Fluid Balance

Osmosis, osmolality, osmolarity, and fluid balance. How hydrostatic pressure and osmotic pressure drive filtration in our capillaries to deliver nutrients and remove wastes. 

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Okay, and welcome to nursing with Dr. Hobbick. Today I am thinking about fluid balance. I know y'all are like, Oh, fluid balance, we're so excited, you should be excited because fluid balances an incredibly important concept to understand for the nurse. I mean, it's important for everybody else to understand it too. But we need to think about fluid balance frequently, not just because our patients could be at risk for fluid overload, or they could be at risk for fluid volume deficit, but because we're going to give them fluid directly into their intravascular space. So let's talk about fluid. The first thing we need to remember is that fluid is not the same thing as water, but rather fluid is water that has things dissolved in it, the things that are dissolved in it represent solutes these are things like sodium chloride, things like proteins, or glucose, those are things that are inside the water, and those things make up a concentration of the fluid. Those are the composition of the fluid. So the number of those is the osmolality or osmolarity of the fluid. We have a fluid amount, which is volume, we have fluid composition, which is what is in the fluid, what electrolytes or solutes are in the fluid. And then we have concentration, the amount of solutes to the amount of water. These are important concepts, because this is how everything else is driven. As far as fluid balance, we need to talk about osmosis, we need to talk about osmotic pole or osmolarity. So first, let's remember where in the body we keep our fluid. There are two main compartments in your body where we keep fluid. One of them is the intracellular fluid. Let's guess where that is. You guessed it inside the cells. intracellular fluid is inside yourself, and that's two thirds of your total body water. Two thirds is inside your cells. The other main compartment is the extracellular compartment, we have fluid that's inside your cells and fluid that's not inside your cells. We actually break that extracellular fluid into three more divisions. Those are the interstitial compartment which is in between and around the cells. We have the intra vascular compartment, which is the fluid that you can find inside your arteries and veins in your vascular system. And then we have the transcellular. Now most of the time, we're talking about intravascular. But we also talk about how the intravascular affects the interstitial, which then affects the intracellular transcellular we don't talk about as much transcellular is fluid that's secreted by epithelial cells. This includes pleural fluid, pericardial, fluid, synovial, fluid, even cerebral spinal fluid. We don't talk about those as much when we're talking about fluid balance as we do the intravascular. And then how that compares or how that impacts our interstitial and intracellular compartments. So one more time we have two main compartments intracellular inside the cells, extracellular outside the cells, and that extracellular is broken into those three divisions have interstitial, which is surrounding ourselves and in between the cells intravascular, which is inside our vessels, our vascular compartment, which is basically our plasma our blood, and then transcellular amount of fluid. This always blows my mind when I talk about this, the amount of fluid in our bodies is That's the sound of my brain exploding. It's so mind blowing every time I think about it. We have about 25 liters of fluid inside ourselves. So always want to think about 12 two liter bottles lined up in front of me. And that's what's inside myself. Remember that intercellular space holds about two thirds of our total body fluid. The other third is somewhere around 15 ish leaders we've got about 10 to 12 liters in the interstitial space which is surrounding the cells and in between them and somewhere around three and a half to six liters in plasma volume. The amount that's transcellular is much smaller, we don't really address that at least not here. I think the orthopedics might disagree, we can have changes in fluid volume, we can have changes in fluid concentration, and these will affect how the fluid behaves where the fluid is in those compartments. Osmosis is the movement of water through a semi permeable membrane to achieve equilibrium of concentration, or osmolarity. Remember as molarity osmolality, that refers to the concentration of the fluid fluid is water with solutes dissolved in it, the more solutes there are, the higher the concentration, I have a semi permeable membrane that the solutes can't get through, but the water can, the water wants everything to be equal, it wants to be equal, let's to be Baby Bear just right, not too high, not too low, the water will move across the membrane if the solutes cannot, in order to achieve equilibrium, you have a semipermeable membrane separating two different concentrations. Let's say on the right hand side, you have 10% Sodium chloride, and on the left hand side, you have 5% Sodium chloride, the water is actually going to move so that there's more water on the more concentrated side so that the two sides will then have an equal concentration of sodium chloride and water. The difference between these two osmolarity is about a concentration per liter osmolality is a osmotic concentration per kilogram. Let's address some other terms that we use for movement of solutes and water. Osmosis refers to the movement of water across a semi permeable membrane, water will move from an area of lesser concentration to an area of greater concentration, working to make that concentration equal on both sides of that membrane. We do have some active transport