Understanding Capillary Exchange Mechanisms

Hello, everybody. In this video, we'll be covering concepts in Part 2 of Chapter 20, and this ties in with the Part 2 PowerPoint. And in this particular video, we'll be looking at different aspects of how capillaries work with blood vessel physiology, and specifically the exchange. of fluid and solutes across the capillary walls in the processes of filtration and reabsorption. So we'll probably chop up part two into a couple of videos, but in your reading, this covers sections 20.3 and 20.4. So there are variations. in circulatory pathways. And this illustration shows five specific variations. We can kind of look at them as three major groupings of variations. The simplest capillary, the simplest pathway, which is one capillary bed shown here in A. The portal system here in B, which involves two capillary beds. And we have these anastomoses here in C, D, and E. We have an arteriovenous anastomosis or a shunt. We have a venous anastomosis, and here's an arterial anastomosis. So we can kind of lump in anastomoses, which we learned about in the previous video here as our third major collection of variations. But let's look at part A first, the simplest pathway. This is the simplest and most common route for blood involving one capillary bed. So from the heart, focusing here on the left side, from the left ventricle, passing blood through the aortic valve into the aorta and into the major arteries, and then branching in the systemic circulation into the arterioles. And here we are at the capillary bed. where gases and nutrients and wastes are exchanged. And then the blood now that has dumped off all that oxygen absorbs carbon dioxide and other metabolic wastes into venules, the smaller branching veins, which lead into larger veins. And we return all of this deoxygenated blood into the right atrium of the heart. So we're passing blood through only one capillary network. From the time blood leaves the heart until the time it returns. So this is the simplest way of thinking about circulation pathways. Portal system is a concept we'll come back to a number of times this semester, where blood is flowing through two consecutive capillary networks before returning back to the heart. And these portal systems. Portal means to carry. Think of a port, portal, like ships in a port are carrying cargo, right? So we're carrying blood through two or more capillary beds. And some areas in the body where we see portal systems are in the brain, between the hypothalamus of the brain and the pituitary gland, specifically the anterior pituitary. the anterior lobe, in the kidneys, as well as between the intestines and the liver. This is the one you're probably more familiar with if you've heard of the hepatic portal system. So we'll be covering more of that here in our circulation studies, as well as when we learn about the digestive system. So With these anastomoses here in part C, we have two vessels other than capillaries that are converging, right? So we see here in this shunt, we have an artery branching to arterioles, which basically merge right into venules that fuse into veins. So we're kind of... redirecting blood around a capillary bed, kind of like a bypass. And then we have our arterial and venous anastomosis. These reflect the branching nature of many blood vessels, especially the veins. There's a lot of branching anastomosis networks in the venous circuit. There's veins that resemble ladders that are running up and down your arm and legs with lots of cross rungs, the steps of the ladder interconnecting. the veins that are running down the lengths of your arms and legs. And arteries have them too. There's often fewer in the arteries. But we see a lot of detours, right? A lot of interconnections that can reroute blood in case of blockages. So capillary exchange. Now the capillaries are the most important blood. in the body. They contain the blood that needs to be processed, right? So this is a communication zone. The capillaries are thin walled. They have intercellular clefts and gaps between those endothelial cells. Some capillaries have fenestrations, pores. We learn about the sinusoidal capillaries with those very large fenestrations and large clefts. So It's only through the capillary walls where exchanges are made between the blood and the surrounding tissue, not in the arteries, not in the veins. And so that's why we say blood in the capillaries is the most important blood, because this is how the blood is actually delivering the necessary substances of oxygen and nutrients and hormones and picking up CO2 and wastes. That's not happening across other vessels. It's only happening here in the capillaries. So capillary exchange is a two-way street. It's a two-way movement of fluid and all of the dissolved components and suspended components, water, oxygen, glucose, amino acids, and all the building blocks, minerals, antibodies, hormones, wastes, carbon dioxide, ammonia. It's the two-way movement of fluid in and out of the blood crossing the capillary walls. And so we can see that reflected here in our illustration. We see water and oxygen, amino acids for protein synthesis, right? Amino acids are the building blocks of proteins. Glucose are headed out, are being filtered out of the blood. And we see... carbon dioxide and waste and water being reabsorbed into the blood. And we'll learn about these processes of blood pressure and osmotic pressure and the differences between them. They're both powerful forces that are involved in capillary exchange. So there's three routes of chemical passage through the capillary wall. And as we learned, the fenestrations are the pores within the endothelial cell that forms up the wall, the capillary. And these pores can be regulated. They can be opened or covered up with a thin membrane to help allow or restrict the passage of small solutes. But this is generally the route where small solutes enter and leave the capillary blood. We have these intercellular clefts, right? These gaps between the endothelial cells, these passages that allow movement between blood and the tissue cells. And if you look closely here, you can see these little vesicles. These are penocytic vesicles. Penocytosis means cell drinking. So these are little membrane dimples that form and enclose small volumes. of blood plasma or extracellular fluid, interstitial fluid that lies outside of the capillary. Interstitial fluid is just another name for tissue fluid. And these penocytic vesicles can then trap the material dissolved in that fluid in a little membrane vesicle and carry it through the cell. And we can use mechanisms. that you learned about in anatomy one like bulk transport right bulk think costco or sam's club big quantities right you're buying larger volumes or multiple items of the same product in one fell swoop right you're not buying one jar of tomato sauce you're buying a six pack right you're buying in bulk And so exocytosis, we're sending those materials out of the cell. And endocytosis, we're bringing materials into the cell. So we're using different processes like diffusion and transcytosis, filtration and reabsorption, which we call bulk flow. So let's address these three transport mechanisms. So diffusion is the most important method of capillary exchange, where we're exchanging gases and small particles like oxygen, carbon dioxide, glucose, amino acids, hormones, etc. across the capillary wall. And as we know, diffusion is a type of passive transport where substances are moving down their concentration gradient. from an area of high concentration to an area of low concentration. So here we see blood moving out of the heart, oxygen-rich blood, right, that's entering into the aorta and then branching out to the major vessels. So we have all this dissolved oxygen that is brought to the systemic capillaries, so it's a high concentration of oxygen. that diffuses out of the blood into the interstitial fluid, and then it diffuses into the systemic tissue cells. Again, moving from an area of high concentration outside the tissue