[Music] now blood flow so this equation you probably remember from your mcat you know for the sake of step one you definitely don't have to integrate it as frequently okay so i'm just gonna go on for about 30 seconds about where this equation comes from and then we'll just go through the most high yield thing down but the idea here is if you remember back from your circuits right so voltage is equal to current times resistance okay it's like ohm's law but it's the same concept as that see the the current is really just the flow so it's the blood flow times the resistance which is equal to the pressure gradient okay so it's essentially the pressure gradient is your voltage the flow is your current resistance is resistance okay because a circuit is basically like a bunch of pipes which are your blood vessels okay so this is the general concept so if i the idea here is if i have a bigger pressure gradient if at one side of you know if i have a tube here if at one side of the tube the pressure is really really high but at the other side of the tube the pressure is pretty low right blood flow is going to go towards the side of lower pressure okay so there's going to be a fa a higher flow if there's a bigger gradient if there's more resistance in this vessel for example if i have a really tiny vessel versus a really big vessel right the really tiny vessel is going to have a higher resistance and so the resistance is primarily going to be associated with the radius but it's inversely proportional see as i decrease the radius here my resistance went up whereas if i have a really big vessel there's a lot more room for flow for laminar flow okay so that's the idea so the first thing is the highest yield thing here is that the radius is going to be the primary determinant of flow and resistance the radius is proportional to flow higher radius more flow okay lower radius less flow all right that's the first thing and it's because it's to the fourth power so if i double the radius of my blood vessel i'm going to get 16 times the amount of flow okay that's a pretty big change right because it'll be 2 to the fourth power if i double the radius i'm going to get 16 times as much flow okay so that is the number one most important thing so that's the first concept so again if they tell you that you know you have a resistance of let's say 32 okay i don't know what the units are let's just say 32 and they say that okay you double your radius so let's say you double the radius right so then it's going to go up by a factor of two two to the fourth power is 16 okay and the radius and the resistance is inversely proportional to the radius right 32 divided by 16. so the resistance would go down to two so again the big point is that the radius is proportional to flow and inversely proportional to resistance okay and again by doubling the radius you're changing the flow going up by a factor of 16 and the resistance going down by a factor of 16. okay so that's the thing to remember now let's go through the second highest yield thing that i want to talk about here second highest yield thing and to do that we're going to have to skip the equation for a second capillaries have the greatest total cross-sectional area okay so this is very confusing for people so let me give you an example let's say that you get a board question and they ask you which of the following has the greatest total cross-sectional area and they give you capillaries and they give you arterials and they give you the aorta and a lot of people especially the first time they get this question they're going to pick the aorta right because you know you're thinking about you're like okay here's my aorta right it's got this massive lumen and then i'm thinking about capillaries that are really tiny how could it possibly be that the capillaries have a greater greater total cross-sectional area than the aorta right the aorta is a massive blood vessel the reason for that is because you have to watch the wording here they're asking for the total cross-sectional area the total cross-sectional area right so that's all of the area of all of the capillaries because there's so many capillaries right so if you think about your blood vessels right so you start off with your aorta okay so you have your aorta and then you're going to have blood vessels branching off right and as you have your abdominal aorta you have all these blood vessels branching off as the aorta continues down here right and eventually you have your iliac arteries okay and as these vessels branch there's going to be additional branches and additional branches and additional branches right that are constantly coming off and eventually you're going to get to the branches of the branches of the branches of the branches which are the capillaries okay and there's so many more capillaries if you add up the total cross-sectional area of all the capillaries coming off all of these vessels it's going to be a lot more area than this one single aorta here okay so the capillaries in general have the greatest total cross-sectional area if we add them up all together because there's so many of them the total cross-sectional area is so large so if i increase that cross-sectional area i'm going to have a lower blood velocity okay so this is a concept where you can go back to equations and make sense of it or you can just kind of remember that the capillaries are going to have the greatest total cross-sectional area and the lowest blood velocity and quite honestly that's really the way i would remember it but the trick is you know if you understand the reverse of this it's actually a little bit easier because remember the aorta is going to have a very fast blood velocity because the pressures are so high okay so the aorta is going to be the opposite of this so the aorta is going to have a low total cross-sectional area even though when you think about it it's tricky because the aorta when you when you see an image of it you might think okay yeah that that lumen looks pretty big but it's actually going to have a low total cross-sectional area because there's only one aorta but because the total cross-sectional area is so low and you have the same pressure going through the aorta as you do all of these capillaries the blood velocity is actually going to be much higher so when we're talking about resistance remember this is a big factor we've kind of already talked about this arterial is the primary regulator of total peripheral resistance okay going back to your physics i know we all love circuits but um this is still comes up on step one so in series circuits you know is basically when you have i'm just going to draw an example here when you have a bunch of resistors kind of in a row you