In this video, I’ll explain how two different pressures (hydrostatic and osmotic pressure) drive fluid out of and back into your blood vessels. These pressures might seem a bit unimportant at first but they’ll help us explain some interesting cases of edema, or tissue swelling. So, enough talking. :) Let’s do it! Welcome to “Physio Flip”! Here we have a tissue, like muscle or nervous tissue, which is being supplied with blood by this network of capillaries. Capillaries, with their thin walls, pores, and gaps are specially designed to allow for the exchange of nutrients, gases, wastes, and fluid between the blood and the tissues. They’re the place in the cardiovascular system where we can get stuff into and out of the blood. For example, as blood flows into the capillary on the arteriole end, you’ll notice that some fluid is filtered out of the blood vessel. However, we normally don’t want that fluid to stay in the interstitial space as this could lead to fluid collecting in the tissues, causing swelling. Many of you have probably experienced this before: if you have ever stayed standing for too long, you might have noticed that your ankles and feet swell up or experience “edema”. This is caused by fluid staying outside the blood vessel, within the interstitial space, causing the tissue to swell. To prevent this swelling from happening, much of the fluid that was filtered out is normally reabsorbed back into the capillary at the venous end. So fluid out, and fluid back in. But why was fluid forced out in the first place, and how does it get reabsorbed? Let’s zoom in first on the fluid being filtered out to see what’s going on. The pressure that drives fluid movement out of the blood in the capillaries is called hydrostatic pressure. Instead of some fancy definition of what hydrostatic pressure is, I’m just going to show you, using an example that some of you may be able to try out at home! Here I have a straw, like what you might use to drink. What I’m going to do is put my finger on this end of the straw, preventing fluid from leaving. Then I’m going to fill up the straw with water. If I were to take this straw and poke a hole in it over here, what do you think would happen? You might have guessed it: water will begin to leak out. Why? Well, all the water in the straw is being pulled down by gravity, which then applies pressure to the water down here by my finger. This pressure is termed hydrostatic pressure, and generates an outward pressure that will drive fluid out of the straw. You might be thinking: “Alright dude, we don’t care about the straw. What’s with the straw?!” Well, what I just showed you is basically the same thing that happens in our capillaries: our cardiovascular system is basically just a long straw with “holes” in the capillaries. In our capillaries, due to the amount of fluid in our cardiovascular system and the hydrostatic pressure in the arteriole end of the capillaries, fluid is forced out by hydrostatic pressure through microscopic pores and channels in the capillary walls. But if hydrostatic pressure were all we had, fluid would collect in the tissues and we’d experience edema, or swelling. Time for our second key term: osmotic pressure. Osmotic pressure is based on 1 key fact: by osmosis, water always flows towards regions in the body that have a higher osmolarity, or a higher concentration of solute particles. For example, let’s say we have some water, surrounding a vessel shown here in yellow. This vessel has pores or small “holes” which allow certain particles to cross. Inside this vessel, I’ll put a high concentration of some particle (shown here in orange). Notice that this particle’s concentration is much higher on the inside of the vessel, compared to the outside. But if this particle is relatively large, like a protein molecule, and cannot leave this vessel through the small pores, that means that the concentration of these large, protein molecules will remain high inside the vessel and remain low on the outside of the vessel. Since water always flows towards regions with a higher concentration of particles, what do you think water is going to do? Yup-in this case, water is going to be reabsorbed back into this vessel, towards the higher concentration of particles. This situation is almost exactly what is happening in your capillaries. In your blood you’ll find a higher concentration of proteins (like albumin) compared to outside the blood vessels in the interstitial space. Because of this higher concentration of proteins in your blood, fluid tends to be reabsorbed back into the blood, driven by osmotic pressure. But for now, what we’ve learned is that hydrostatic pressure tends to drive fluid out of your blood vessels, osmotic pressure tends to drive it back in. Quick side note: this osmotic pressure, caused by large molecules like proteins, is called colloid osmotic pressure (or oncotic pressure). So, as I mention osmotic pressure in this video, I’m really talking specifically about “colloid osmotic pressure”. Also, we do have other solutes in our blood besides proteins, like Na+ and Cl-. However, these do not contribute to the osmotic pressure or the reabsorption of fluid. If you’re curious as to why, checkout the comments below! Zooming back out, let’s put it all together to see how hydrostatic and osmotic pressure filter and reabsorb fluid at the capillaries. On the arteriole end of the capillary, the hydrostatic pressure is higher than the osmotic pressure. This causes fluid to be forced out into the interstitial space. But as hydrostatic pressure decreases as we move through the capillary, the osmotic pressure eventually becomes equal to and even greater than the hydrostatic pressure, causing nearly all of the filtered fluid to be reabsorbed back into the capillary at the venous end. You might notice that there is still a small fraction of fluid sitting in the tissues. Not all of it was reabsorbed back by osmotic pressure. That’s where our final player comes in: the lymphatic system. Vessels of the lymphatic system will help reabsorb the remaining small fraction of filtered fluid that wasn’t reabsorbed by osmotic pressure, helping us to prevent swelling or edema. There you have it! In the next videos, we’ll review a few cases where fluid does collect within the tissue, caused by abnormal changes in hydrostatic pressure or osmotic pressure. For example, I’ll explain in detail why your ankles swell up when you sit or stand for too long.