondary active transport is quite often the ion that's used the ion whose energy is harnessed is going to either be sodium or hydrogen ions and if with secondary active transport always always is going to involve two different ions one's going to provide the gradient provide the energy the other is going to use it and there are two types there are Sim Porters and antiporters importers both the ion providing the energy and the ion to be transported are moving in the same direction either into the cell or out of the cell with antiporters the ion that's providing the energy and the ion that's using it are going in opposite directions one's going in and one's going out now each type of symporter is an antiporter as well is going to have an upper limit on how fast it can work and that's called the transport maximum so um once you go above that you can no longer have any more reabsorption as I said all of water reabsorption occurs through osmosis and about 90 percent of the filtered water that's reabsorbed by the kidneys is reabsorbed along with sodium chloride and glucose usually where where sodium goes chloride is going to follow and so will water reabsorption together with the solutes and the tubular fluid again is obligatory it's obliged to follow so the at the end collecting duct based upon what your hydration situation is you may have the facultative reabsorption if you're dehydrated ADH is going to cause you to take more water back into your blood if you're over hydrated you're going to produce dilute urine glucosauria happens because the renal symporters can absorb glucose fast enough if your blood glucose levels above 200 mids per ml the transport maximum is exceeded and some glucose is going to stay in the urine and that's called glucosauria anytime you see Uria at the end of a word it means urine the most common cause of glucosauria is diabetes mellitus because the insulin activity either insulin isn't being produced or the cells are refractory to it and your blood sugar is too high but there's also a genetic disorder that produces a defect in this importer that reduces its ability to work majority of solvent water reabsorption is going to happen in proximal convoluted tubule and most of it is going to involve sodium ions the sodium Transporters in the in the PCT promote reabsorption of a hundred percent of most of the organic solutes like glucose amino acids 80 to 90 percent of bicarbonate which is the buffering agent hco3 minus um 65 of water sodium and potassium 50 of chloride and variable amounts of calcium magnesium and phosphate normally a hundred percent of filtered glucose amino acids lactic acid water soluble vitamins and other nutrients are reabsorbed in the first half of the proximate proximal convoluted to tubuled by sodium ions and quarters sodium hydrogen antiporters are going to achieve the sodium reabsorption return filtered bicarbonate and water to the peritubular capillaries so the proximal convoluted tubules are continually producing hydrogen ions to fuel these antiporters and they combine the hydrogen the carbon dioxide in water remember when we make ATP at the end we had the waste products that carbon dioxide water and we combine those to get carbonic acid the same thing happens here the water and CO2 are combined together to produce carbonic acid and that dissociates into the hydrogen ions and bicarbonate bicarbonate then can go into the blood to help buffer the blood the diffusion of chloride into the interstitial fluid through the paracellular root makes the tubular fluid more positive than interstitial fluid and this promotes passive paracellular reabsorption of sodium potassium calcium magnesium all which have positive positive charges the reabsorption of sodium and some other solutes creates an osmotic gradient and promotes reabsorption of water so if we look here you can see that this sodium supporters so we're going to transport something with sodium in this case we're transporting glucose so we're using because there's more sodium here than there is in the cell when these ion channels open sodium is going to rush in and when it does it's going to transport glucose with it glucose is going to use that energy to enter into the cell and then it's going to leave through facilitated diffusion and go into the capillary the sodium is going to be pumped out of the cell using active transport primary active transport the energy from the ATP and this helps keep the intracellular sodium levels low if they get too high this important wouldn't work because it's only working because there's more sodium outside than inside so we have to keep it low in here and that's why we have to pump it out and here you can see um the sodium antiporters reabsorbing sodiums creating hydrogen ions Okay so here the sodium is the sodium gradient is used to pump hydrogen ions out all right so we have carbon dioxide diffuses in either from the capillary or from the lumen and combines with water to produce carbonic acid and carbonic acid dissociates pretty rapidly into hydrogen ions and bicarbonate the bicarbonate is going to be removed through facilitated diffusion and it's going to go into the capillary to help buffer the blood and the hydrogen ions are going to be pumped out um for every hydrogen ion that's secreted into the tubular fluid one filtered by carbonate eventually returns to the blood um ammonia and the ammonium ion are secreted in the proximal convoluted tubule so these are both filtered at the glomerulus and then secreted by the proximal convoluted tubule cells into the tubules when you take amino acids and you remove the amino group um you generate ammonia and bicarbonate okay so the cells in the proximal convoluted tubule deaminate glutamine and that gives us ammonia as well as bicarbonate at the pH inside the tubule cells most of the ammonia quickly binds hydrogen becomes the ammonium ion and that can substitute for the hydrogen ion on the sodium hydrogen ion antiporters and then it's secreted secreted into the tubular fluid then the sodium bicarbonate symporters provide a route for the reabsorbed sodium in the newly formed bicarbonate under the bloodstream