In this video, we're going to continue our lesson on hemodynamics as we discuss altering resistance in blood vessels. And so, really, there are 3 important factors that affect resistance of blood flow through the cardiovascular system. And recall from our previous lesson videos that resistance refers to any opposition to blood flow, or in other words, how difficult it is for blood to flow through the blood vessels of the cardiovascular system. And, ultimately, resistance is a measure of the amount of friction that the blood encounters as it flows through the cardiovascular system. And so recall that the greater the resistance is, the more difficult it is for the blood to flow through the cardiovascular system. And so, again, there are 3 important factors that affect resistance that we have numbered down below in our text, 1, 2, and 3. And these are, blood viscosity, or how thick the blood is, blood vessel length, or how short or how long the blood vessel is, and blood vessel diameter or how wide or how narrow the lumen is of the blood vessel. And so notice down below in the table, we have a column for each of these three important factors and how they affect resistance. Now before we continue, I want to address the elephant in the room, which is this delicious looking milkshake or smoothie in the top right of the screen. And so you may be thinking, why is this in the lesson? And the reason is that if you've ever had a really thick milkshake or a really thick smoothie with a straw, then even though you may not realize it yet, you already have a pretty good understanding of how these three factors impact resistance. And so notice that in this milkshake or smoothie, that there are 2 different types of straws sticking out of it, which you can think of as blood vessel straws. And so notice that one of these straws is really tall and narrow, kind of like a coffee stirrer straw. And the other straw is short, but broad and wide. Now when you try drinking a really thick milkshake or a really thick smoothie with a tall and narrow straw like this one, you're going to encounter a lot of resistance. And this is because the really thick milkshake or the really thick smoothie is going to flow really, really slowly through this tall and narrow straw. Now on the other hand, when you try drinking a really thick milkshake or a thick smoothie with a short but broad and wide straw, you're not going to encounter much resistance at all. And this is because the really thick milkshake or smoothie is going to flow through this straw relatively quickly. And so your milkshake and smoothie drinking experiences can actually be helpful when it comes to understanding how these three important factors affect resistance. So let's take a look at the very first factor once again, which is blood viscosity. And again, viscosity is really just the thickness of the blood, and so the greater the viscosity, the more thick the blood is. And so if we were to say that water had a relatively low viscosity, because water is pretty fluid, then a substance such as a thick milkshake, or a thick smoothie, or even honey would be substances that are very viscous because they are very thick. And so the more viscous the fluid is, the harder it is to get that fluid flowing through the vessels. And so just like a milkshake, the greater the viscosity or the more thick the fluid is, the more resistance will be encountered, and so the harder it will be for the fluid to flow through the vessel or to flow through the straw. Now the next factor that we have here is blood vessel length, which, of course, refers to how short or how long the blood vessel is. Now, the longer the blood vessel is, the more opportunities there are for blood to encounter resistance with the blood vessel walls, and so the more resistance there will be. And so, again, just like drinking a milkshake, the longer the blood vessel is or the longer the straw is, the more resistance will be encountered. Now, last but not least, we have blood vessel diameter, which, again, refers to how wide or how narrow the blood vessel is. And it turns out that this factor is the most easily altered physiologically, and this is because we know that many blood vessels have smooth muscles in their walls that allow them to contract to narrow down their diameters, and then the smooth muscle can relax to widen their diameters. Now, again, just like drinking a smoothie, the larger the diameter of the blood vessel or the larger the diameter of the straw, the less resistance is encountered. And so it turns out that blood viscosity and blood vessel length, both are directly proportional to resistance. So the greater the viscosity and the greater the length, the more resistance. However, blood vessel diameter is inversely proportional to resistance, and so the greater the diameter, the less resistance there is. So let's take a look at this image down below, which is really just a table, and notice that in this table, again, we have columns for viscosity, vessel length, and vessel diameter. And what you'll notice is that, this row that you see here are all going to be, factors that would lead to lower resistance. Whereas the second row that you see here are going to be factors that lead to higher resistance. And so, again, when it comes to blood viscosity, it is directly proportional with resistance. And so the lower the viscosity, the lower the resistance. And notice that the blood flowing through this blood vessel is very, very fluid, and so it's kind of flowing similar to how we would expect water to flow through. Since water is not very viscous, it's very, very fluid. Now, down below, what we have is a blood vessel with higher viscosity, and so the blood flowing through here is very, very thick and has the consistency of honey, if you will. Now the next one here is blood vessel length. And, again, the shorter the blood vessel is, the less resistance there will be because there will be fewer opportunities for the blood to encounter friction with the walls. And the longer the blood vessel is, the more opportunities for the blood to encounter friction with the walls, and so that will create higher resistance. And then last, but not least, vessel diameter is inversely proportional with resistance. So the larger the diameter is, the fewer opportunities there are for resistance and friction, and so, that will lead to lower resistance. And the smaller the diameter is, the more opportunities for friction there will be between the blood and the blood vessel walls. And so it's going to be a lot harder for the blood to flow through the smaller diameter than it is the larger diameter. And so this here concludes our lesson on altering resistance in blood vessels, and moving forward, we'll be able to apply these concepts and problems. So I'll see you all in our next video.