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Current time:0:00Total duration:11:50

Putting it all together: Pressure, flow, and resistance

Video transcript

and I'm going to talk to you about pressure resistance and flow so we're going to try to make sure you feel real comfortable with all three of these things by the time we're done so we start with the harps and off of the heart is the aorta this is the largest artery in the body and this is one branch of the aorta I didn't draw a lot of the other ones is the brachial artery and the blood is flowing from the aorta into the brachial artery and let's say that the blood is trying to make its way out to a fingertip for example so on its way out there it makes its way to an arteriole and the blood continues flowing and it goes into a capillary bed and vessels are too small to draw so I just kind of do that thing and it then goes into the other half of the capillary bed where now the blood is deoxygenated so I'm going to draw that as blue that's the part where now the blood is without oxygen and then it continues to go and get collected into a venule which sounds a little bit like the arterial on the other side right and we've got a vein over here and then finally the blood gets collected in a large vein called the vena cava and they're actually two vena cavas so this will be the superior vena cava there's also an inferior vena cava and the blood flow through this half is as you would guess kind of continues to go around and if I was to try to figure out the pressures the blood pressures at different points along this system I'm going to choose some points that I think would be interesting ones to check so it would be good probably to check what the pressure is right at the beginning and then maybe at all the branch points so what the pressure is as the blood goes from the aorta to the brachial artery maybe as it ends the brachial artery and enters the arteriole maybe the beginning and the end of the capillaries also from the venule to a vein and also wrapping it up what the pressure is at the end now these numbers or these pressures can be represented as numbers right like what is the millimeters of mercury that the blood is exerting on the wall at that particular point in the system and earlier we talked about systolic versus diastolic pressure and there we wanted to use two numbers because that's kind of the range the upper and the lower range of a pressure but now I'm going to do it with one number and the reason I'm using one number instead of two is that this is the average pressure over time so the average pressure over time for me you know keep in mind my blood pressure is pretty normal somewhere around 120 over 80 in my arm so the average pressure in the aorta kind of coming out would be somewhere around 95 and in the artery in the arm probably somewhere around 90 again that's what you would expect somewhere between 80 and 120 so 90 is the average because it's going to be not exactly 100 because remember it's spending more time in diastole and relaxation than in systole so it's going to be closer to 80 for that reason and then if you check the pressure over here by this X probably something like let's say 80 and then as you cross the arterial the pressure Falls dramatically so it's somewhere closer to 30 and then here is about 20 here's about 15 let's say 10 over here and then at the very end it's going to be close to a 5 or so so here let me shred that e and 10 and 5 and the unit's here are millimeters of mercury so I should just write that pressure in millimeters of mercury that's the unit's that we're talking about so the pressure Falls dramatically right so from 95 all the way to 5 and the heart is a pump so it's going to instill a lot of pressure in that blood again and pump it around and around and that's what keeps the blood flowing in one direction so now let me ask you a question let's see if we can figure this out let's see if we can figure out what the resistance is in all of the vessels in our body combined so we talked about resistance before but now I want to pose this question see if we can figure it out so what is total body resistance and that's really the the key question I want to try to figure out with you we know that there is some relationship between radius and resistance we talked about vessels and tubes and things like that but let's really figure this out and make this a little bit more intuitive for us so to do that let's start with an equation and this equation is really going to walk us through this this puzzle so we've got pressure P equals Q times R really easy to remember because the letters follow each other in the alphabet and here actually instead of P Lipa Delta P which is really change in pressure so this is change in pressure and a little doodle that I that I always keep in my mind to remember what the heck that means is if you have a little tube the pressure at the beginning let me say start PS is for start and the pressure at the end can be subtracted from one another and that gives you PS minus PE equals Delta P so the change in pressure is really the change from one part of the tube to the end of the tube and that's the first part of the equation so next we've got Q so what is Q this is flow and more specifically it's blood flow and this can be thought of in terms of a volume of blood over time so let's say minutes so how much volume how many liters of blood are flowing in it in a minute or two minutes or whatever number of minutes you decide and that's kind of a hard thing to figure out actually now what we can figure out what we can figure out is that Q the flow will equal the stroke volume and I'll tell you what this is just after I write it and stroke volume times the heart rate so what that means is that basic if you have if you can if you can know how much blood is in each heartbeat so if you know the volume per heartbeat and if you know how many beats there are per minute then you can actually figure out the volume per minute right because the beats would just cancel each other out and it just turns out it happens to be that I am about 70 kilos that's me I'm 70 kilos and for a 70 kilogram person the stroke volume is about 70 milliliters so for a 70 kilo person you can expect about 70 milliliters per beat and as I write this let's say my heart rate is about 70 beats per minute I feel pretty calm and so it's not too fast so the beats cancel as we said and I'm left with 70 milliliters times 70 per minute so that's about 4900 milliliters per minute or if I'll simplify that's a 5 I'll say about so the flow is about 5 liters per minute okay so I figured out the blood flow and that was simply because I happen to know my weight and my weight tells me the stroke volume and I know that there's a change in pressure we got to figure that out soon and lastly this last thing over here is resistance and I'm just gonna I know I've said it before I just want to point out to you again the resistance is going to be proportional to 1 over R to the fourths and so just remember that this is an important issue R is radius and that's the radius of the vessel so let's figure out this equation and how let's figure out the the variables in this equation and how it's going to help us solve the question I asked you know what is the total body resistance okay so if I have to figure out total body resistance let me clear out the board I've got let's say the heart over here I'd like to do the heart in red and it's pumping blood at my aorta all right so blood is going out out of a order and then it's going a branching branching here branching here let's say and then it's going to branch some more and it's going to bring some more and you can see where this is going it's going to keep branching and eventually every branch kind of collects on the venous side all the blood is kind of kind of filtering back in slowly into venules and veins and finally into a vena cava and I should really draw this going like that the blood is going to go back into the vena cava so that's my system and I got to figure out what the total body resistance is here so if I have a system drawn out for myself and I happen to know that here I said 95 and here I said the pressure was 5 then Delta P equals 95 minus 5 which is 90 and I know that there are five liters of blood flowing through per minute and that was my cue so I could say 90 equals five liters per minutes actually let me take a step back from that instead of 90 let me write the unit's 90 millimeters of mercury equals five liters per minute that was my flow that's my cue and I've got Delta P here and my resistance is the unknown so I'll just leave that as R so let's just solve for R so I'll move my flow to the other side so R equals I'll put it here 90 divided by five which is 18 and the units are a little funky but I'll just write them out anyway millimeters of mercury time's minutes divided by leaders so this is the answer to my question what is the total body resistance well we know what the pressures are at the beginning and end of our system and we know that the flow has to be around five liters per minute because that's based on my weight and my heart rate therefore the resistance must be 18 millimeters of mercury times minutes over liters whatever that set of Units means to you it's kind of an abstract thing but but basically I want to demonstrate to you that this powerful equation helps us solve for what would otherwise become a tricky problem to figure out