cell to an area of low concentration inside the tissue cell. These tissue cells are metabolizing, doing their daily activities, and they're generating carbon dioxide waste, which builds up to a high concentration. So CO2 now diffuses out of the tissue cell from an area of high concentration in the cell to an area of low concentration in the interstitial fluid. So that quantity builds up and now it's going to diffuse into the capillary here on the venule end, the venous end, where the CO2 is going to acidify. It's going to become, as it dissolves in water, it's going to become carbonic acid, HCO3. And so that's a common form of carbon dioxide transport in the blood as HCO3. There's that carbon atom. And so we're going to move CO2 as carbonic acid from an area of high concentration in the interstitial fluid to an area of low concentration in that venous blood. off it goes to the right atrium where it can then enter into the pulmonary circuit. So this can only occur if the solute can permeate the plasma membranes. Permeate means penetrate. They can pass through the plasma membranes of our endothelial cells in the capillary or find passages large enough to pass through like fenestrations or intercellular clefts. So you There are certain chemicals that are more easy for capillaries to diffuse than others. And these are lipid soluble substances. So if you are lipid soluble, you can dissolve in lipids, right? You are able to dissolve in more nonpolar hydrophobic substances. So the steroid hormones are lipid soluble, oxygen and CO2, the respiratory gases are lipid soluble. they can easily diffuse through the plasma membrane. Why? Well, remember plasma membrane structure, the phospholipid bilayer. We have the polar phosphate heads on the outside of the membrane shown by these circles, right? They're charged with positive and negative charge. They're polar. So they're more hydrophilic on either side. But the majority of the plasma membrane is made up fatty acid tails, which are non-polar, no net charge, and they're hydrophobic. So small gases like oxygen and CO2 can pass through because they're small. Hydrophobic molecules. like our steroid hormones can pass through because steroid hormones are lipids. They're derived from cholesterol. So they can get through. Small polar molecules like water or ethanol can also pass through without any trouble. But the chemicals that are tougher to diffuse across the membrane are water-soluble. They dissolve in polar substances like water, where there's both positive and negative charges. So large polar molecules like glucose can't cross membranes without additional help. Amino acids, charged molecules, ions, electrolytes like calcium, sodium, chloride, and hydrogen ions, right? They can't get through without assistance. So here. the membrane itself is a barrier. So water-soluble substances to get through without protein assistance, like through a carrier protein or a protein channel, they must find filtration pores and intercellular clefts, and then they're able to get through more easily. But large particles like proteins and red blood cells They're too big. They can't fit across the membrane. They can't easily fit through a protein channel. So they're held back. Mention transcytosis as another mechanism, where this is where the endothelial cells that form the capillary wall, they're picking up material on one side of their membrane by penocytosis, cell drinking, or very specific receptor-mediated endocytosis, where they're taking a specific chemical into the cell. They're kind of targeting it, okay, like a low-density lipoprotein in the digestive tract, an LDL. And they're transporting those vesicles across the cytoplasm of their cell. You can see here, here's some penocytosis or endoreceptor-mediated endocytosis. And so trans means across, right? So the vesicle moves across the cell and then we discharge the material through exocytosis on the opposite side of the membrane and sending it out into the interstitial fluid. So chemicals that move by transcytosis are often challenging chemicals to transport. Fatty acids, albumin, which is a major blood plasma protein, some hormones like insulin. that are more protein-based rather than lipid or steroid-based. As you guys know, and you'll learn more, not all hormones are steroids. There's different types of hormones. Some have proteins in their construction. They're protein-based. So they'll be transported through transcytosis more commonly. If they can't get... out of the capillary through other mechanisms. So let's look at this process of bulk flow, which involves the two forces of filtration and reabsorption. Okay. So first off, filtration is always movement out of the blood into the interstitial fluid, into the tissue fluid. Reabsorption is always movement into the blood. from the tissue fluid, from the interstitial fluid back into the blood. It's a passive process where large numbers of ions, molecules, or particles in a fluid can move together in the same direction. That's why we call it bulk flow. It involves large numbers, not just one ion, but 10,000 ions in one bulk quantity. And it's passive. So the cell's not the body is not using up any ATP to carry out these processes. So the fluid is going to filter out at the arterial end of the capillary bed. Here's the arterial, and it's going to branch into the capillary bed. This is where we see the net filtration of materials leaving the blood. And then at the venous end, is where we see most of the reabsorption. Blood pressure is higher at the arterial end and blood pressure is lower at the venous end. So that encourages... the directional movement. Higher blood pressure helps push material out through filtration, and a lower blood pressure at the venous end encourages the reabsorption of material back into the blood as we drain the capillary beds into the venule. So they're following pressure gradients. Both processes occur from an area of higher pressure to an area of lower pressure. pressure, and they'll continue as long as a pressure difference exists. If for some reason, the pressures would balance out, you wouldn't have much filtration or reabsorption. And these processes are important for regulation of relative volumes of blood and interstitial fluid. ISF is the abbreviation for interstitial fluid. So what is blood hydrostatic? pressure, BHP. It's basically blood pressure, but we know blood contains water in the plasma, and we know water has a pressure. And hydrostatic, hydro is water. So this is the pressure that is the physical force exerted against a surface by a liquid. So you see here in the illustration, all these blue arrows are representing the hydrostatic. pressure of the water within the blood plasma pushing out against the capillary wall. So a great analogy is a garden hose. So we turn on the water hose, turn on the water, water pressure increases. Now imagine that there's holes in the hose, right? Some hoses are porous to promote watering at the roots rather than spraying on top of the plant where we're watering at the roots where the water is absorbed. And so this hose has little holes and we see water squirting out of the hose. This is like the solution being filtered out of the blood following these blue arrows. Pretend the capillary is a hose, right? And we have fluid squirting out through the capillary. And remember, capillaries are adapted with intercellular clefts, which are leaky, the fenestrations, the pores, which allow fluid out. So the fluid squirting out is creating the tissue fluid within the interstitial spaces around the blood capillary. So the filtration process is the pressure-driven movement of fluid and the solutes that they contain from the blood capillary into the interstitial fluid. Filtration is always out of the blood and into the tissue fluid. So That's the major force of filtration. But there's a second force called IFOP, I-F-O-P, which stands for interstitial fluid osmotic pressure. But let's look first here at the diagram. We'll see variations of this diagram. So look at the blood that is flowing from