IMS 1 TBL 4 Fluid Compartments PDF

Hello. My name is Mike Ferenczi. I will be giving you this lecture on the fluid compartments. The adult body consists of approximately 60 percent water. The water is found in a number of separate compartments, which we shall be examining in this presentation. There are important differences in th...

Hello. My name is Mike Ferenczi. I will be giving you this lecture on the fluid compartments. The adult body consists of approximately 60 percent water. The water is found in a number of separate compartments, which we shall be examining in this presentation. There are important differences in the solutes dissolved in the water of each compartment, and these give each compartment their own distinct qualities. The solutes are all functionally important. We will be learning about the key solutes in each compartment. The word homeostasis is a word which describes the mechanisms to maintain the solute distribution in the compartments. Homeostasis is a mechanism to maintain the status quo in each of our fluid compartments. First, we need to understand a number of important concepts such as membranes, permeability, diffusion and Brownian motion, osmosis, osmolarity and osmolality, tonicity, transport and exchange across membranes, exchange across capillary walls, pH, electrical charge, electrolytes and several more. Some of these concepts you will already have encountered in some of the earlier. presentations and TBL Sessions in the introduction to medical sciences. Some concepts we will explore in this presentation. So to recap, let's talk about biological membranes. IMS 1 TBL 4 Fluid Compartments They usually consist of a lipid bilayer. The bilayer consists primarily of phosphatidylcholine. Lipids consist of a glycerol, phospholipid or glycol lipid, and also contains sterols. The lipid bilayer is about five nanometres thick. That is five times ten to the minus nine metres. In the top right of this image, you can see an electron micrograph showing the plasma membrane, and you can see it looks like a bilayer because it has three layers, or colours: a dark, light and dark region across it. Again, it's five nanometer in diameter (in thickness). And however, it's not just the phospholipids. Lots of signalling proteins and other proteins attach to the membrane, and some of the proteins go right through the bilayer. Here, the image shows a signalling protein crossing the lipid bilayer. The bilayer is represented here as these grey structures and the protein shown in different colours with different colours representing the alpha helices that form the core of these proteins. Saturated lipids form this very regular bilayer. But if there are some unsaturated lipids, then the unsaturated lipids decrease the regularity of the bilayer and give it extra flexibility. IMS 1 TBL 4 Fluid Compartments Lipid membranes are semi permeable, and some types of molecules can diffuse easily across the membrane. For example, they can travel from the intracellular space to the extracellular space or vice versa. Lipid membranes are hydrophobic. Hydrophobic means water hating. Water and charged or polar molecules do not diffuse freely through the membrane. But at the other end, fat soluble molecules can diffuse through biological membranes relatively easily. But their solubility in plasma, extracellular fluid or interstitial fluid is limited, so the fat soluble molecules go through the membrane. But there is not much fat-soluble molecules dissolved in the water component. Large charged molecules are trapped inside or outside the cells. The semi-permeable membrane forms the border between the inside of cells and the outside, the extra cellular space. In reality, all the molecules do cross the membrane, but it's all a matter of time scales. Some molecules diffuse very slowly and others move very quickly. Water I mentioned earlier doesn't get through the membrane very well because it is charged as shown in this model. Here it is, in fact, polar. It has a positively charged region and a negatively charged region. And because of these charges, it does not diffuse through the membrane very quickly. IMS 1 TBL 4 Fluid Compartments But if you wait long enough, it does diffuse. In this slide, you don't really need to remember these numbers, but these are the numbers which show how quickly various molecules will flow through membranes, expressed in centimetres per second. Gases such as oxygen and carbon dioxide travel very quickly through the membrane, and ethanol also diffuses quite quickly, faster than water, slightly faster than water. Water diffuses hundred times more slowly than carbon dioxide, but there is an awful lot of water, The water concentration of liquid water is fifty five moles per litre, so huge concentration. So