 In this video, we're going to be talking about circulation through the organs. Now, if you haven't watched our videos about flow, pressure, and resistance, I'd recommend watching those first because that'll give you some necessary information that you're going to need to understand some of the concepts in this video. If you take a look down here, you can see some examples of circuits or vessels that are in series and in parallel. Now, in our video on resistance, we discussed how you can determine the total resistance for vessels that are in series or in parallel. Take a look at this diagram over here. This diagram is a depiction of how the blood supplies to the major organs of the body are organized. If you compare this diagram to the ones over here, does it look more like they're in series or in parallel? Hopefully, you should be able to see that these vessels are arranged in parallel. And that's really the take-home point of this video, that the blood supplies to the major organs of the body are arranged in a parallel circulation. And if you think back to our video on resistance, you'll remember that we can calculate the resistance of vessels in parallel by adding up the inverse of the resistances of the individual blood supplies to get the inverse of the total resistance. And you'll also remember by adding vessels in parallel, you tend to decrease the total resistance of your system. In another way to think about this would be what if the opposite were true? What if the major organs were arranged in a series circulation? You'll remember that the total resistance of vessels in series is calculated by adding up the total resistance of the individual vessels. So the total is necessarily larger than each individual resistance. You don't drop the resistance by adding vessels in series. So if that were true and the organs were arranged in a series circulation, then according to our pressure gradient equation, delta P equals Q or flow times resistance, we'd have a much higher resistance. And to maintain the same amount of flow, we'd need a much higher pressure gradient. That means that the heart would have to work much harder to supply that pressure gradient. So that's one of the advantages of having the organs arranged in a parallel circulation. And hopefully that helps you remember this fact. Now that we understand the structure of the circulatory system, let's discuss some of its specific elements. When blood leaves the heart, it travels through the aorta. That's part of the arterial system. From there, it travels through various elements of the arterial system into the arteriolar system. From there, it enters the capillaries where it participates in gas exchange. Then it travels through the venules and then the veins. And there are some important properties of these vessels that you need to understand. For example, the arterials are the highest resistance element of the circulatory system. Now if we take a look at our pressure gradient equation here, delta P for the pressure gradient equals Q or the flow times the resistance, we can rewrite it as flow equals the pressure gradient over resistance. And we'll see something interesting. Now we already mentioned that the arterioles are going to have a higher resistance. One important thing to know about flow is that it's conserved in the circulatory system. So that means moving from the arteries to the arterioles and the capillaries, etc. The flow is equal. So if we're trying to maintain the same flow with a higher resistance, that means we need a higher pressure gradient. In other words, we're going to have a larger drop in pressure across the arteriolar system than in any of the other systems because of this higher resistance. Again, delta P is a pressure gradient. So we're talking about the difference in pressure between the beginning and the end of the arteriolar system. And it's going to be larger because of that larger resistance. Another thing you need to know is that the capillaries have the largest cross-sectional area in the circulatory system. Now if we take a look at our flow equation here, Q or the flow equals the cross-sectional area A times velocity V, we can rewrite that as the velocity equals the flow over the cross-sectional area. Once again, flow is conserved. If we're trying to maintain the same amount of flow, we increase our cross-sectional area, what happens to our velocity? It decreases. The blood slows down. This is actually advantageous because it means the blood spends more time in the capillaries and it has more time to exchange oxygen and carbon dioxide with the alveoli and the peripheral tissues. And these are just a couple examples of why when you see things like arterioles have the highest resistance and capillaries have the highest cross-sectional area, that you go back and look at the mathematical relationships between different variables because the step one question writers really like to test you on those things rather than just the fact that arterioles have the highest resistance.