the arteriole into the capillary bed. So we're looking here at a small piece of the capillary bed, just basically one capillary. So The blood pressure measured as BHP, blood hydrostatic pressure, that enters into the capillary is at its highest. It's going to be 35 millimeters of mercury. So the BHP is pushing fluid out of the capillaries into the interstitial fluid. That's the white space around the capillary. And then as the blood moves through the capillary across the bed, the BHP quickly drops. as blood moves from the higher pressure arterial end at 35 millimeters of mercury, right, which is generated initially by the pumping action of the heart. That's where that higher pressure comes from, from the left ventricles contraction to the lower pressure venule end of the capillary, which is around 16 millimeters, about half of the BHP value. at the arterial end. So what is the role of the second force? Well, this is a minor force, this IFOP, interstitial fluid osmotic pressure. Okay. And this also gets into what's the difference between blood hydrostatic pressure and osmotic pressure. So this value is low. Here it's one millimeter of mercury. It's negligible, right? Because when fluid leaves the capillary. It's quickly picked up by surrounding lymphatic capillaries of the lymphatic system. So they're little vessels kind of like blood capillaries, but they're absorbing tissue fluid. And so like a sponge, those lymphatic capillaries quickly absorb any tissue fluid. So it actually leaves very little actual tissue fluid and little protein in the tissue spaces. That's what osmotic pressure is all about. Osmotic pressure is the fluid pressure that's generated by the solutes like proteins that the fluid contains. So if there's little fluid, there's going to be little protein. And so that's not going to be a big draw, as we'll be learning and reinforcing. Water. wants to flow towards areas of higher solutes, of higher protein. And because IFOP is very low, low fluid, low protein, it is a very minor force. So it's not going to be factoring too much into filtration. The major force, number one force by far is BHP promoting filtration. But we have to account everything when we look at capillary exchange. So we do factor in typically one millimeter of pressure generated by IFOP, representing that little bit of fluid and that little bit of protein that's in the tissue fluid. Think of it like a magnet. It's not really a magnet, but it's just a way to visualize. It's like a magnet that's pulling fluid out of the... blood capillary. Those proteins are kind of like little magnets. Remember with osmosis, right? We learned in anatomy one that water flows where the salts goes, or we say water shoots towards the solutes. So water is shooting out of the blood capillary towards the solutes, towards the proteins here within the interstitial fluid. But because there's a little fluid, there's a little protein, there's not much to shoot at. There's not much protein there to draw water out of the blood. So that's the powerful forces that are, BHP in particular is the powerful force, right? That's promoting filtration here at the arterial end. But... At the opposing venous end where reabsorption is occurring, right, we have two other forces, BCOP, B-C-O-P, and I-F-H-P. So we learned reabsorption is the pressure-driven movement from the interstitial fluid back into the blood capillaries, okay? So reabsorption is always back into the blood capillary. So... Here at the venous end, we have a BCOP measure, blood colloid osmotic pressure. Remember, colloids are mixtures that contain very large chemicals like proteins that they don't settle. They stay within the volume of fluid. They're not... sinking to the bottom. Like if you mix a handful of dirt from the garden to a bucket of water and stir it up, right? Everything is mixed together temporarily. And then you let that bucket of muddy water sit for an hour. And then you'll see all that sediment at the bottom of the bucket. Okay. Those are suspensions. So those materials are influenced by gravity and they're going to sink to the bottom of the bucket. But not colloids. Colloids stay suspended within the volume of the fluid. Milk and blood plasma are common colloids. Think of milk sugar, milk protein, milk fat are within the volume of milk. Same thing with plasma. Plasma proteins and other large chemicals like antibodies are within the volume of the blood plasma. So these are big plasma proteins that are too large to cross the capillary wall. So they're staying in the blood. They're non-penetrating proteins. They don't penetrate out. of the vessel. So think of these plasma proteins as the cops, the police that are overseeing reabsorption, okay? And so it's a significant number. It's measured at 26 millimeters of mercury and it's fairly constant across the entire capillary bed, okay? Why does it remain constant, right? Well, those plasma proteins aren't leaving. They're too big. So they stay within the blood throughout the entire capillary bed. So that's a pretty stable measurement for BCOP at 26 millimeters. We're focusing on it here at the venous end because this is where the reabsorption is primarily taking place. So BCOP deals with the force from water's tendency to move across the semipermeable membrane, the cell membrane. Toward the solution with the greater concentration of non-penetrating solutes, which are the plasma proteins in the blood. So remember osmosis, water shoots toward the greater solutes. So you see the arrow, the longer arrow. Water wants to flow out of the tissue fluid, the interstitial fluid, and into the blood where all of those big plasma proteins. It's like you eat a salty pretzel. and you're thirsty, right? This pretzel is making me thirsty. Water follows salt or sugar or big plasma proteins, okay? So that's the nature of osmosis, okay? So we have that as the most significant force, BCOP, promoting reabsorption of water into the blood. The other secondary reabsorption. factor is IFHP. This stands for interstitial fluid hydrostatic pressure, not blood pressure, right? This is tissue fluid hydrostatic pressure. So this is the water pressure within the tissue fluid, okay? And we know there's little tissue fluid, so there's little pressure. It's usually very low. We're measuring here at zero, that little arrow. There's... It's a negligible force. It only becomes a significant factor if we have edema and the volume of tissue fluid is much higher than IFHP is going to be much higher. Okay. So this represents the fluid pushed back into the blood capillaries just from that fluid pressure of the tissue fluid. Okay. But it's... normally close to zero and only becomes a concern in edemic states. As I said, the fluid volume in the ISF, the tissue fluid is very low. So we have to factor it in though, right? When we look at the numbers, but again, just to reinforce, remember BHP is the most powerful force driving, promoting filtration out of the blood and B-COP. is the number one powerful force driving reabsorption back into the blood. So here is an overview of the whole shebang, okay? As we're reinforcing hydrostatic pressure, BHP is forcing fluid out of the capillary blood here at the arterial end of the capillary. And here... At the venous end, osmotic pressure forces fluid into the capillary blood. So we're seeing these arrows moving into the venous end of the capillary. So we can crunch some numbers and calculate net filtration pressure, or NFP. And these are concepts that we'll return back to again when we study the urinary system. And the job of the kidneys to carry out these processes as they're filtering and processing blood and transforming the waste into urine. So the net filtration at the arterial end of the capillaries. So what factors influence that? Well, we know blood hydrostatic pressure, BHP, and IFOP. the interstitial fluid osmotic pressure. So we take 35 plus one for IFOP, we add together BHP plus IFOP, that gives us 36, and we subtract. the reabsorption. So we're going to subtract from that BCOP, the powerful reabsorption