even if only a few molecules of water can diffuse through the membrane, because of the high concentration there is a reasonable flow of water through the membrane. It is interesting to look at sodium and potassium. These are small charged ions and their permeability coefficients are incredibly small: 10 to the minus 14 cm/s. So these ions really do not flow naturally through the membrane. This slide shows Brownian motion. Every small molecule suspended in the liquid will move according to Brownian motion. You can maybe observe this when you see dust particles in the beam of sunlight. You may have observed these dust particles moving, apparently in random directions. This is called Brownian motion. Molecules dissolved in a solvent move in a random, three dimensional fashion. IMS 1 TBL 4 Fluid Compartments Driven by this Brownian motion, the random forces generated by the thermally excited water molecules can account for the motion. The spontaneous random movement will lead to equilibration so that all solvent volumes will end up with the same concentration of solutes. So this spontaneous random movement will lead to equilibration, all solvent volumes will end up with the same concentration of solutes. As you can see in the diagram, the solutes over time with equilibrate. Osmosis. Spontaneous movement or diffusion of water or other solvent molecules to selectively permeable membrane is described by the word osmosis. So movement is spontaneous, driven by Brownian motion. The movement is directional: to equilibrate the concentration of solvent and solute molecules on both sides of the membrane. No energy is required. The process is driven by the concentration gradient and the kinetic energy of the molecules, as you saw in the simulation of the previous slide. When pure water is added to blood, plasma becomes diluted or hypotonic compared to the inside of the red blood cells and water will flow into the red blood cells to reduce the difference in tonicity, causing the red blood cells to swell and possibly even to burst. So there are two words which are very similar: osmolarity and osmolality. IMS 1 TBL 4 Fluid Compartments We will talk about the difference between these two on this slide. So osmolality, the concentration of all solutes in the liquid, such as plasma measured as osmoles per kilogram of solvent, and the solvent is usually water whereas osmolarity is the concentration measured as osmoles per litre of solvent. So in one case, it's a kilo of solvent and the other case it is a litre of solvent. Now, a litre of water weighs approximately one kilogram of solvent. So in biological systems, the difference is not important. But you will see in the literature both measurements being referred to. Osmolarity is affected by the temperature and the pressure, but not the osmolality. Osmolality is, as we said, a concentration as a function of kilograms of solvent. So it considers a mole per unit weight whereas the osmolarity depends on the volume of the liquid, which will be affected by its temperature and by the pressure. The calculations are based on the concentration of sodium ion, which is a major solute of the extracellular liquid. And on blood glucose and on blood urea nitrogen (BUN), these are the major components of the extracellular space or liquid and is a major contributor to osmolarity. In a clinical setting people refer to the clinical osmolarity as two times the concentration of sodium ion, IMS 1 TBL 4 Fluid Compartments plus the concentration of glucose divided by 18 plus the concentration of blood urea nitrogen divided by two point eight. These numbers 18 and two point eight arise because glucose and blood urea nitrogen are usually measured in milligrams per decilitre and for the osmolarity calculation to be valid, we need to convert these numbers into millimoles per litre. So these factors of 18 and 2.8 allow us to do that. Tonicity is similar to osmolarity, but osmolarity is a property of a solution containing solutes whereas tonicity is the osmotic property of this solution compared to the osmotic property of another solution. So the tonicity is never an absolute number, but it's relative to another solution. Then plasma may be hypotonic, which means low tonicity compared to the intracellular content of the red blood cells. If it has fewer solute molecules, than in the intracellular space of the red blood cells, the solvent, which is water, will diffuse into the red blood cells to make the tonicity equal on both sides of the membrane. That will cause the red blood cells to swell and possibly to burst. Hypertonic plasma, that is plasma, which contains a greater concentration of solutes, will make the red blood cells shrink. So we can look at the distribution of solutes in the different compartments, IMS 