know just for an example let's say this is my aorta and then i have my brachiocephalic artery here so this is going to be going to the right side and then i have my right subclavian artery okay and so what i'm doing here is i'm just showing you vessel after vessel after vessel right and this might eventually you might go to your axillary artery etc okay and so this is just kind of going we're going to the next artery to the next artery and eventually we're going to get to some distal artery probably in the hand okay as we continue going down this pathway from the aorta the brachiocephalic arteries the right subclavian artery to the axillary artery okay and so as we're going through each one of these right that's going to be in series so everything is going in series so the resistance the total resistance is going to be their resistance from the aorta the resistance from the brachiocephalic artery the resistance from the subclavian artery the resistance from the axillary artery all the way down and that will provide us with a total resistance that the blood has to travel through so there'll be a pressure gradient and a resistance that is going to be dependent on all the vessels that we have to go through in series okay so the concept is again if you go from your aorta to your arteries to your arterials all the way down to the capillaries as you're doing that you're going in series you're going in one kind of unit whereas in parallel for example might be okay well here's your brachiocephalic artery here's your left common carotid artery here's your left subclavian artery you know here's your celiac trunk here's your superior mesenteric artery so in other words all these vessels are kind of branching off at different points we're not going in series they're just all branching off of this one vessel at different places so to figure out the resistance in this situation between each of these vessels we would have to write this equation in parallel which is one divided by the total resistance which would be equal to one over the resistance of each of these vessels that would give us the total resistance amongst all these vessels okay and again just remember and usually you don't have to do a lot of plugging in with this equation but just remember this gets confusing a lot of times people write this equation like this they'll put total resistance is equal to 1 over r1 plus 1 over r2 and they'll just kind of go on and do that remember that's not correct to do this correctly the total resistance is going to be it's going to be 1 over the total resistance so for example if you had you know 1 4 plus 1 half is equal to 1 over rt this is just an example right 1 4 plus 1 half is really just 3 three-fourths is equal to one over rt so then rt would be equal to four over three so this is why it gets kind of tricky so just always remember that this is one over the total resistance okay now this is the money point to remember in series the blood flow is going to be constant through each resistor okay so the blood flow through the arteries through the arterials through the capillaries it's going to be constant the blood flow through the aorta through the brachiocephalic artery through the subclavian artery through the axillary artery it's going to be constant okay so that's the big thing to remember for ceres in other words as this blood is flowing right it's not going to change its flow rate between each one of these vessels it's just going to flow from one vessel to another based on their pressure gradient and the resistance now when we're talking about vessels in parallel for example you know again let's just say that we have some vessel here and every vessel that branches off of this single vessel right this was our aorta for example these vessels will all have the same pressure okay so all the vessels in parallel will have the same pressure okay so that's remember the p in parallel p and pressure so they're all going to have the same pressure so the pressure is constant in parallel so again the flow is constant in series the pressure is constant parallel it kind of makes sense i like thinking of things in series right because if i have you know my aorta going all the way down to my veins i know that the pressure in the aorta is not going to be the same pressure in a single vein for example if i follow that blood all the way down its path and its trajectory i know that the pressure is going to change through each single you know substituent a vessel that i go through as i go through the aorta through the brachiocephalic artery eventually to a capillary eventually to a vein if i'm just sitting on a blood on a blood cell following it all the way through the pressure is going to change throughout my route whereas if i look at these blood vessels that are branching off in parallel they're all going to have the same pressure okay there's no reason why one would have a different pressure because they're all originating from that single same structure the pressure is going to be constant but however the flow could be different depending on at what point that vessel originates now the last two things i want you to remember in this flow equation is that though again we have the change in pressure in the numerator here and all i all i really did to create this equation i took this q and isolated it right so i just divided pressure by resistance so i just have q is equal to change in pressure divided by resistance okay and so this is the change in pressure right here the resistance which i just showed you what it's proportional to is just plugged into this equation here and that's where you get this from it's because it's just the change in pressure and the resistance is on the bottom so all i'm doing is i'm taking i'm flipping this so i get r to the fourth in the numerator and then i get the viscosity and the length in the denominator now n again is viscosity okay and we think i wrote it here viscosity is how thick the blood is essentially so in board questions there's a couple things this can be related to usually it's going to be a patient that has polycythemia or a patient that has anemia and they might be saying well how does the polycythemia affect the flow remember polycythemia would be higher hematocrit a higher viscosity lower flow patient has anemia they might have a higher flow okay because the hematocrit might be lower think of it as like the thickness of the blood if blood's really thick it's not going to flow as easily and if also if a vessel is longer if i have a cylinder that's longer there's more opportunities for that blood flow to encounter resistance okay so length is going to be proportional to resistance longer blood vessel more opportunity to encounter resistance