factor, that's 26 millimeters. And then we'll add to that whatever our IFHP, which is usually zero or one, here we're using zero. So it's going to be 36 minus 26, which gives us a sum total of 10 millimeters of mercury of pressure. That's the net filtration. pressure. So we're factoring in filtration and reabsorption, and we're left with 10 millimeters of mercury. So 36 millimeters of pressure going out, 26 millimeters of pressure going in. So the net overall filtration pressure is 10 millimeters of mercury. And here at the venous end, we're measuring the net pressures promoting reabsorption. So here we're going to factor in at the venous end, our blood hydrostatic pressure, which is lower, right? At the end of the capillary, but it's at 16. And we're going to add that to our IFOP, which is still one, okay? That's 17. And we're going to subtract from that our BCOP, which we know is the same across. the entire capillary. That's 26 plus our IFHP, which is also the same. It's just the tissue fluid hydrostatic pressure, which is zero. So we have 17 minus 26, that's going to be negative nine. So the positive numbers represent filtration out of the blood. The negative numbers represent reabsorption into the blood. So we see 10 millimeters of pressure of fluid leaving, as we see here at the arterial end, leaving the blood. And we have negative nine, nine millimeters of pressure reabsorbing in. So those numbers are about equal, right? One millimeter of pressure difference, very, very minor. So these numbers pretty much balance themselves out. So some reminders here, the positive number, 10, indicates the net movement of water and small solutes like sodium and chloride out of the blood and into the interstitial spaces. And this is the process of filtering that is producing the tissue fluid. And here at the venule end, the negative number, negative nine, indicates the net movement of water back into the blood. through reabsorption. So some water is always absorbed at the venule end of the capillary because of osmosis, right? Water is shooting towards the solutes, towards BCOP of those plasma proteins that are in the blood throughout the vessel from the arterial end to the venule end. BCOP's the same, 26 and 26 at either end. So that's our net reabsorption. So normally there is nearly as much fluid reabsorbed as filtered. And typically on average, 85% of the fluid filtered is brought back in to the blood through reabsorption. It's like when you clean out your garage, right? Most of the stuff goes back in, right? We leave some of the junk out on the curb for the trash or have a garage sale, but most of it's going to go back in, right? So that's what we're looking at here. So here's our arterial, here's our capillary. So here's the arterial end of the capillary. So we have 33 to 36 millimeters of pressure headed out, 20 to 26 headed in. And so the net filtration pressure is 10 to 13 millimeters out. We just, we said 10 just to simplify. And then at the, the Venule end, the venous end, right, where pressure is lower, 13 to 17 millimeters of pressure out through filtration. Reabsorption is higher, 20 to 26 millimeters of pressure in. So we subtract that and that gives us roughly seven to nine. We said nine millimeters net reabsorption back in. So at the arterial end, the net pressure is outward at. 10 millimeters and fluid leaves the capillary. due to filtration. That's our net filtration pressure, 10 millimeters. And at the vein ascends, we have our net reabsorption pressure at negative nine. As fluid is moving inward from the tissue fluid into the blood capillary through the reabsorption and then into the venules and the veins, it goes. So the numbers are pretty much equal, as much fluid reabsorbed as filtered. So which statement is true of capillary exchange? Interstitial fluid hydrostatic pressure, IFHP, pushes fluid from the capillaries to the interstitial fluid. Blood hydrostatic pressure, BHP, is lower at the venous end of a capillary. Blood colloid osmotic pressure, BCOP, is due to dissolved ions. Blood hydrostatic pressure is lower at the arterial end of a capillary. The answer is B. BHP is lower at the venous end of the capillary, which is true, right? Higher at the arterial end, they said it's around 36 and roughly 16 at the venous end. So we refer to this balancing act of fluid exchange as Starling's hypothesis. This states that the fluid movement due to filtration across the capillary wall is dependent on the balance between the hydrostatic pressure gradient of the fluid pressure in blood and the tissue fluid and the osmotic pressure gradient of those proteins within the tissue fluid and the proteins in the blood. plasma or plasma proteins. So fluid that is not reabsorbed, which is around three liters per day for the whole body, enters the lymphatic vessels, which eventually merge into veins. And then they're brought back into all that fluid and it's brought back into the body's general circulation. So here's an application. question to consider. A person who has liver failure cannot synthesize the normal amount of plasma proteins like albumin. How does a deficit of plasma proteins affect BCOP, blood colloid osmotic pressure? And what is the effect on capillary exchange? Okay, so let's break this down, right? Here's what we just reviewed. with normal capillary exchange, normal net filtration pressure of 10 millimeters of fluid pushed out of the blood. And here's our net reabsorption pressure of negative nine millimeters of mercury of fluid that's reabsorbed back into the blood. Okay. So if we're saying that BCOP is low, well, let's factor that in. Okay. It's not zero. right? There's a deficit, a reduction of plasma proteins, but it's not zero. So we can, we can factor that in here. So we can say, okay, 16 plus one, that's going to be the same. So the blood pressure at the venous end, blood hydrostatic pressure, 16, right? Plus one with the interstitial fluid osmotic pressure, IFOP. Okay. We'll add that together. Okay. That's 17. And now let's reduce the B-COP, which is normally at 26 throughout the capillary. But let's reflect that deficit. Let's bring that down to 20, 26 to 20, okay? Plus zero for IFHP. We'll keep that the same right now. So now we have 17 minus 20 gives us negative three. Oh, that's lower than negative nine. So that means we're not reabsorbing as much fluid into the blood. And there's more fluid accumulating in the interstitial fluid, which results in edema, where we can see swollen feet, swollen ankles, and lower legs. So the B-COP is lower than normal in a person with a low level of plasma proteins, right? Negative 9 is normal. Negative 3 reflects edema. So capillary reabsorption back into the blood is very low, and edema is the major result. So let's match these terms. A lot of terminology here in capillary exchange. So let's match the following. Pressure generated by the pumping of the heart pushes fluids out of the capillaries. Look over our answers here. That answer is B, blood hydrostatic pressure, BHP. The pressure created by proteins present in the interstitial fluid. It pulls fluid out of capillaries. That is E, the interstitial fluid osmotic pressure, IFOP. That's our other filtration factor. The balance of pressure determines whether blood volume and interstitial fluid remains steady or change. That's our net filtration pressure. That's the big picture summary overview of capillary exchange to see whether blood volume and the tissue fluid volume are at balance or are changing. Number four, the force due to the presence of plasma proteins pulls fluid into the capillaries from interstitial spaces. That's D, that's our BCOP, blood colloid osmotic pressure. And number five, the pressure due to fluid in interstitial spaces pushes fluid back into the capillaries. That's C. That's our IFHP. That's the second force behind BCOP promoting reabsorption, the interstitial fluid hydrostatic pressure. Okay, we're going to stop here in this video, and then we will finish up the chapter in our next video. So please let me know if you have any questions. Thank you.

