Hello. Welcome to Byte Size Med. This video is on the different body fluid compartments. The human body is around 50 to 70 percent water. Now this number isn't fixed because the amount of water would vary depending upon different factors, like age, body fat percentage and things like that. This water content of the body is the total body water and it gets distributed inside cells and outside cells. If we consider the water content of the insides of all the cells of the body as one compartment, that is the intracellular fluid or the ICF. And all of the water outside the cells together forms the extracellular fluid, that's the ECF. And what's between the two? A cell membrane. So the cell membrane is what separates the intracellular fluid from the extracellular fluid. Most of the fluid is inside the cells, that's two-thirds the total body water. So the remaining one-third would be the extracellular fluid. If we assume an average of 60 percent of the body weight is total body water, then 40 percent is the intracellular fluid and 20 percent is the extracellular fluid. 60-40-20. The ECF is further divided into fluid inside vessels and fluid outside vessels. Inside the vessels would be the plasma, outside is the interstitial fluid. The interstitial fluid is larger, being around three quarters of the ECF. So the plasma is a quarter. Now what's between these two? A capillary membrane. Now blood can be considered to be a compartment of its own, because it has both an ECF and an ICF. It has plasma and it has cells, like red blood cells. The fluid inside the RBCs is the ICF and the fluid in the plasma, that's the ECF. The fraction of blood that's RBCs is called the hematocrit. There's also fluid in the pericardial cavity, the peritoneal cavity, there's the cerebrospinal fluid. Now these kinds of fluids, they form another compartment called the transcellular compartment, which should be a small part of the extracellular fluid. For now, we're going to focus on the ICF, the ECF, the interstitial fluid, and the plasma. So yes, these compartments have water, but they also have solutes. So between the ECF and the ICF is the cell membrane and between the plasma and the interstitial fluid is the capillary membrane. Fluid from the plasma gets filtered through the capillary membrane to form the interstitial fluid. So the interstitial fluid is an ultrafiltrate of the plasma, and its composition is similar to the plasma. The capillary membrane, it has pores which lets solutes through, except for proteins. Proteins are too large and so don't pass through the membrane. They stay in the plasma. So the interstitial fluid has very little protein. Cations are positively charged, anions are negatively charged. Proteins are negatively charged. Because they can't move across the capillary membrane, by the Gibbs-Donnan effect to maintain balance, the plasma will have more smaller cations like sodium and lesser smaller anions like chloride. But this effect isn't much. So the composition of the two is quite similar, making the predominant cation in the ECF sodium, the predominant anions would be chloride and bicarbonate. Along with plasma proteins in the plasma, the cell membrane is selectively permeable. It's freely permeable to water, but is impermeable to a lot of solutes. So the composition of the ICF and the ECF are different. In the ICF, the major cations are potassium and magnesium, versus sodium of the ECF. Here inside the cell, the anions are organic phosphates like ATP and ADP, and proteins, So proteins are inside the cells and in the plasma, but the interstitial fluid has very little protein. Potassium is higher inside the cells, while sodium is higher outside. But in both compartments, the concentration of cations and anions are equal, keeping them overall electrically neutral. So the volume of total body water is equal to the volume of the ICF plus that of the ECF, while the ECF volume is equal to the volume of interstitial fluid plus the plasma volume. These three, that's the total body water, the ECF and the plasma volume can be measured by placing an indicator in the compartment and allowing it to distribute evenly. Then its dilution is assessed. That's the indicator- dilution principle. Different indicators are used depending upon what we're choosing to measure. Like for total body water, it would be something that crosses the cell membrane and so gets distributed between the two compartments. For the ECF, it should not cross the cell membrane and for the plasma volume, it should not cross the capillary membrane nor should it enter the RBCs. But these two, the interstitial fluid and the intracellular fluid, they are calculated indirectly. The ICF will be the total body water minus the ECF, and the interstitial fluid is the ECF minus the plasma volume. If we have the plasma volume and the hematocrit, then we can calculate the blood volume. That's the plasma volume over one minus the hematocrits. So the compartments have solutes and a solvent. Now these solutions, across the cell membrane, have to be at equilibrium. The ECF osmolarity has to be equal to the ICF osmolarity. The cell membrane is semi-permeable. It's freely permeable to water, but is impermeable to most solutes. So if there's any imbalance, water will move across the membrane to bring back equilibrium. This movement of water is by osmosis. So those are the different body fluid compartments. If this video helped you, give it a thumbs up and subscribe to my channel. Thanks for watching and I'll see you in the next one! :)