this requires ATP adenosine triphosphate to move electrolytes across the cell membrane against the concentration gradient, electrolytes those solutes they want to diffuse diffusion is a passive motion, where they move from an area of high concentration to an area of low concentration. I always describe this as if I have a candle and I light my candle and you walk over by where the candle is lit, it smells really strongly. But if I walk far away, it smells less strong. However, when I first lit the candle, it didn't smell at all farther away. Those particles of scent diffused, obviously with air movement, but in a passive process, nothing is really grabbing them and moving them around. Away from that area of high concentration, our electrolytes try to do the same thing. And water at the same time. If the electrolytes are solutes can't move across the membrane, the water will everything is working towards this equilibrium filtration is the movement across the membrane under pressure from higher to lower pressure. That will become important when we talk about filtration, and hydrostatic pressure and osmotic pressure. The other things we need to think about are the fact that as far as a body, a body can have a decrease in volume of fluid. But that means that there's no change in concentration, they can have an increase in volume of fluid with no change in concentration, they could have an increase in concentration, or a decrease in concentration with no change in volume. Or they could have a decrease in volume with an increase in concentration or osmolality. These terms refer to fluid volume excess and increase in volume but no change in concentration, fluid volume deficit, a decrease in volume with no change in concentration. We can have hypernatremia, which is an increase in osmolality or concentration, decrease in osmolality or concentration. And most of the time when we're talking about true dehydration, not always but most of the time. This is a decrease in volume and an increase in concentration. Clinically dehydration, according to some textbooks is the combination of a fluid volume deficit and hypernatremia. You might be sitting there going well Dr. Hobbick. Why are you talking about sodium? I'm talking about sodium because so yum is the most abundant cat ion it is the major player as far as a positively charged ion in our intravascular compartment when we're testing our blood, and we're testing for electrolytes, that's really one of the big ones that we're looking at, contributes the most to the concentration. No, we do also have a lot of chloride, that sodium is our major player. It's one of the reasons why when we use sodium chloride normal saline, we're replacing the electrolytes that are already the most abundant in the intravascular compartment. Let's consider filtration. You imagine we have a capillary and this capillary, I mean, you know that our arteries in our veins are attached they come together, but our capillaries, you have one side of the capillary is arterial, one side of the capillary is venous have four forces at play here. If you imagine your capillary you are representing the intra vascular space inside the capillary, there's fluid inside there, that is in the intravascular space. But on the other side of the capillary membrane, is the interstitial space. There's fluid in the interstitial space as well. Fluid has pressure, hydrostatic pressure is a pushing force. Inside the capillary, the hydrostatic pressure is pushing against the outer wall of the capillary. On the outside of the capillary hydrostatic pressure in the interstitial space is pushing against the wall of the capillary. Now as you can imagine, with the heart pumping behind this fluid that's inside the capillary and the arterial end, we have more pressure, the hydrostatic pressure is higher and if you recall, we can actually squeeze our arteries and what happens if you squeeze the arteries, you know that hose that you used to play with, you turn the hose on and you stick your finger over the end what happens when you made the hose exit the hole smaller, the pressure increase we constrict our arteries we increase that pressure that hydrostatic pressure on the arterial end, the hydrostatic pressure is pushing. We also have our solutes in side of our blood, which contribute to us Modic pressure as monic pressure is a pulling force. Osmotic pressure is where the water goes, you may have heard an instructor say to you that water follows salt, but it's not really that simple. It's that water follows solutes. Remember we said that Osmosis is the movement of water across a semi permeable membrane in order to reach a equilibrium in concentration. If there's more solutes on one side of that capillary membrane, it will pull water to the side that has the most solutes my hydrostatic pressure on the arterial end of the capillary is pretty high, it's pushing outwards a lot higher than the asthmatic pressure is pulling in fluid is pushed out of the capillary. This moves that fluid between the vascular and the interstitial space on the arterial and the hydrostatic pressure overcomes the osmotic pressure inside the vessel and pushes that fluid out. We have hydrostatic pressure, nice amount of pressure in the interstitial space, but it's nowhere near what it is inside the vessel. We're losing water on the arterial end of the vessel. But our albumin, our solutes, say can't get through that membrane, then we've lost water but not solutes, which means the concentration gets higher on the venous and because we've lost that water on the arterial end. Now the hydrostatic pressure is lower on the venous end, but the osmotic pressure is higher because the concentration increased. Now we start to pull fluid back into the venous end, and it brings with it waste products. Those are then taken on to the liver to the kidneys, where they need to go and