 Now, let's discuss the topics of flow rate and pressure gradients as they apply to the cardiovascular system. When we discuss flow rate in a vessel, we're really talking about a volumetric flow rate. And that'll become important in a second when we cover the calculation. But by volumetric flow rate, what I mean is that the units are in milliliters per second. Okay? You can see that right here. Let's take a look at how we do the calculation for that flow rate. Our volumetric flow rate is equal to a cross-sectional area of the blood vessel, and that's in centimeters squared times the velocity of blood through the blood vessel, which is in the units of centimeters per second. And now what happens when we multiply centimeters squared times centimeters per second? We get centimeters cubed per second, cubic centimeters per second, and what's another unit that's equal to a cubic centimeter? That would be a milliliter per second. And that's how we arrive at our milliliters per second for our volumetric flow rate here. And I think it's really important to pay attention to the units in this case, because it'll make the calculation a lot more intuitive. Now, you'll sometimes see the calculation for flow rate written like this. Q equals AV, where Q represents the flow rate. A is the cross-sectional area, and V is the velocity of blood through the blood vessel. I'd like to take the opportunity to do a really high-yield physiology tie-in here. That's the fact that if you consider all the different types of blood vessels in the circulatory system, the capillaries actually have the highest cross-sectional area of all those different types of vessels. And again, this should make a little bit of intuitive sense, because between the alveoli and the lungs and all those peripheral tissues, the capillaries are the vessels that are participating in gas exchange. They have a lot of cross-sectional area, and thus a lot of surface area to participate in gas exchange. That's really important for these vessels specifically. You can see a mini example of that here in this image on the right. If you were to add up the total cross-sectional area of all these capillaries, it would be a lot more than if you added up all the cross-sectional area of either the arterios or the arteries or the veins, again, because they're participating in gas exchange with these tissue cells. Now, what does that increased cross-sectional area in the capillary system have to do with the flow rate? Again, a moment ago we mentioned that the way we calculate flow rate is Q equals AV. Flow rate is equal to the cross-sectional area times the velocity of blood through the blood vessel. So if we rewrite that, we can write it as B equals Q over A, or the velocity of blood through the blood vessel is equal to the flow divided by the cross-sectional area. And one thing you need to know about flow between the different vessels in the body is that it's conserved. That means that moving from the arterial system to the arteriolar system to the capillary system, we are conserving flow. That means that the flow is constant. So what does that mean for the capillaries? That means we've increased our surface area and we've left our flow constant. That means our velocity is going to go down. That means we're slowing down the blood when we reach the capillaries, okay? And what does that allow? It allows for the maximum amount of gas exchange. The blood is sticking around longer in the capillaries and it has more time to exchange oxygen and carbon dioxide with the tissues and with the alveoli. Let's tie everything together by mentioning the concept of pressure gradient. You can see the calculation for pressure gradients here. It's delta P, the pressure gradient is equal to Q, the flow through the blood vessel times R, the resistance in that blood vessel. The pressure gradient is the driving force that pushes blood through the circulatory system. So now based on this little calculation here, we know that if we want to move to a blood vessel that has a higher resistance, without changing our flow, we have to increase our pressure gradient. We also know that if we want to change either of our variables that go into the flow rate, either the cross-sectional area or the velocity, we need a higher pressure gradient as well. So if you want to push blood faster through a blood vessel, you need a higher pressure gradient. If you want to push blood over a larger cross-sectional area, you also need a larger pressure gradient.