1 TBL 4 Fluid Compartments so the extracellular compartments which consist of plasma, lymph and interstitial fluids have a similar composition, but plasma has more proteins than interstitial fluid and intracellular fluid is quite different because it has a lower sodium. So we'll be talking about that later, but have a look at the table. So on the left hand side of the table in the first column. You see the name of all of the solutes, and some are positively charged, such as sodium, potassium, calcium and chlorine ions. Also, organic phosphate and proteins. And in plasma, which is extracellular space, we have a high concentration of sodium and a low concentration of potassium. Significant amounts of calcium. A high concentration of chlorine ions. And we need negative charges to balance the positive charges here. And we also have organic phosphates and proteins, which also contribute to negative charges. And you will notice that proteins here have 17 negative charges for each protein molecule. And that's because the protein is a very large molecule, which on balance at pH7.4, which is the pH in the plasma, these proteins will acquire negative charges, so it's only one millimole per litre, but in terms of charges, this corresponds to about 17 millimoles per litre. IMS 1 TBL 4 Fluid Compartments So the 17 plus 10 plus 105 roughly balance the positive charges that we have in for the sodium, potassium and calcium. Inside cells, and here we take the example of muscle cells, we have much less sodium than in the plasma, but a much higher concentration of potassium, hardly any calcium, and we shall talk more about the role of calcium in activating muscle contraction, a small amount of chlorine, because the negative charge here is contributed largely by the organic phosphates which are present inside the cells, such as adenosine triphosphate and the proteins, which, as I mentioned earlier, are highly negatively charged. And the pH inside muscle cells is somewhat more acidic than in the plasma. So the proteins carry multiple charges, which are on balance negative at normal pH. But it's not a charge that contributes to a osmolarity. It is a total number of particles. As I mentioned before, the proteins inside and outside the cells are there at low concentration, but do contain a large number of electrical charges. Electrical charge determines the electrical gradient, or the electrical field across the cell membrane. Charged solutes are called electrolytes that allow water to conduct electricity. So let's review the factors which affect the cell gradient. Tonicity controls the cell membrane gradient, IMS 1 TBL 4 Fluid Compartments that is the non-diffusible ions on both sides of the membrane. Or rather, the concentration of non diffusible ions on both sides of the membrane. The concentration of each species of solute determines osmolarity and concentration of gradients, move solutes and water across semi permeable membranes. Both cations, such as sodium ions and anions, such as chlorine ions, contribute to osmolarity as they are each species of solutes. Concentration of unbalanced electrical charges determines electric fields and membrane potential. The pH controls the ionisation state of molecules. Changes in cell volume depend on osmolarity in and out of the cell. Tonicity also depends on whether the membrane is impermeable to some solutes. The permeability considerations may depend on the timescale that I mentioned earlier. Now we look at the permeability of real cells and the control of their value. Water moves through the cell membrane slowly. Proteins and phosphates are impermeant and are at higher concentration inside cells than out. Water tends to move into the cells and make them swell to dilute the impermeant anions. But cells don't burst, because there is an active pump called the sodium potassium pump, which maintains a lower sodium inside the cells compared to the outside. IMS 1 TBL 4 Fluid Compartments The pump makes the membrane effectively impermeant to sodium. So net movement of sodium is accompanied by water so there's no net movement of sodium accompanied by water. So sodium is effectively an impermeant external solute, which balances internal, impermeant solutes. Red blood cells do not have mitochondria, so they do not produce ATP, which is necessary to fuel the sodium potassium pump, so they burst when placed in the hypotonic solution. The membrane potential is a measure of the imbalance of electric charge across a cell membrane. This is a voltage recorded between the intracellular space and the extracellular space where the reference electrode is placed, where the reference electrode is placed. So this shows the membrane potential. The membrane potential is somewhere between minus 60 to minus 90 