 Let's talk about vascular resistance. Vascular resistance is just the resistance that we need to overcome to effectively push blood through a vessel in the body, okay? And there's a few important things to know about vascular resistance, and they're all illustrated in this formula here. Now let's write it out a little bigger so we can take a closer look. We have resistance equals 8 times the symbol eta. The symbol eta is a coefficient for the viscosity of the fluid in our system, which in this case is blood, times the length of the vessel over pi r to the fourth power. Now there's no formula sheet on your assembly exams, so you're definitely going to want to commit this formula to memory, but it's very unlikely you'll get a question that asks you to directly calculate resistance, but you might get a question that tests you on the relationship between the different elements in this formula. With that in mind, let's consider our viscosity. Again, the resistance is a function of viscosity, and what determines the viscosity of blood? That would be the hematocrit. The hematocrit is the percentage of blood that's made up of actual red blood cells, so things that increase hematocrit like polycythemia would increase resistance, while things that decrease hematocrit like anemia would decrease resistance, and it's a linear relationship, so we know if we double the viscosity, for example, we'll double the resistance. Now how about length? Length is another linear relationship, so if we double the length, we'll have double the resistance. That's pretty easy. Then we've got the radius, and you'll notice that the radius here is to the fourth power, so what happens if we double the radius? Our resistance now becomes one-sixteenth of what it was before, so one thing you should be aware of is that the radius has more effect on the resistance than either the viscosity or the length of the blood vessel. So now that we know how to calculate resistance, let's consider another scenario. The body isn't just one blood vessel with one length and one radius, it's a bunch of different blood vessels, and they all have different lengths in radii, so how do we account for that? Well, let's look at whether those vessels are in series or in parallel. You can see examples of series and parallel vessels down here. Here are some in series and here are some in parallel. Now you might recall from studying physics that there are formulas that help you calculate the total resistance for circuits that are in series and in parallel, and the good news is these same formulas apply to vascular resistance, so let's take a look. For circuits in series and vessels in series, we can simply add up the individual resistances to get the total, and for vessels in parallel, we can add up the inverses of the resistances that will give us the inverse of the total resistance. Now let's work through an example to help us understand this. Now let's imagine we have three vessels in series like we see here, and each has a resistance of two. So what's the total going to be? It's going to be two plus two plus two equals six. It's just the sum of each individual resistance. So what if we have three vessels in parallel like we see here, and each has a resistance of two? So what's the total going to be? It's going to be one over two plus one over two plus one over two is one over the total. That means one over the total equals three over two, and the total equals two thirds for three vessels in parallel. So you can see the adding vessels in parallel actually lowers the resistance. One final thing to mention about resistance is that the arterioles are the highest resistance vessel in the circulatory system. This is something that I've gotten a lot of questions on, and one thing I think you should really remember about resistance.