 The presence of fluid in a container results in the generation of a hydrostatic pressure on the base and walls of the container due to weight of the fluid. Similarly in blood vessels, the presence of blood creates a hydrostatic pressure on the walls of the blood vessels. There is a push force and tends to move water outside the blood vessel. At a tearier end of the capillary, this pressure is 32 millimeter mercury. Some fluid is also present in interstitial space. However, due to pull by the tissues, a negative hydrostatic pressure is generated in interstitial space and this negative pressure is 3 millimeter mercury minus 3 millimeter mercury. There is another force which tends to draw in water and that is osmotic pressure. Osmotic pressure is determined by total number of solutes in a solvent. In body, the solutes are proteins and ions. Since ions can freely cross the capillary membrane, they are not effective in generating osmotic pressure on only one side of the membrane. If you remember, the definition of osmosis itself says that movement of water across a semi permeable membrane. If the solutes are also moving, then movement of water will be ineffective. So, we do not take into account the presence of ions while considering effective osmotic pressure. Capillary membrane, however, is impermeable to proteins. The proteins are mostly present inside the capillary and exert an osmotic pressure of 28 millimeter mercury. However, this impermeability to proteins is not absolute and some proteins leak out into interstitial space and they exert an osmotic pressure of 8 millimeter mercury in interstitial space. Now, let us work which force moves water with side. Hydrostatic pressure in capillary will push water to outside the capillary. So, this one push water outside the capillary. Hydrostatic pressure in interstitial space, since it is negative, instead of push, it acts as a pull force and causes movement of water outside the capillary. So, this pressure is also causing movement of water outside the capillary. Osmotic pressure in capillaries will tend to pull water inside capillaries. So, this osmotic pressure is pulling the water. Osmotic pressure in interstitial space will pull water into interstitial space. So, basically this one is also causing movement of water outside the capillaries. Now, let us see by the balance of the forces with side will water move. So, forces promoting filtration. So, we have to add all these forces which are promoting filtration. This one, this one and osmotic pressure. So, it will be 32 plus 3 plus 8. Now, here we are not using negative sign because we have already said that this hydrostatic pressure is promoting the filtration. So, total forces promoting filtration will be 43 millimeter mercury, 43 millimeter mercury. There is only one force opposing filtration from the capillaries and that is osmotic pressure of the capillaries that is 28 millimeter mercury. So, the net driving force for water is 15 millimeter mercury which is we have to subtract the 2 and we will get 15 millimeter mercury that is causing movement of water away from the capillaries. Till now we discussed what happens at arterial end of the capillaries. We saw that 15 millimeter mercury force is promoting filtration at the arterial end of the capillary. Now, we will see what happens at the venous end of the capillary. Vanes are compliant to vessels that means their walls are not rigid. The hydrostatic pressure on vane side is quite less because of that. So, in venous end hydrostatic pressure is 10 millimeter mercury and rest all forces are same because the proteins have not filtered while the blood has passed through the capillaries. Osmotic pressure remains same. Hydrostatic pressure is same. Now even when fluid is filtered hydrostatic pressure in the interstitium remains same because there is quite a negative pull in the interstitial space. So, the filtered fluid does not make much difference in the hydrostatic pressure of the interstitial space and osmotic pressure also remains the same. So, similar to the arterial end forces causing outward movement of water will be hydrostatic pressure both hydrostatic pressures, osmotic pressure of interstitial space. Thus forces promoting filtration will be you have to add all these forces. So, 10 plus 3 plus 8 it will come to 21 millimeter mercury. While the force which opposes filtration is 28 millimeter mercury. So, if you take a balance of forces it will give you minus 7 millimeter mercury. That means it is towards drawing enough water into the capillary. So, there is inward pull of water at the venous end. That means a water which is filtered at arterial end is reabsorbed back at venous end. But see now at arterial end net outward force was 15 millimeter mercury and at venous end net inward force was 7 millimeter mercury. So, some fluid remains in interstitial space every time that passes through a capillary. That fluid is removed by another circulation known as lymphatics. Lymphatics carry that excess fluid from the interstitial space. When can fluid accumulate inside interstitial space? If we know this we can very well predict that when can extra fluid accumulate in interstitial space. That extra fluid accumulation in interstitial space is known as edema. So, let us see how can edema occur? So, we are talking about physiological basis of edema. We saw that hydrostatic pressure is promoting filtration and osmotic pressure is drawing in water. Static pressure increases too much then the balance of forces will shift towards filtration. So, the physiological systems will be overwhelmed. I mean lymphatics may not be enough. The lymphatic circulation may not be enough to carry the excess filtered water. So, increased hydrostatic pressure will lead to edema. Similarly, decreased on cautic pressure leads to edema. Then if you saw the previous diagram we have drawn these lining epithelial cells. There is some space in between the cells through which water passes. What happens if the space between the cells increases? That is known as increased capillary permeability. Increased capillary permeability means with the same net forces more water will filter into the interstitial space. And next is also blocked lymphatics because the some amount of water which remains in interstitial space is being removed by lymphatics. So, blocked lymphatics will also lead to edema. So, when these cases occur, increased hydrostatic pressure occurs like in case of hypertension where there is increased blood pressure or there is excess fluid accumulation. Excess fluid accumulation occurs in many cases like in heart failure it can occur also sometimes it can be iotrogenic. That means when the doctor is giving the fluid to the patient if it is not monitored properly and extra excess fluid is given then it can lead to edema because hydrostatic pressure will increase. Then decreased on cautic pressure that is due to decreased protein concentration. So, we have initially told that proteins are the ones which contribute to this on cautic pressure. So, there is decreased protein concentration in blood. So, when can protein concentration in blood can decrease either the person is eating less. So, either in malnutrition or there is decreased absorption in the intestine. So, person is eating properly but these amino acids are not being absorbed in the intestine. So, there may be malabsorption, malnutrition, malabsorption or there can be decreased synthesis of proteins. So, liver is the main organ which is synthesizing protein. So, any liver disease where there is decreased synthesis. So, liver disease can cause decrease on cautic pressure and edema or there is increased loss of proteins from the body that occurs in kidney diseases. Normally proteins are not filtered in nephron but in some kidney diseases there can be filtration of proteins in the nephron and they will be lost from the body. Increased capillary permeability can occur in case of local injury. We all might have experiences whenever there is local injury there is swelling of that site. So, that is because of increased capillary permeability or there can be infection that can also cause increased capillary permeability. Blocked lymphatics can occur in case of philariasis which is a parasitic infestation and also there is when there is in cancer when there is metastasis of cancerous cells to lymph nodes and it can also cause blockage of lymphatics. So, metastasis of cancer. One more thing before we finish this video. There are two types of edema. There is transudate and exudate. Transudate means that it is the water which is being accumulated inside the interstitial space. But in exudate along with water there is movement of proteins also from the capillary. So, we know that normally these spaces between the endothelial cells of the capillary wall are not large enough for proteins to pass. But when there is increased capillary permeability, local injury or infection these spaces can increase and the proteins can also pass. So, exudate is water plus proteins while transudate protein concentration in the collected fluid is not high.