millivolts with inside negative compared to the outside of the cell. There is imbalance of ions. Potassium is high inside the cell and sodium is high outside the cell. So what is the membrane potential? Can we work it out? The membrane potential is a measure of the imbalance of electric charge across the cell membrane. As I mentioned earlier, this is caused by the negative charges of the non-diffusible intracellular proteins and phosphates and the non-equilibrium ion distribution of sodium and potassium maintained by active pumps, IMS 1 TBL 4 Fluid Compartments in particular the sodium-potassium pump. And these next two lines remind you of the imbalance in terms of ion, sodium and potassium with sodium being high on the outside, and potassium being high on the inside. The sodium potassium pump is responsible for maintaining that charge imbalance. And that's because the sodium-potassium pump is electrogenic. In other words, it creates a difference in electric charge because it pumps three positive charges out in exchange for only two positive charges in. So the sodium potassium pump moves out more sodium out of the cells than it brings in, into the cell. So three sodium out for two potassium in. That's a charge imbalance and that contributes to creating a membrane potential. But it requires energy, which is available by the hydrolysis of ATP to ADP. And if energy fails, then the sodium potassium pump will not be able to be active. The membrane potential will disappear and the cell will die. So the Nernst equation is shown on the right of this slide. There's also a diagram showing the sodium potassium pump, which shows three sodium ions being pumped out for two potassium ions being pumped into the cell. And this diagram shows that ATP IMS 1 TBL 4 Fluid Compartments is hydrolysed to ADP and phosphate to fuel this energy-requiring process for moving sodiums out and potassiums in. If we look at this equation, Em is the membrane potential. The electric potential, which is a difference in ionic charge across the membrane. R is the universal gas constant, which is 8.3J/mol. T is the absolute temperature or the thermodynamic temperature in degrees Kelvin. Zero degrees Kelvin is zero absolute, which is -273.15 degrees Celsius, and 37 degrees C corresponds to 310 Kelvin. z is a number of moles of electrons transferred across the membrane. In another word it is the valency of the ions. If we consider here the distribution of sodium on both sides of a membrane, then the valency of sodium is one. And F is the Faraday's constant, which is 96,485 Coulombs per mole. This is the concentration of sodium on the outside. And this is a concentration of sodium on the inside, in moles per litre. And this is log base ten of this ratio, and 2.303 is a factor to convert natural logarithm to log base 10. This ratio RT/zF for sodium at 37 °C comes to 0.0267, the outside concentration of sodium is 0.15 molar. The inside concentration of sodium is 0.01 molar IMS 1 TBL 4 Fluid Compartments So calculating through this equation, we get a membrane potential of 0.07 volts, which corresponds to 72.3 millivolts. And we can develop this equation further to include all the other ions and to have therefore a more precise calculation of the membrane potential as long as one knows the concentration of ions which contribute to the membrane potential. So the Nernst equation describes how a difference in electrolyte concentration on both sides of a semi-permeable membrane results in the membrane potential. For biological cell membranes the equation shows that the membrane potential is proportional to the logarithm of the ratio of the impermeant cation concentrations inside and outside the cell. So let's go back to fluid compartments if we consider a 70 kilo man, the largest compartment is an intracellular fluid that is inside all the cells of the body. And it's all this fluid that is therefore confined by cell membranes. 55% of our body is water, which is about 23 litres for a 70 kilo man containing 60 percent water. Water, which is not intracellular, is extracellular, so the extracellular compartment can be broken down into several spaces, there is the interstitial fluid, which is the water in between cells, including lymph, there is 36 percent or 15 litre of the total body liquid in the interstitial fluid. IMS 1 TBL 4 Fluid Compartments There is blood plasma corresponding to seven percent or three litres of body water and the trans cellular fluids. These are specialised fluid compartments, such as a cerebrospinal fluid in the spinal cord, the choroid plexus and in the brain ventricles. Aqueous and vitreous humour in the eyeballs. The synovial fluid in the joints. Saliva, gastric juice, urine. All these are transcellular fluids and that correspond to just roughly