foreign blood flow refers to the volume of blood traveling through a blood vessel an organ or the entire body over a period of time and it can be measured as liters per minute as that blood flows it encounters various factors that resist flow and movement of blood notice vascular resistance the first factor that contributes to vascular resistance is blood viscosity you can think of viscosity as the fluid's thickness or how sticky it is this relationship is directly proportional which can be represented as resistance is proportional to viscosity shown as the Greek letter ETA so this means that as viscosity goes up it's harder for the liquids molecules to slide past each other and so the resistance goes up just imagine a heaping stack of pancakes then grab some maple syrup even when you flip the syrup upside down it doesn't really come out and resist moving right away slowly it gloops out and doesn't really Splash but just coats those pancakes in a delicious film of sugary goodness oh yeah okay now say you have another stack and you grab some orange juice instead and then you pour that orange juice immediately comes out and pretty much goes everywhere this is because the juice is less viscous than the syrup and so there's going to be less resistance to movement taking a look at blood there's lots of large proteins in cells so that makes it pretty viscous and so it moves a lot slower than just plain water or orange juice blood viscosity doesn't change much over time but certain conditions like polycythemia which is where a person has too many red blood cells can increase viscosity and conditions like anemia where a person doesn't have enough red blood cells can decrease viscosity a second factor that affects resistance is total blood vessel length just like viscosity the relationship is directly proportional this can be represented as resistance disproportional to L for length so simply put shorter vessels have less resistance and longer vessels have more resistance because there's more friction resisting flow this means that as a child grows into an adult their blood vessels will get longer and their peripheral resistance will go up a third factor that affects resistance is blood vessel radius which in this case is inversely proportional to the resistance to the fourth power this means that as a vessel's radius goes down its resistance really goes up unlike viscosity and length the radius can change from minute to minute especially the radius of arterials which can vasoconstrict like when you're lying at home on the couch which would decrease the radius and increase resistance or vasodilate like when you're running around outside playing frisbee which would increase the radius and decrease resistance now the equation relating all these variables is resistance R is equal to eight times the viscosity ETA times length L divided by pi times radius R to the fourth power now keep in mind that resistance is also related to blood pressure and blood flow by the relationship blood flow Q is equal to change in pressure Delta p over resistance r so let's apply this to a real life situation and let's say that a person has a blood flow of 300 milliliters per minute going through their corroded artery and then they suddenly develop a blockage of exactly half the artery which can happen with a stroke what would happen to blood flow well a 50 blockage means that the radius is now one half of what it was and looking at our equation since nothing else has changed plugging in one half R for the original R we get one half R to the fourth power or 1 16 R to the fourth meaning resistance goes up by 16 times assuming that blood pressure doesn't change right away subbing in this new 16 times greater resistance we see that the blood flow drops by 16 times from 300 milliliters per minute to 300 over 16 which is equal to 19 milliliters per minute which is a huge drop for just having the radius so that's how resistance works for a single vessel but what if you have a bunch of different sized vessels in a row each with their own resistance and you wanted to figure out the total resistance well since they're all in a row one after another we say that they're in series and you simply add the individual resistances all together to get a total resistance this total is called the serial resistance as an example let's think about blood heading out to a cell in your toe and back after leaving your heart it has to go through an artery then an arterial and then through a capillary alongside the toe cell and then back through a venule and finally through a vein to reach the heart again if we add up all those individual resistances our total will equal the total resistance faced by the blood flow going to the toe cell and then back to the heart assuming there are no branches in the system and no other cells to worry about which isn't really that realistic but that's okay for now another thing to know about this is that when we arrange resistance in series like this the blood flow through each part of the system is actually the same even though the resistance at each level will differ mostly based on the length and radius of each vessel like I said this wasn't really the most realistic because it doesn't account for the millions of Branch points in the circulatory system and for that we have to think about the concept of parallel resistance in short this is when two vessels branch and move blood in parallel and then meet up again which is exactly what happens in the circulatory system since they start at the same point and then there's a lot of branching and then ultimately all the blood meets up again in the right atrium to calculate the total resistance for these portions where the blood vessels split we use 1 over R total is equal to one over R1 plus one over R2 and so on so let's do another example and use both of these equations let's say that we wanted to figure out the total resistance here in this case we have five different individual resistances to worry about let's say that we've got a larger but longer blood vessel with a resistance of six and then it branches into three other vessels the really thin vessel in the middle has a resistance of 10 and the slightly larger ones on either side both have a resistance of five then all the branches come back together and connect to a short wide vessel with a resistance of two now these resistances do have units but for simplicity's sake I'm not showing them but in case you're curious resistance is measured in millimeters of mercury times minutes over liters to figure out the total resistance for this whole system we need to break this down into two parts and we're going to add up the parallel resistance of the three vessels in the middle first so you've got one over five plus one over ten plus one over five which is two over ten plus one over ten plus two over ten which is five over ten which remember in the equation is equal to 1 over R total so if we flip 1 over R total and also 5 over 10 we get our total equals 10 over 5 which equals 2. so this is the resistance of this chunk of parallel vessels and this makes sense because with parallel resistance the total is always less than the resistance of any one component now that we have the combined resistance of the parallel guys the remaining three components are just in series so we add up six plus two plus two and that's 10 or 10 millimeters of mercury times minutes over liters and this happens to be around the actual resistance you see in the entire systemic circulation by comparison the pulmonary circulation has only about one tenth of that resistance and it's closer to one millimeters of mercury times minutes over liters it's also worth mentioning that just like before the blood flow through these bits in series is the same along with the total flow through this whole parallel system the flow through each of these individual parallel vessels though is not the same since the blood has to split its flow through each of the vessels although the sum of these three flows must equal the total flow alright as a quick recap vascular resistance is a measure of the resistance that must be overcome to push blood through the vessels and could be represented by the equation eight times viscosity Ada times length L divided by pi times R to the fourth so it's directly proportional to the viscosity and the length and inversely proportional to the radius to the fourth power resistance in series is equal to the sum of each resistance and one over the resistance in parallel is equal to the sum of the inverse of each resistance helping current and future clinicians Focus learn retain and Thrive learn more