 Cerebrospinal fluid is like the extracellular fluid of brain. So let us see how the cerebrospinal fluid is formed and how it flows and what are its functions with some clinical applications. Well, this cerebrospinal fluid forms in the majority in the lateral ventricles. So here this schematic diagram is showing the lateral ventricles and these are two lateral ventricles. Then from lateral ventricle it flows via foramen of Mondro to the third ventricle. So here it is third ventricle, this is the foramen of Mondro and some amount is also synthesized in the third ventricle. Actually there is a plexus, coroid plexus which is lining these ventricles, all the ventricles, lateral ventricle, third ventricle and fourth ventricle and it is being synthesized by this plexus. So we told that it moves into the third ventricle via foramen of Mondro. Then from third ventricle it moves to fourth ventricle by means of equiduct of sylvius and again some amount is added in the fourth ventricle. Then it passes from the fourth ventricle into the central canal of the spinal cord right. So there is some amount in the central canal of the spinal cord and then there are some spaces again some foramans are there, literally there is foramen of Lushka and immediately there is foramen of Magindy and via these foramans it enters into the subarachnoid space and first it is you see it is entering into the cisterns which are present below this cerebellum. Basically cisterna magna is there and via this cistern it flows into the subarachnoid space. So everywhere here there is CSF okay and this is the subarachnoid space and also in the central canal of the spinal cord. So that is where CSF is being formed basically in the walls of the ventricles by the coroid plexus and how it is flowing and it is present in the subarachnoid space. So that is only about the formation but it has to be absorbed also and how it is absorbed for this let us see another diagram. So here we are seeing a diagram where this is basically the meninges okay dura matter is there then arachnoid matter is there in between there is some space very less space basically dura matter arachnoid matter are almost close together but below the arachnoid we have subarachnoid space and that is what I was talking that CSF is present in the subarachnoid space so here the CSF is there and then this layer is the pyrometer okay and you see this layer of the pyrometer is continuing around the blood vessels right so these are the blood vessels also present in subarachnoid space and then they dip into the substance of the brain right so this is the brain tissue. So here also in the peri vascular space there is this space around the blood vessels this is the peri vascular space where also some amount of CSF is present and it is very important because here if there is any leakage of the proteins then it is via this space that they return back into the CSF okay that is one thing but we were talking about the absorption of CSF you see here CSF is present in the subarachnoid space where it is reached via the ventricles so in the arachnoid there are microscopic projections known as microscopic willi willi are there and lot of willi lot of willi combine together to form the arachnoid granulations arachnoid granulations okay so these granulations they actually project into the subdural space so here these granulations project and there are dural venous sinuses so from here the CSF is absorbed and it enters into the dural venous sinuses via these arachnoid granulation so it completes the cycle of CSF formation as well as CSF absorption so how much CSF is actually present in total throughout the ventricles in the central canal of spinal cord and in the subarachnoid space approximately 150 ml of CSF is present however the rate of formation of CSF is quite high it is approximately 550 ml per day right so you see that this cycle of formation and reabsorption of CSF continues throughout the day and it recycles approximately three to four times per day you see 150 ml into four is how much it is 600 ml right 600 ml and we are telling that in a day 550 ml per day is being formed so this recycling of CSF is taking place now anywhere where fluid is present we talk about the fluid pressure so we should know about the CSF pressure as well so normal CSF pressure is 70 to 180 millimeter of water okay we are not saying mercury okay 70 to 180 millimeter of water is the CSF pressure now this CSF pressure can increase also and can decrease also right so there is a range we are talking now the CSF pressure value is maintained by the rate of reabsorption of CSF that is you see if we see it in a graphical format here x-axis is showing the outflow pressure in millimeter CSF that is equivalent to millimeter water only and the flow of the CSF ml per minute is shown in the y-axis so what you see that if we are considering the CSF pressure you see the rate of formation is almost constant it is not dependent on the CSF pressure on the other hand when the CSF pressure is increasing the rate of absorption of CSF is increasing almost linearly right and this is the point of equilibrium so that is like the average CSF pressure where formation is equal to the absorption if pressure is increasing above that absorption is increasing so that the pressure is brought back to normal and if pressure is decreasing below that then absorption is decreasing and you see at 68 millimeter CSF pressure the absorption is becoming zero so below 68 millimeter CSF pressure there is no absorption of CSF so basically CSF pressure is regulated by rate of absorption of CSF and why this is so because we were talking about the arachnoid granulations here basically the CSF pressure is one way CSF flow is one way it cannot happen on the other side and when the pressure is more there are kind of valves which open more so the flow into the arachnoid granulation and hence into the dural venous sinuses increases fine now let us see how this CSF is actually formed and what is the composition of CSF so if we compare the composition of CSF with plasma what we see that osmolarity of CSF is same as that of the plasma right but there is increase in chloride and decrease in potassium ions so what is happening actually the coroid plexus which is lining the ventricles from there there is secretion okay so plasma comes out but there is reabsorption of some amount of potassium ions that is why the potassium concentration is decreasing and there is secretion of some amount of chloride ions so that is why the chloride ions are increasing and remember there is something known as blood CSF barrier that means everything which is present in blood will not enter into CSF and CSF is forming so that is the reason that the protein concentration you see it is very less that is only 30 milligram per deciliter while in case of plasma it is 7 gram per deciliter that is actually 7000 milligram per deciliter okay similarly glucose is less okay so there are certain transporters by which the glucose enters but it is much less it is 60 milligram per deciliter and in plasma it is 100 milligram per deciliter however the osmolarity is same why the osmolarity is same because there can be free flow of water so depending on the ions which are there suppose one side it becomes hyper tonic the water will move and ultimately the osmolarity on both sides will become same so in brief what we have to remember here