So there are variations. in circulatory pathways. And this illustration shows five specific variations. We can kind of look at them as three major groupings of variations. The simplest capillary, the simplest pathway, which is one capillary bed shown here in A.

The portal system here in B, which involves two capillary beds. And we have these anastomoses here in C, D, and E. We have an arteriovenous anastomosis or a shunt.

We have a venous anastomosis, and here's an arterial anastomosis. So we can kind of lump in anastomoses, which we learned about in the previous video here as our third major collection of variations. But let's look at part A first, the simplest pathway.

This is the simplest and most common route for blood involving one capillary bed. So from the heart, focusing here on the left side, from the left ventricle, passing blood through the aortic valve into the aorta and into the major arteries, and then branching in the systemic circulation into the arterioles. And here we are at the capillary bed.

where gases and nutrients and wastes are exchanged. And then the blood now that has dumped off all that oxygen absorbs carbon dioxide and other metabolic wastes into venules, the smaller branching veins, which lead into larger veins. And we return all of this deoxygenated blood into the right atrium of the heart.

So we're passing blood through only one capillary network. From the time blood leaves the heart until the time it returns. So this is the simplest way of thinking about circulation pathways. Portal system is a concept we'll come back to a number of times this semester, where blood is flowing through two consecutive capillary networks before returning back to the heart. And these portal systems.

Portal means to carry. Think of a port, portal, like ships in a port are carrying cargo, right? So we're carrying blood through two or more capillary beds.

And some areas in the body where we see portal systems are in the brain, between the hypothalamus of the brain and the pituitary gland, specifically the anterior pituitary. the anterior lobe, in the kidneys, as well as between the intestines and the liver. This is the one you're probably more familiar with if you've heard of the hepatic portal system. So we'll be covering more of that here in our circulation studies, as well as when we learn about the digestive system.

So With these anastomoses here in part C, we have two vessels other than capillaries that are converging, right? So we see here in this shunt, we have an artery branching to arterioles, which basically merge right into venules that fuse into veins. So we're kind of...

redirecting blood around a capillary bed, kind of like a bypass. And then we have our arterial and venous anastomosis. These reflect the branching nature of many blood vessels, especially the veins. There's a lot of branching anastomosis networks in the venous circuit.

There's veins that resemble ladders that are running up and down your arm and legs with lots of cross rungs, the steps of the ladder interconnecting. the veins that are running down the lengths of your arms and legs. And arteries have them too.

There's often fewer in the arteries. But we see a lot of detours, right? A lot of interconnections that can reroute blood in case of blockages.

So capillary exchange. Now the capillaries are the most important blood. in the body.

They contain the blood that needs to be processed, right? So this is a communication zone. The capillaries are thin walled. They have intercellular clefts and gaps between those endothelial cells. Some capillaries have fenestrations, pores.

We learn about the sinusoidal capillaries with those very large fenestrations and large clefts. So It's only through the capillary walls where exchanges are made between the blood and the surrounding tissue, not in the arteries, not in the veins. And so that's why we say blood in the capillaries is the most important blood, because this is how the blood is actually delivering the necessary substances of oxygen and nutrients and hormones and picking up CO2 and wastes.

That's not happening across other vessels. It's only happening here in the capillaries. So capillary exchange is a two-way street.

It's a two-way movement of fluid and all of the dissolved components and suspended components, water, oxygen, glucose, amino acids, and all the building blocks, minerals, antibodies, hormones, wastes, carbon dioxide, ammonia. It's the two-way movement of fluid in and out of the blood crossing the capillary walls. And so we can see that reflected here in our illustration.

We see water and oxygen, amino acids for protein synthesis, right? Amino acids are the building blocks of proteins. Glucose are headed out, are being filtered out of the blood.

And we see... carbon dioxide and waste and water being reabsorbed into the blood. And we'll learn about these processes of blood pressure and osmotic pressure and the differences between them.

They're both powerful forces that are involved in capillary exchange. So there's three routes of chemical passage through the capillary wall. And as we learned, the fenestrations are the pores within the endothelial cell that forms up the wall, the capillary.

And these pores can be regulated. They can be opened or covered up with a thin membrane to help allow or restrict the passage of small solutes. But this is generally the route where small solutes enter and leave the capillary blood. We have these intercellular clefts, right?

These gaps between the endothelial cells, these passages that allow movement between blood and the tissue cells. And if you look closely here, you can see these little vesicles. These are penocytic vesicles. Penocytosis means cell drinking. So these are little membrane dimples that form and enclose small volumes.

of blood plasma or extracellular fluid, interstitial fluid that lies outside of the capillary. Interstitial fluid is just another name for tissue fluid. And these penocytic vesicles can then trap the material dissolved in that fluid in a little membrane vesicle and carry it through the cell.

And we can use mechanisms. that you learned about in anatomy one like bulk transport right bulk think costco or sam's club big quantities right you're buying larger volumes or multiple items of the same product in one fell swoop right you're not buying one jar of tomato sauce you're buying a six pack right you're buying in bulk And so exocytosis, we're sending those materials out of the cell. And endocytosis, we're bringing materials into the cell. So we're using different processes like diffusion and transcytosis, filtration and reabsorption, which we call bulk flow. So let's address these three transport mechanisms.

So diffusion is the most important method of capillary exchange, where we're exchanging gases and small particles like oxygen, carbon dioxide, glucose, amino acids, hormones, etc. across the capillary wall. And as we know, diffusion is a type of passive transport where substances are moving down their concentration gradient. from an area of high concentration to an area of low concentration. So here we see blood moving out of the heart, oxygen-rich blood, right, that's entering into the aorta and then branching out to the major vessels. So we have all this dissolved oxygen that is brought to the systemic capillaries, so it's a high concentration of oxygen.

that diffuses out of the blood into the interstitial fluid, and then it diffuses into the systemic tissue cells. Again, moving from an area of high concentration outside the tissue cell to an area of low concentration inside the tissue cell. These tissue cells are metabolizing, doing their daily activities, and they're generating carbon dioxide waste, which builds up to a high concentration. So CO2 now diffuses out of the tissue cell from an area of high concentration in the cell to an area of low concentration in the interstitial fluid.

So that quantity builds up and now it's going to diffuse into the capillary here on the venule end, the venous end, where the CO2 is going to acidify. It's going to become, as it dissolves in water, it's going to become carbonic acid, HCO3. And so that's a common form of carbon dioxide transport in the blood as HCO3.

