 Dr. O'Hare, everybody, Dr. O'Hare. This video we're gonna talk about capillaries. So let's start with what are they, right? So arteries carry blood away from the heart, veins carry it back, but capillaries are like the functional unit of your circulatory system. What's the point of carrying blood around unless you can't deliver things and pick up things? So capillaries are gonna be where we deliver nutrients, deliver oxygen, and then pick up waste products, carbon dioxide, metabolic waste, et cetera. And then we do the process over and over and over again. We've already talked about the role of the heart, continually moving that blood around. So here we see a capillary bed. First thing I want you to notice was called the precapillary sphincter. So these are going to generally be closed, right? If every precapillary sphincter was open and all your blood was in your capillaries, there wouldn't be any blood left. So the majority of the time, most of the branches of capillary beds are closed. And then they're allowed to open when they're needed. That's how your body can determine should I send more blood to my muscles right now because I'm exercising, or should I send more blood to my gut because I'm adjusting a huge meal, these types of things. The other benefit of these precapillary sphincters is they basically allow blood to kind of spurt into your capillary beds. It's not continuous flow. And the reason that's important is because fluids are going to take the path of least resistance. So if blood were just to continually flow through capillaries, it would choose the path that seemed to work the best for it where there was the least resistance. So this spurting of blood makes sure that an entire capillary bed gets used and all of your cells get oxygen and get to deliver their waste products. All right, but the key thing I wanna talk about here is what actually happens at capillary beds. This is a very, very important image because this describes how capillary beds work, but also ties into things we're gonna cover soon about the lymphatic system. So here we see a capillary bed. And just so you know, it's not 50-50, it's actually there's slightly more of an arterial end of a capillary than a venous end, but not a huge deal here. But what we're looking at is so in the left-hand side, the arterial side of a capillary, it's primary job is filtration. And that's because there's enough pressure coming through here that it can filter out liquid. It's called capillary hydrostatic pressure. So there's enough hydrostatic pressure flowing through this capillary that it's gonna squeeze or push fluid out of the arterial end of a capillary. So that's why you see that net filtration pressure is a positive number. It's filtering fluid out. So it's gonna filter out about 24 liters of fluid a day. And that fluid becomes interstitial fluid, the fluid between your cells. It bathes your cells with nutrients and oxygen and the like, right? So that's what's happening on the left-hand side. Then we have this place where there's balance in the middle. And then on the venous side, you're gonna see what's called blood colloid osmotic pressure. So osmosis is the movement of water, water following solutes. So in a normal healthy person, there's gonna be enough solute in the blood, in the capillary bed here to suck back in about 20.4 liters. So 24 liters of fluid is filtered out. 20.4 liters is gonna be sucked back in. So this is assuming you have enough blood cells and plasma proteins in your blood. And the most important plasma protein here would be albumin. So as long as you have enough albumin in your blood, you will suck back that 20.4 liters a day. So you look at, let's say someone is starving in protein malnourished. You've ever seen an image of a starving child that's skinny. Then you have a starving child that's skinny that has a pot belly. Well, the child with the pot belly is worse off because the child that's just skinny still must have enough protein in their body where they can still make albumin and still suck this fluid back in. The child that's skinny with a pot belly, that's fluid accumulation called acides. They don't even have enough protein available to make albumin. So yes, 24 liters of fluid might be being filtered out but less than 20.4 liters is being reabsorbed. Here in the United States, what you might see is someone with liver failure. So people with liver failure, they might accumulate liters of fluid a week that has to be drained off and that's because their damaged liver, it can't make the plasma protein albumin. So it could be. So in those two cases, in one case, there isn't the resources to make albumin. In the other case, the liver is too damaged to do so but either way, you're filtering out 24 liters a day, maybe even more if you have like high blood pressure or something but you're reabsorbing less and that's why that fluid is accumulating. Let's go back to a normal healthy person. Normal healthy person is gonna filter out that 24 liters of fluid a day. They'll reabsorb 20.4 liters of it because of the net filtration pressure being a negative number here on the right hand side but where does the other 3.6 liters go? Well, it becomes lymphatic fluid. So if you think about lymphatic fluid is basically recycled blood plasma. The plasma is being squeezed out of the capillaries where it becomes interstitial fluid and then most of it's gonna be sucked back in and become plasma again and then head back to the heart but that 3.6 liters that doesn't make its way back into the capillary becomes your lymphatic fluid and we'll cover this next chapter but it's still gonna be filtered back to the heart. It's gonna work its way to the right atrium but until it does so, it's lymph or lymphatic fluid and as it travels through your lymphatic system including your lymph nodes and your lymphoid organs, right? It's basically a way for your body to clean and monitor what's going on in your blood that it can't do in the circulatory system because the blood's moving too fast. So lymphatic fluid's very important. You need that 3.6 liters of fluid to become lymph rather than just going back into the capillaries. Okay, that is the function of capillaries and the production of lymphatic fluid. I hope this helps. Have a wonderful day. Be blessed.