 First step in formation of urine is ultrafiltration of plasma from glomerular capillaries into the tibials. The rate at which there is filtration of plasma at the level of the glomerular capillaries is known as glomerular filtration rate and it is normally 125 ml per minute or if expressed in terms of per day then it is 180 litres per day. Approximately 700 ml of plasma flows into the renal vessels. So if we calculate that how much of this plasma is getting filtered in terms of fraction we need to divide the glomerular filtration rate divided by the renal plasma flow. So it will be 125 ml per minute divided by 700 ml per minute is equal to it comes to approximately 20%. So that means that 20% of plasma which enters into the renal vessels gets filtered. But what are the factors which determine that how much of this plasma will filter? Well there are two things. One is the driving forces across the capillary and second is the characteristics of the capillary that how much it is permeable and how much is its surface area. Okay so first let's discuss what are these forces? Well you might remember the forces which drive the movement of the water across a capillary. Basically these are hydrostatic pressures and the oncotic pressure. Where we say that hydrostatic pressure is the push force that is moves water away from it and the oncotic pressure is the pull force that is pulls water towards it is it. So here also these forces operate. So these forces are present on either side of the capillary wall that is in the capillaries and in the bowman's capsule. So there is hydrostatic pressure of the capillary which is due to the fluid which is present in the capillary. Then there is oncotic pressure in the capillary which is due to the plasma proteins which are there. Then there is hydrostatic pressure in the tubules or we can say bowman's space because of the filtered fluid isn't it? And then there is oncotic pressure in the tubules or in the bowman's space which obviously will not be much or we can say it is zero because plasma proteins are not filtered. So among these which are the forces which promote filtration and which are the forces which oppose filtration? So for determining this you can apply your knowledge about hydrostatic pressure and oncotic pressure. So see hydrostatic pressure of the glomerulus is a push force so it will be the force promoting filtration. Then oncotic pressure of the tubules or oncotic pressure of the bowman's space is the pull force which will again pull water towards the tubules. So again it's a force promoting filtration. Then oncotic pressure of the capillaries is a force opposing filtration and also hydrostatic pressure of the tubules is the force opposing filtration. So how much is the net driving force? When you see now by subtracting the forces opposing filtration from that of forces promoting filtration we can determine the effective filtration pressure or EFP. So we'll just do that and for that we need to know the actual values of these forces. So hydrostatic pressure in the capillaries is 60 millimeter mercury. Then oncotic pressure in the tubules is zero since there are no proteins in the tubule. Then hydrostatic pressure in the tubules is 18 millimeter mercury while the oncotic pressure in the capillaries is 32 millimeter mercury. So we'll just add this 18 plus 32 is equal to 50 millimeter mercury. So on subtracting it comes to 10 millimeter mercury. So the effective filtration pressure is 10 millimeter mercury. So that is the net driving force for filtration. Okay now before we proceed just remember one thing here that as filtration goes on through the capillary what happens these values of oncotic pressure change along the length of the capillaries. So here on the afferent arterial end the oncotic pressure is little less compared to those and the efferent arterial end of the capillary because as filtration proceeds the oncotic pressure will rise isn't it due to the increase in the concentration of the proteins. So obviously this effective filtration pressure also changes along the length of the capillary. So the filtration is also different along the length of the capillary isn't it. But to determine that how much is the effective filtration pressure we actually take mean values around the length of the capillary. So this calculation is based on that okay. So these are the forces which drive plumber filtration. Now there is second factor as we talked about in the beginning that the characteristics of the capillaries that how much is it permeability and how much is the surface area of the capillaries. Because see if there is a capillary say which is very less permeable what will happen the force will not be very effective for filtration but in case of glomerular capillaries this permeability or what we call as hydraulic conductivity is quite high. So obviously if the permeability is high filtration will be more and secondly the surface area of the capillary if surface area is very less then the filtration will be less. So you know that glomerular capillaries is basically a bunch of the capillaries isn't it. So this causes increase in the surface area of the capillaries. So these characteristics of the capillaries are combined together in a coefficient known as filtration coefficient. So we can say that GFR is equal to effective filtration pressure into filtration coefficient. So we saw that effective filtration pressure is 10 millimeter mercury and this filtration coefficient is about 12.5 ml per minute per millimeter mercury. So if we calculate it comes to 125 ml per minute of GFR. Okay so what is the significance of knowing all this? See this aspect that is a filtration coefficient is normally not used for day-to-day regulation of the GFR. It is by changing these forces this effective filtration pressure that body regulates GFR which will be a subject of another video perhaps. However there are certain diseases which affect this particular filtration coefficient like in case of uncontrolled hypertension or diabetes. There is basically thickening of the glomerular capillary membrane and there is loss of this permeability. Also due to the destruction of the capillaries there is loss of the surface areas. So in these diseases what will happen? Over a long term if they are uncontrolled there will be decrease in the glomerular filtration rate which will ultimately affect the excretion of the waste products. So that's all regarding the factors that determine the GFR. Well thanks for watching the video. 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