There's that carbon atom. And so we're going to move CO2 as carbonic acid from an area of high concentration in the interstitial fluid to an area of low concentration in that venous blood. off it goes to the right atrium where it can then enter into the pulmonary circuit. So this can only occur if the solute can permeate the plasma membranes. Permeate means penetrate.

They can pass through the plasma membranes of our endothelial cells in the capillary or find passages large enough to pass through like fenestrations or intercellular clefts. So you There are certain chemicals that are more easy for capillaries to diffuse than others. And these are lipid soluble substances.

So if you are lipid soluble, you can dissolve in lipids, right? You are able to dissolve in more nonpolar hydrophobic substances. So the steroid hormones are lipid soluble, oxygen and CO2, the respiratory gases are lipid soluble.

they can easily diffuse through the plasma membrane. Why? Well, remember plasma membrane structure, the phospholipid bilayer. We have the polar phosphate heads on the outside of the membrane shown by these circles, right? They're charged with positive and negative charge.

They're polar. So they're more hydrophilic on either side. But the majority of the plasma membrane is made up fatty acid tails, which are non-polar, no net charge, and they're hydrophobic. So small gases like oxygen and CO2 can pass through because they're small.

Hydrophobic molecules. like our steroid hormones can pass through because steroid hormones are lipids. They're derived from cholesterol. So they can get through.

Small polar molecules like water or ethanol can also pass through without any trouble. But the chemicals that are tougher to diffuse across the membrane are water-soluble. They dissolve in polar substances like water, where there's both positive and negative charges.

So large polar molecules like glucose can't cross membranes without additional help. Amino acids, charged molecules, ions, electrolytes like calcium, sodium, chloride, and hydrogen ions, right? They can't get through without assistance. So here. the membrane itself is a barrier.

So water-soluble substances to get through without protein assistance, like through a carrier protein or a protein channel, they must find filtration pores and intercellular clefts, and then they're able to get through more easily. But large particles like proteins and red blood cells They're too big. They can't fit across the membrane. They can't easily fit through a protein channel.

So they're held back. Mention transcytosis as another mechanism, where this is where the endothelial cells that form the capillary wall, they're picking up material on one side of their membrane by penocytosis, cell drinking, or very specific receptor-mediated endocytosis, where they're taking a specific chemical into the cell. They're kind of targeting it, okay, like a low-density lipoprotein in the digestive tract, an LDL.

And they're transporting those vesicles across the cytoplasm of their cell. You can see here, here's some penocytosis or endoreceptor-mediated endocytosis. And so trans means across, right?

So the vesicle moves across the cell and then we discharge the material through exocytosis on the opposite side of the membrane and sending it out into the interstitial fluid. So chemicals that move by transcytosis are often challenging chemicals to transport. Fatty acids, albumin, which is a major blood plasma protein, some hormones like insulin. that are more protein-based rather than lipid or steroid-based.

As you guys know, and you'll learn more, not all hormones are steroids. There's different types of hormones. Some have proteins in their construction.

They're protein-based. So they'll be transported through transcytosis more commonly. If they can't get... out of the capillary through other mechanisms.

So let's look at this process of bulk flow, which involves the two forces of filtration and reabsorption. Okay. So first off, filtration is always movement out of the blood into the interstitial fluid, into the tissue fluid. Reabsorption is always movement into the blood.

from the tissue fluid, from the interstitial fluid back into the blood. It's a passive process where large numbers of ions, molecules, or particles in a fluid can move together in the same direction. That's why we call it bulk flow. It involves large numbers, not just one ion, but 10,000 ions in one bulk quantity.

And it's passive. So the cell's not the body is not using up any ATP to carry out these processes. So the fluid is going to filter out at the arterial end of the capillary bed.

Here's the arterial, and it's going to branch into the capillary bed. This is where we see the net filtration of materials leaving the blood. And then at the venous end, is where we see most of the reabsorption. Blood pressure is higher at the arterial end and blood pressure is lower at the venous end. So that encourages...

the directional movement. Higher blood pressure helps push material out through filtration, and a lower blood pressure at the venous end encourages the reabsorption of material back into the blood as we drain the capillary beds into the venule. So they're following pressure gradients.

Both processes occur from an area of higher pressure to an area of lower pressure. pressure, and they'll continue as long as a pressure difference exists. If for some reason, the pressures would balance out, you wouldn't have much filtration or reabsorption.

And these processes are important for regulation of relative volumes of blood and interstitial fluid. ISF is the abbreviation for interstitial fluid. So what is blood hydrostatic?

pressure, BHP. It's basically blood pressure, but we know blood contains water in the plasma, and we know water has a pressure. And hydrostatic, hydro is water.

So this is the pressure that is the physical force exerted against a surface by a liquid. So you see here in the illustration, all these blue arrows are representing the hydrostatic. pressure of the water within the blood plasma pushing out against the capillary wall. So a great analogy is a garden hose. So we turn on the water hose, turn on the water, water pressure increases.

Now imagine that there's holes in the hose, right? Some hoses are porous to promote watering at the roots rather than spraying on top of the plant where we're watering at the roots where the water is absorbed. And so this hose has little holes and we see water squirting out of the hose.

This is like the solution being filtered out of the blood following these blue arrows. Pretend the capillary is a hose, right? And we have fluid squirting out through the capillary. And remember, capillaries are adapted with intercellular clefts, which are leaky, the fenestrations, the pores, which allow fluid out.

So the fluid squirting out is creating the tissue fluid within the interstitial spaces around the blood capillary. So the filtration process is the pressure-driven movement of fluid and the solutes that they contain from the blood capillary into the interstitial fluid. Filtration is always out of the blood and into the tissue fluid.

So That's the major force of filtration. But there's a second force called IFOP, I-F-O-P, which stands for interstitial fluid osmotic pressure. But let's look first here at the diagram. We'll see variations of this diagram. So look at the blood that is flowing from the arteriole into the capillary bed.

So we're looking here at a small piece of the capillary bed, just basically one capillary. So The blood pressure measured as BHP, blood hydrostatic pressure, that enters into the capillary is at its highest. It's going to be 35 millimeters of mercury.

So the BHP is pushing fluid out of the capillaries into the interstitial fluid. That's the white space around the capillary. And then as the blood moves through the capillary across the bed, the BHP quickly drops. as blood moves from the higher pressure arterial end at 35 millimeters of mercury, right, which is generated initially by the pumping action of the heart. That's where that higher pressure comes from, from the left ventricles contraction to the lower pressure venule end of the capillary, which is around 16 millimeters, about half of the BHP value.

at the arterial end. So what is the role of the second force? Well, this is a minor force, this IFOP, interstitial fluid osmotic pressure.