one litre. So the total fluid is 23 litres of intracellular fluid plus 15 litres of interstitial fluid, plus the blood plasma at three litres plus one litre of transcellular fluids, which is 42 litres in all. 42 litres for 70-litre man, a 70 kilo man, 60 percent of total body weight. There is a small difference between men and women in terms of fluid compartments, women body water is slightly less overall than in men because of the higher adipose tissue content in women, which is 55 percent compared to 60 percent water. Age matters, water content as the body decreases with age. This is particularly noticeable in the skin and that's why we have such a large moisturiser industry that tries to maintain water content of the skin. Older people have a tendency to drink less with adverse effects. Clinically, this is called dehydration. Dehydration can be measured from the osmolarity of blood or from the specific gravity or osmolarity of urine. IMS 1 TBL 4 Fluid Compartments By definition, hypertonicity of serum is where this sodium ion concentration is greater than 145 millimoles per litre. In sports medicine one uses the osmolarity of urine, and it is defined as osmolality greater than 700 milliosmoles per kilogram water or a urine specific gravity of 1.02. But urine measurements are less reliable because they are strongly affected by when you exercised last, when you've been drinking last, and how much you've been drinking and so on. So urine osmolarity is not a very precise measure of of dehydration. However, chronic or repeated dehydration leads to a number of problems, such as urinary tract infections, lung infections, kidney stones, kidney failures and seizures. It's particularly dangerous for babies, children and older adults. It's caused by diarrhoea and vomiting. Often, the consequence of infections and fevers. Think of cholera. Think of food poisoning. Intensive exercise in hot weather can cause excessive sweating and more water loss. Medication can cause water loss, diabetes as well. People with diabetes mellitus, type two diabetes will tend to urinate very frequently. Or not drinking enough. IMS 1 TBL 4 Fluid Compartments So now let's consider the very important water flow across capillaries because this is an essential process in all our bodies, which is important to provide nutrients to all our cells in different capillary beds all around our tissues. So this image shows two diagrams of capillaries one on the left, you can see capillary endothelial cells lining or forming the wall of a small capillary, which has a diameter about the same size as a diameter of a red blood cell. And this is from 3 to 8 micrometers in diameter. And these endothelial cells have a nucleus and there's a basement membrane around the cells. And then there are additional cells here called pericapillary cells. And the important thing to remember about capillary endothelial cells is that they have gaps between them. Molecules can flow through, particularly small molecules, proteins, not so much, but small molecules will flow across from the plasma into the interstitial space through the gaps between the cells, and this is shown on this cross section of a capillary where again, we can see an endothelial cell, which is very thin. Only half a micron thick and between two neighbouring cells IMS 1 TBL 4 Fluid Compartments there is a gap. Very small, 10 to 20 nanometres wide with a tight junction somewhere here, which connects it to the endothelial cell, but still it allows small molecules, smaller than four or five nanometre to flow through, so water, for example, will flow through these gaps. And here the pericyte is the same as a pericapillary cell shown on this slide here. And the glycocalyx acts as a sort of membrane lining the inside of the capillary wall and contributes to the physical strength of the capillaries and acts as some sort of filter. And then the basement membrane surrounds the capillary, and will also provide particular diffusion properties to the capillary. So we need to consider a number of processes which contribute to the movement of water and molecules across the capillary wall. Oncotic pressure is also called the colloid osmotic pressure, and it is a pressure exerted by large molecules that do not cross the capillary wall easily. So understanding oncotic pressure is key to understand why water gets back into the bloodstream. We will get back to that a little bit later, but imagine that we've got plasma which contains a lot of plasma protein and that, as we've seen before, this plasma protein can carry negative charge, IMS 1 TBL 4 Fluid Compartments and these plasma proteins are too large to flow out of the capillaries, but water is small enough to flow out, so water will leak out of the capillaries. But if too