chapter 26 lecture three is renal physiology in the production of urine and this focuses on filtration so remember we have filtration reabsorption secretion so in filtration you're going to filter part of the plasma in reabsorption you're going to put some water and things that are useful back into the blood and in secretion you're going to remove any extra waste from the blood and secrete it into the urine so excretion rate is the rate of filtration plus secretion minus reabsorption remember filtration secretion add to the urine reabsorption subtracts from the urine so if you wanted to determine the rate of excretion of drug x you would measure how much is in the filtrate um how much is in the urine and how much is in the blood again filtration happens the renal corpuscle reabsorption secretion happened in the renal tubule the fluid that enters the capsular space is glomerular filtrate and the fraction of the plasma that becomes the filtrate is the filtration fraction and the filtration the filtrate is produced by blood pressure the filtration fraction in a healthy person is about 20 percent of the plasma we filter about 48 gallons a day and of that all of it is reabsorbed except one to two quarts the things that enhance filtering thinness of the membranes large surface areas and high blood pressure again the blood pressure is kept high because the difference in diameter between the afferent nephron arterioles and here we can see the filtration membrane we have the capillary and remember this is fenestrated capillary so it has little holes in it they're not big enough for blood cells to go through but they're big enough for other things and then we have the basement membrane and we have the pores that are the filtration slits that are formed by the gaps between the pedicels and the potassites i said before i talk about the messengeal cells mesengeal cells are found amongst the cells in the capillaries okay they're found between the capillary cells and these are going to be important whether they are relaxed or contracted it's going to either increase decrease the rate of filtration so these guys help to regulate filtration the um filtered substances are going to move from the bloodstream and they're going to go through those three barriers first the pores then the basement membrane and then the filtration slits in the pedicels of the podocytes so we're going to be forcing fluids and solutes through membrane by pressure and here you can see the potassites remember that's the visceral epithelium here's another view of these three barriers now the first one again is we have the fenestrated capillaries they have small holes in it this is going to prevent the blood cells and the thrombocytes so all the formed elements are going to be kept in here because of that what escapes through here is going to be large proteins and medium-sized proteins along with all the stuff that's dissolved small proteins all the different solutes the medium proteins are going to be trapped by this basement membrane and then the medium proteins are going to be trapped by the slip membranes filtration slits of the pedicels and all that's going to go back into the blood and we'll leave through the app through the efferent arterial anything that goes through there is going to become part of the filtrate small proteins are still going to be returned but they're returned later on through the process of pinocytosis the net filtration pressure is the total pressure that promotes filtration so when we look at these different pressures here there's three pressures involved there's the glomerular blood hydrostatic pressure capsular hydrostatic pressure and the blood colloid osmotic pressure so blood hydrostatic pressure is simply the the pressure of the blood moving through the blood vessel think of a garden hose and you turn the water on and it's under pressure inside there because the friction of the blood of the water moving through the hose the capsular hydrostatic pressure is the pressure that's exerted by the fluid that's in the capsule and it's going to push back against the blood hydrostatic pressure so the blood hydrostatic pressure is going to promote filtration just like if you had a garden hose that had some holes in it and turned the water on the water would squirt out the holes so the capsular is going against it the blood hydrostatic pressures is pushing it out the third one is the blood colloid osmotic pressure and that's based upon proteins that are found in the blood and that is always going to try to draw water back into the blood to dilute the proteins just like solutes in an osmotic apparatus you have water in it you put solute on one side solute can't go through the membrane but water can water moves through to try to dilute it so this is going to draw water in to try to dilute those plasma proteins so of these three pressures two of them are going to go against filtration one is going to promote it in order to have filtration this the glomerular blood hydrostatic pressure is going to have to be higher than the sum of these two and in a normal person it is usually the glomerular blood hydrostatic pressure is around 55 millimeters of mercury the chp is around 15 and the blood colloid osmotic pressure is around 30. so we end up with a net filtration pressure in a normal person of about 10 millimeters of mercury if you have kidney disease or if you have liver disease in which you're not making plasma proteins if you have kidney disease in which the capillaries become too permeable and proteins can enter the filtrate that is going to decrease the blood colloid osmotic pressure and if we go back if this blood colloid osmotic pressure let's say that this was 10 instead of 30 right then we would have 55 minus 25 and we would have a much higher filtration rate so we would have an increase in the net filtration rate and net filtration pressure and the glomerular filtration rate so you would lose fluid because of that so the glomerular filtration rate is the amount of filtrate formed in all of the renal corpuscles of both kidneys per minute in an average adult male it's about 125 mils per minute in order for us to maintain homeostasis we need to keep that constant if it's too high we're going to lose a lot of important things because it's going to go too fast to be able to make exchanges if it's too low we're not going to remove enough waste because not enough things are going to be filtered through those those three layers so we need to keep it about 10 millimeters of mercury the the pressure at about 10 millimeters mercury in order to get that uh glomerular filtration rate that's ideal if there are changes in that net filtration rate it affects the glomerular filtration rate and if the gbhp goes below 45 it's going to stop so if we look back here if this rate this blood pressure goes below below 45 then we would have 45 minus 45 and there would be no pressure no pressure at all to promote filtration even if it's if it's close to 45 it's still not going to be an ideal situation the glomerular filtration rate is going to be pretty good as long as your mean arterial blood pressure is between 80 and 180.