Okay. And this also gets into what's the difference between blood hydrostatic pressure and osmotic pressure. So this value is low.

Here it's one millimeter of mercury. It's negligible, right? Because when fluid leaves the capillary.

It's quickly picked up by surrounding lymphatic capillaries of the lymphatic system. So they're little vessels kind of like blood capillaries, but they're absorbing tissue fluid. And so like a sponge, those lymphatic capillaries quickly absorb any tissue fluid.

So it actually leaves very little actual tissue fluid and little protein in the tissue spaces. That's what osmotic pressure is all about. Osmotic pressure is the fluid pressure that's generated by the solutes like proteins that the fluid contains. So if there's little fluid, there's going to be little protein.

And so that's not going to be a big draw, as we'll be learning and reinforcing. Water. wants to flow towards areas of higher solutes, of higher protein. And because IFOP is very low, low fluid, low protein, it is a very minor force. So it's not going to be factoring too much into filtration.

The major force, number one force by far is BHP promoting filtration. But we have to account everything when we look at capillary exchange. So we do factor in typically one millimeter of pressure generated by IFOP, representing that little bit of fluid and that little bit of protein that's in the tissue fluid.

Think of it like a magnet. It's not really a magnet, but it's just a way to visualize. It's like a magnet that's pulling fluid out of the...

blood capillary. Those proteins are kind of like little magnets. Remember with osmosis, right?

We learned in anatomy one that water flows where the salts goes, or we say water shoots towards the solutes. So water is shooting out of the blood capillary towards the solutes, towards the proteins here within the interstitial fluid. But because there's a little fluid, there's a little protein, there's not much to shoot at. There's not much protein there to draw water out of the blood.

So that's the powerful forces that are, BHP in particular is the powerful force, right? That's promoting filtration here at the arterial end. But... At the opposing venous end where reabsorption is occurring, right, we have two other forces, BCOP, B-C-O-P, and I-F-H-P. So we learned reabsorption is the pressure-driven movement from the interstitial fluid back into the blood capillaries, okay?

So reabsorption is always back into the blood capillary. So... Here at the venous end, we have a BCOP measure, blood colloid osmotic pressure. Remember, colloids are mixtures that contain very large chemicals like proteins that they don't settle. They stay within the volume of fluid.

They're not... sinking to the bottom. Like if you mix a handful of dirt from the garden to a bucket of water and stir it up, right?

Everything is mixed together temporarily. And then you let that bucket of muddy water sit for an hour. And then you'll see all that sediment at the bottom of the bucket. Okay. Those are suspensions.

So those materials are influenced by gravity and they're going to sink to the bottom of the bucket. But not colloids. Colloids stay suspended within the volume of the fluid.

Milk and blood plasma are common colloids. Think of milk sugar, milk protein, milk fat are within the volume of milk. Same thing with plasma. Plasma proteins and other large chemicals like antibodies are within the volume of the blood plasma. So these are big plasma proteins that are too large to cross the capillary wall.

So they're staying in the blood. They're non-penetrating proteins. They don't penetrate out. of the vessel.

So think of these plasma proteins as the cops, the police that are overseeing reabsorption, okay? And so it's a significant number. It's measured at 26 millimeters of mercury and it's fairly constant across the entire capillary bed, okay? Why does it remain constant, right? Well, those plasma proteins aren't leaving.

They're too big. So they stay within the blood throughout the entire capillary bed. So that's a pretty stable measurement for BCOP at 26 millimeters. We're focusing on it here at the venous end because this is where the reabsorption is primarily taking place.

So BCOP deals with the force from water's tendency to move across the semipermeable membrane, the cell membrane. Toward the solution with the greater concentration of non-penetrating solutes, which are the plasma proteins in the blood. So remember osmosis, water shoots toward the greater solutes. So you see the arrow, the longer arrow. Water wants to flow out of the tissue fluid, the interstitial fluid, and into the blood where all of those big plasma proteins.

It's like you eat a salty pretzel. and you're thirsty, right? This pretzel is making me thirsty.

Water follows salt or sugar or big plasma proteins, okay? So that's the nature of osmosis, okay? So we have that as the most significant force, BCOP, promoting reabsorption of water into the blood.

The other secondary reabsorption. factor is IFHP. This stands for interstitial fluid hydrostatic pressure, not blood pressure, right?

This is tissue fluid hydrostatic pressure. So this is the water pressure within the tissue fluid, okay? And we know there's little tissue fluid, so there's little pressure.

It's usually very low. We're measuring here at zero, that little arrow. There's...

It's a negligible force. It only becomes a significant factor if we have edema and the volume of tissue fluid is much higher than IFHP is going to be much higher. Okay. So this represents the fluid pushed back into the blood capillaries just from that fluid pressure of the tissue fluid. Okay.

But it's... normally close to zero and only becomes a concern in edemic states. As I said, the fluid volume in the ISF, the tissue fluid is very low.

So we have to factor it in though, right? When we look at the numbers, but again, just to reinforce, remember BHP is the most powerful force driving, promoting filtration out of the blood and B-COP. is the number one powerful force driving reabsorption back into the blood.

So here is an overview of the whole shebang, okay? As we're reinforcing hydrostatic pressure, BHP is forcing fluid out of the capillary blood here at the arterial end of the capillary. And here...

At the venous end, osmotic pressure forces fluid into the capillary blood. So we're seeing these arrows moving into the venous end of the capillary. So we can crunch some numbers and calculate net filtration pressure, or NFP. And these are concepts that we'll return back to again when we study the urinary system. And the job of the kidneys to carry out these processes as they're filtering and processing blood and transforming the waste into urine.

So the net filtration at the arterial end of the capillaries. So what factors influence that? Well, we know blood hydrostatic pressure, BHP, and IFOP.

the interstitial fluid osmotic pressure. So we take 35 plus one for IFOP, we add together BHP plus IFOP, that gives us 36, and we subtract. the reabsorption.

So we're going to subtract from that BCOP, the powerful reabsorption factor, that's 26 millimeters. And then we'll add to that whatever our IFHP, which is usually zero or one, here we're using zero. So it's going to be 36 minus 26, which gives us a sum total of 10 millimeters of mercury of pressure. That's the net filtration. pressure.

So we're factoring in filtration and reabsorption, and we're left with 10 millimeters of mercury. So 36 millimeters of pressure going out, 26 millimeters of pressure going in. So the net overall filtration pressure is 10 millimeters of mercury. And here at the venous end, we're measuring the net pressures promoting reabsorption. So here we're going to factor in at the venous end, our blood hydrostatic pressure, which is lower, right?