much water leaks out then the concentration of protein will tend to draw water back in. So we will be looking at this in more detail in some of the slides. So let's remind ourselves that lipid-soluble substances pass through the endothelial cells, so small gas molecules such as oxygen and CO2 diffuse pretty much freely across the endothelial cells. Small water-soluble substances pass through these pores between the cells as we know that these are 5 nanometre in diameter. So plenty big enough for water, sodium, potassium, glucose and amino acids. Exchangeable proteins are moved across by vesicular transport, so it is possible for large proteins to travel across, but they need a proper special mechanism, such as through vesicles. The plasma proteins generally cannot cross endothelial cell membranes and cannot get through the pores between the cells. So let's consider a capillary bed. A capillary bed is shown in the centre of this diagram. We have got an arterial capillary on the left hand side, shown in red, and in purple, we have the venule, the venous side blood vessels, and in-between we have the capillary bed with these relatively permeable capillaries. IMS 1 TBL 4 Fluid Compartments Pc is the hydrostatic pressure in the arterial or venous vessels. Pc, 35 on the arterial side and 15 on the venous side. It's higher here than on the venous side because on this side we have the activity of the heart which pumps the blood and creates this hydrostatic pressure. Pi is a pressure in the interstitial space, and here it's negative because the blood, the heart does not pump the interstitial fluid and we end up with a slightly negative hydrostatic pressure in the interstitial space. Pi, the letter PI represents the oncotic pressure, so PIc is the oncotic pressure in the arterial or venous vessels. So PIc is the oncotic pressure in the capillary of the venous side and the oncotic pressure of the arterial side, and you can see that the oncotic pressure in the venules or arterioles is the same. And that's because the concentration of proteins on the arterial side and on the venous side is the same, and it's largely albumin, which is a protein which contributes to this. In the interstitial fluid, however, we have very low oncotic pressure because there's hardly any protein in this interstitial space. In fact, we saw that the capillary walls are slightly permeable, so by the time the plasma reaches the venules, IMS 1 TBL 4 Fluid Compartments a little bit of the protein will have been pushed out of the capillaries. So the oncotic pressure on the venous side of the capillary bed is slightly higher than on the arterial side. So if we look at these different pressures, the hydrostatic pressure and the oncotic pressure on the arterial side, we see that the pressures add up to push water out. But on the other side, on the venous side, actually there's much less hydrostatic pressure. But the oncotic pressure remains very high, so that means that the pressure gradient is reversed. And on the venous side, the hydrostatic pressure is such that it brings water back into the bloodstream. So on this side, we have water out of the capillary into the interstitial space and then on this side because of the loss in hydrostatic pressure. The water flows there, this pressure is too low to push water out, but the oncotic pressure, sucks water back in because the water wants to dilute the protein inside the venous side, so water flows from the interstitial space into the venules, so this process is really very, very important because it explains why it's possible for our body to maintain a constant blood volume. We lose water in our tissues. But then we regain water because of the oncotic pressure. Very important concept. IMS 1 TBL 4 Fluid Compartments And when the fluid balance is disturbed, we have disease. An example of this is oedema. Oedema is a swelling of a tissue because of excess interstitial fluid, which could be caused by the imbalance of forces causing fluid to move between the blood, plasma, interstitial and lymphatic vessels, which return excess interstitial fluid to the circulation. It's usually a consequence of increased capillary wall permeability to plasma protein. Indeed, if we lose the plasma proteins, the oncotic pressure in the venules will be too low and water will not flow back into the venules. The forces include the hydrostatic pressure from the cardiac cycle. So again, if we have cardiac deficiency, there may not be enough hydrostatic pressure to push the water out and to get the water back. So, the osmotic pressure, we have to remember is due to the proteins in the capillaries, mainly albumin, and we call that the colloid osmotic pressure, which is the same as the oncotic pressure. Thank you. IMS 1 TBL 4 Fluid Compartments