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Current time:0:00Total duration:3:23

Video transcript

let's say that you've got a blood vessel here and it's a long ish blood vessel and we'll give it a resistance of eight and it has three branches say two big ones and kind of a tiny one in the middle and this goes straight across and this has a resistance of ten and these biggish ones they have resistance of let's say half that so they're about five and on this side they all come together again and enter a short vessel this has a resistance of three so my question to you is what is the total resistance of blood going in here and out here so it's going to have to go through this eight bit and then it has three choices here here here or here but eventually they all come together again into that three bit and then exit out the other side so what is the total resistance so what is RT for this that's the question and what I'm going to do is I'm going to divide into two parts part one I'm going to figure out in part one what the resistance is for this part right in here so I can do that using an equation I introduced in the last video which was you can basically take R T which is total resistance for that yellow box equals 1 over 1 over 5 plus 1 over 10 plus 1 over 5 and I can look at that and tell you the common denominator is going to be 10 right for all 3 and you know the numerator I've got 2 1 and 2 so putting it all together I've got 1 over what is that 5 over 10 and that equals 10 over 5 which equals 2 so that tells me that the resistance in this middle yellow box is 2 and that makes sense with our rule because we said that when things are in peril the total resistance is going to be less than any component and in fact two is less than five ten and five right less than any of those numbers individually so we've got now in part two we have three things in series right we basically have something like this we have eight and we have two and we have three so we've got basically three things in a series and so we simply add those up so I'm going to say RT now equals 8 plus 2 plus 3 and so RT equals 13 so if I want to know what is my total RT by total resistance I would say it is 13 so that's the answer to this problem and what I want to get you thinking about is total resistance for the body the human body which has obviously more than you know just a few vessels like I have in this diagram we have literally thousands and thousands of vessels