At the end of the capillary, but it's at 16. And we're going to add that to our IFOP, which is still one, okay? That's 17. And we're going to subtract from that our BCOP, which we know is the same across. the entire capillary. That's 26 plus our IFHP, which is also the same.

It's just the tissue fluid hydrostatic pressure, which is zero. So we have 17 minus 26, that's going to be negative nine. So the positive numbers represent filtration out of the blood.

The negative numbers represent reabsorption into the blood. So we see 10 millimeters of pressure of fluid leaving, as we see here at the arterial end, leaving the blood. And we have negative nine, nine millimeters of pressure reabsorbing in.

So those numbers are about equal, right? One millimeter of pressure difference, very, very minor. So these numbers pretty much balance themselves out. So some reminders here, the positive number, 10, indicates the net movement of water and small solutes like sodium and chloride out of the blood and into the interstitial spaces.

And this is the process of filtering that is producing the tissue fluid. And here at the venule end, the negative number, negative nine, indicates the net movement of water back into the blood. through reabsorption.

So some water is always absorbed at the venule end of the capillary because of osmosis, right? Water is shooting towards the solutes, towards BCOP of those plasma proteins that are in the blood throughout the vessel from the arterial end to the venule end. BCOP's the same, 26 and 26 at either end.

So that's our net reabsorption. So normally there is nearly as much fluid reabsorbed as filtered. And typically on average, 85% of the fluid filtered is brought back in to the blood through reabsorption. It's like when you clean out your garage, right? Most of the stuff goes back in, right?

We leave some of the junk out on the curb for the trash or have a garage sale, but most of it's going to go back in, right? So that's what we're looking at here. So here's our arterial, here's our capillary. So here's the arterial end of the capillary.

So we have 33 to 36 millimeters of pressure headed out, 20 to 26 headed in. And so the net filtration pressure is 10 to 13 millimeters out. We just, we said 10 just to simplify. And then at the, the Venule end, the venous end, right, where pressure is lower, 13 to 17 millimeters of pressure out through filtration.

Reabsorption is higher, 20 to 26 millimeters of pressure in. So we subtract that and that gives us roughly seven to nine. We said nine millimeters net reabsorption back in.

So at the arterial end, the net pressure is outward at. 10 millimeters and fluid leaves the capillary. due to filtration.

That's our net filtration pressure, 10 millimeters. And at the vein ascends, we have our net reabsorption pressure at negative nine. As fluid is moving inward from the tissue fluid into the blood capillary through the reabsorption and then into the venules and the veins, it goes. So the numbers are pretty much equal, as much fluid reabsorbed as filtered.

So which statement is true of capillary exchange? Interstitial fluid hydrostatic pressure, IFHP, pushes fluid from the capillaries to the interstitial fluid. Blood hydrostatic pressure, BHP, is lower at the venous end of a capillary. Blood colloid osmotic pressure, BCOP, is due to dissolved ions.

Blood hydrostatic pressure is lower at the arterial end of a capillary. The answer is B. BHP is lower at the venous end of the capillary, which is true, right?

Higher at the arterial end, they said it's around 36 and roughly 16 at the venous end. So we refer to this balancing act of fluid exchange as Starling's hypothesis. This states that the fluid movement due to filtration across the capillary wall is dependent on the balance between the hydrostatic pressure gradient of the fluid pressure in blood and the tissue fluid and the osmotic pressure gradient of those proteins within the tissue fluid and the proteins in the blood. plasma or plasma proteins. So fluid that is not reabsorbed, which is around three liters per day for the whole body, enters the lymphatic vessels, which eventually merge into veins.

And then they're brought back into all that fluid and it's brought back into the body's general circulation. So here's an application. question to consider.

A person who has liver failure cannot synthesize the normal amount of plasma proteins like albumin. How does a deficit of plasma proteins affect BCOP, blood colloid osmotic pressure? And what is the effect on capillary exchange?

Okay, so let's break this down, right? Here's what we just reviewed. with normal capillary exchange, normal net filtration pressure of 10 millimeters of fluid pushed out of the blood.

And here's our net reabsorption pressure of negative nine millimeters of mercury of fluid that's reabsorbed back into the blood. Okay. So if we're saying that BCOP is low, well, let's factor that in.

Okay. It's not zero. right? There's a deficit, a reduction of plasma proteins, but it's not zero.

So we can, we can factor that in here. So we can say, okay, 16 plus one, that's going to be the same. So the blood pressure at the venous end, blood hydrostatic pressure, 16, right?

Plus one with the interstitial fluid osmotic pressure, IFOP. Okay. We'll add that together. Okay.

That's 17. And now let's reduce the B-COP, which is normally at 26 throughout the capillary. But let's reflect that deficit. Let's bring that down to 20, 26 to 20, okay?

Plus zero for IFHP. We'll keep that the same right now. So now we have 17 minus 20 gives us negative three.

Oh, that's lower than negative nine. So that means we're not reabsorbing as much fluid into the blood. And there's more fluid accumulating in the interstitial fluid, which results in edema, where we can see swollen feet, swollen ankles, and lower legs. So the B-COP is lower than normal in a person with a low level of plasma proteins, right? Negative 9 is normal.

Negative 3 reflects edema. So capillary reabsorption back into the blood is very low, and edema is the major result. So let's match these terms.

A lot of terminology here in capillary exchange. So let's match the following. Pressure generated by the pumping of the heart pushes fluids out of the capillaries. Look over our answers here. That answer is B, blood hydrostatic pressure, BHP.

The pressure created by proteins present in the interstitial fluid. It pulls fluid out of capillaries. That is E, the interstitial fluid osmotic pressure, IFOP. That's our other filtration factor.

The balance of pressure determines whether blood volume and interstitial fluid remains steady or change. That's our net filtration pressure. That's the big picture summary overview of capillary exchange to see whether blood volume and the tissue fluid volume are at balance or are changing.

Number four, the force due to the presence of plasma proteins pulls fluid into the capillaries from interstitial spaces. That's D, that's our BCOP, blood colloid osmotic pressure. And number five, the pressure due to fluid in interstitial spaces pushes fluid back into the capillaries.

That's C. That's our IFHP. That's the second force behind BCOP promoting reabsorption, the interstitial fluid hydrostatic pressure. Okay, we're going to stop here in this video, and then we will finish up the chapter in our next video. So please let me know if you have any questions.

Thank you.

Transcript for:Understanding Capillary Exchange Mechanisms

Transcript for:
Understanding Capillary Exchange Mechanisms