eventually comes back around to the heart of the lungs to be reactivated and sent back out to the body. When you're thinking about what kind of fluid we're putting into this intravascular space. If I add a hypertonic fluid to the intravascular space hypertonic means that the concentration is higher than that of normal body fluid. That means that I've increased the concentration inside the vessels and water can be pulled into the vessels because the concentration the osmotic pressure has been increased. hypotonic fluid would do the opposite, it would dilute the blood causing water to move out into the interstitial space. When we affect the interstitial space, it will then affect the intracellular space, the water will move back and forth across those compartments. The last thing for us to think about as far as fluid balance in this really short podcast is how the fluid gets there and how it gets out. Obviously, intake is water that we drink or other fluids, jello counts as intake. Don't ask me why. Ice, remember, ice is going to be half because it doesn't take up the full volume of the cup or the measuring tool, ice cream, soups, not chunky soups, though, we would count all of these things as intake, we also need to count any IV fluids that you get the patient, any irrigation that is done, consider a patient who's on continuous bladder irrigation. That means that they have a large bag of fluid that is running into a catheter that goes through their urethra and into their bladder. And then all of that fluid plus the urine and whatever else, maybe blood is drained out of that another Lumen or hole in that catheter into a collection bag. We have to count that irrigation fluid is intake so that when we subtract intake and output, we know how much of the output is irrigation and how much is patient's own body fluids. We would count all of these things as intake. How does the fluid get out? Well, I mean, we all know that fluid gets out as urine, right? If you have urine, if you have liquid stool, we don't count the stool that's formed or you know, constipated stools. But we also count vomit. Sweat is another way for the fluid to get out. Of course, tears and our breath. I'm currently live in Florida. And I joke around that they don't know that their breath has water in it because they can't see it when it gets really cold because it doesn't get cold enough here for that. That's just a joke. I know the Floridians know what that is. We have that breath vapor that's constantly coming out, we have evaporation off of our skin. Those things that we can't see we can't sense are called insensible losses. Those we can't measure, I can't measure how much water you lose through your breath. I can't measure how much comes off your skin. But I do know that if you're breathing faster, you're going to lose more water. If you have a fever, more evaporation will take place I can expect in a normal course the patient could lose anywhere from 500 milliliters to a liter or more of fluid per day. In those insensible losses. We also have fluid that we take in that we can't measure a lot of food is up to 85% fluid or water. And I can't measure that you think about an apple or an orange. Think about those chunky soups, we can't really measure that water, but we know it's there. When we need to measure a patient's fluid balance, we need to measure intake and output, we need to make sure that we are getting an accurate measurement of intake and output. And one of the most sensitive indicators of fluid status is going to be a daily weight. So important to get that weight on the same scale at the same time under the same conditions every day, making sure that your texts or AIDS or whoever is weighing the patient knows you have to have the correct number of blankets or sheets on the bed, nothing extra. And ideally we want to get that weight before they've had any breakfast. We can compare that weight and a kilogram or 2.2 pounds of weight equals a liter of fluid. So my patient has gained three pounds overnight. That's over a liter of fluid. That's pretty bad. We need to do something we need to let the provider know. The other things that we can monitor to help us know about our patients fluid status are the patient's sodium level, the patient's hematocrit and their urine specific gravity. If the patient has too little body fluid, they've lost water, the concentration of the blood will go up. That means the sodium will go up because when we measure it we're measuring how much is sodium is in this much of blood. hematocrit is a measure of the percentage of the blood that is a solid and plasma if your hematocrit is high. Then you have hemo concentration, a lot of solids and not enough fluid. If your hematocrit is low. Then you have hemo dilution you have a lot of fluid For a lesser amount of solids, we monitor that hematocrit. You'll also need the hemoglobin to give you an idea of what else is going on there. If the hemoglobin is the same across three samples, but the hematocrit changes, we know that it's a fluid balance change. Rather than say a patient who's bleeding. We all know that the kidneys work very hard to maintain fluid balance. The kidneys will dilute the urine to get rid of excess fluid or concentrate the urine to retain fluid and not be putting out as much this will affect the urine specific gravity urine specific gravity will go up. If we don't have enough fluid in the urine is being concentrated, and it will go down if the urine is being diluted, letting you know what the kidneys are thinking is happening and how they're trying to compensate. I hope you enjoyed this discussion of fluid balance. And if you're confused and definitely recommend Khan Academy has some really good videos on this concept. And I hope you have a wonderful day. Thanks for spending the last 20 minutes with me on nursing with Dr. Hobbick. See you next time.