that because of the blood CSF barrier which is there everything which is present in blood cannot enter into the CSF and that is why there is very less protein in CSF and glucose is little less right responcentration you see it is almost similar except there is secretion of chloride ions and reabsorption of potassium ions from the CSF okay so with this now let us discuss the functions of CSF so what are the functions of CSF first of all you see I said in the diagram below that there was brain and in the subarachnoid space so its kind of brain is here okay and in the space there is CSF so brain is actually surrounded by a fluid okay and this surrounding of the brain by the CSF creates buoyancy for the brain and this is very important because it is protective because you see if the fluid is not there then this brain will rest on itself it will rest on itself and there will be compression of the nerves at the bottom right so brain is not compressed by its own weight because of the presence of the fluid so buoyancy is protective second that it has another way of protective function in the sense that when the head moves okay so when the head is moving brain just doesn't come and hit the skull because it is kind of floating in the CSF the specific gravity of CSF and brain is same almost similar so it kind of floats in CSF also we saw in the diagram before that that arachnoid layer is there right so from there there are certain projections which are coming and they are holding the brain these are known as arachnoid trabacula arachnoid granulations we saw that they are for absorption of the CSF but this is arachnoid trabacula just a second we'll just see the picture so this was the picture and you see these are the arachnoid trabacula which are holding the brain so even if it moves little bit these arachnoid trabacula hold it in place so maintenance of buoyancy so that the brain is not compressed by its own weight and also it is kept floating third CSF also acts as a lymphatics we talked about the peri vascular space right so if there is any leakage of proteins from the blood vessels which is always there or some RBCs also can come there suppose there is some damage so from there from the peri vascular space ultimately the fluid is carried to the CSF and then it will be absorbed so the lymphatic function is brought about by CSF and finally it has some homeostatic rule as well okay so these are the functions of CSF now let us discuss some applications of CSF physiology in which very important you might have heard is about countercoupe injury but before going into countercoupe we will talk about some other aspects as well so first important thing when we talk about cerebrospinal fluid is hydrocephalus what is hydrocephalus that is collection of fluid in the cranial cavity is hydrocephalus and this is occurring either maybe due to decreased absorption of CSF decreased absorption of CSF which may occur if there is some problem in the arachnoid granulations which we discussed so in if the arachnoid will I are blocked then they will not be able to absorb the CSF into the venous sinuses. Second the pathway which we discussed from the later ventricles to the third ventricle to the fourth ventricle anywhere there if there is a block then that will cause the accumulation of the CSF in that particular area and the basically the recycling which we were talking that will not happen and how much is the amount synthesized per day it is 550 ml per day so that is going to be synthesized and everything is going to be accumulated wherever the block is before that everything will accumulate so that also causes hydrocephalus and that is due to a block now these causes have some other names as well where this decreased absorption it is known as communicating hydrocephalus communicating because there is no block we see and all the areas are communicating with each other or also known as external hydrocephalus and when there is block in the pathway it is known as non-communicating hydrocephalus or also known as internal hydrocephalus okay so whenever it is there what we see is that a CSF pressure is going to rise right and obviously the cranial cavity is a closed space so if the pressure of something is being raised it is on the expense of the other tissues so blood vessels will be compressed the brain tissue will also be compressed so that is hydrocephalus second application is when we need to do lumbar puncture now lumbar puncture is basically investigation where we draw a sample of CSF by inserting a needle just below the spinal cord in the vertebral column so there we will insert a needle and we draw a sample of the CSF and why we need to draw it suppose there is some infection like meningitis is there encephalitis is there and we need to find out what kind of infection is there so we need to study the CSF parameters in that case lumbar puncture is done now suppose if there is increase in intracranial pressure right we are not aware of it maybe some increase in intracranial pressure is there and we do lumbar puncture what will happen suddenly due to the pressure all the fluid will come out and when the fluid is coming out along with that the brain tissue will also come out of the system so that is known as brain herniation hernia of the brain tissue will occur okay so it is very important that before lumbar puncture we have a look into the eye and that is by means of fundoscopy before lumbar puncture always fundoscopy is done because if there is increase in the intracranial pressure that is going to suppress the veins which are coming out of the eye okay and when they are suppressed what will happen to the eye vessels they will appear engorged and if they appear engorged that means there is some external pressure which is compressing the outflow from the veins and that is why the vessels are getting engorged so that will give you an indication about the intracranial pressure if intracranial pressure is high then lumbar puncture is not done then third is the countercoupe injury what is countercoupe injury is that suppose somebody receives a blow on the skull right so instead of the brain tissue getting injured on the side of the blow the brain tissue gets injured on the other side and why is that so let me draw the brain tissue suppose this is the brain tissue okay now when there is a blow what will happen this skull will start moving to the opposite side but because this brain tissue is floating in the CSF okay so the movement there will be some inertia okay it will float slowly it will move slowly but the skull moves very fast okay so it is accelerating and because of the difference in this movement the skull is moving very fast the brain is moving less fast there develops a vacuum on the opposite side right now when suddenly this skull movement stops what will happen the vacuum which is there now there is no accelerator force for maintenance of the vacuum before the skull was moving the vacuum was maintained now suddenly that has disappeared okay so this vacuum is going to pull this brain tissue towards the skull so there will be a sudden pull of the brain tissue and this is going to go and strike the skull okay so it is not because of the blow with the injury is occurring but because of the hitting of the brain the neural tissue to the skull on the opposite side so that is known as the counter coop injury if injury occurs on the side of the blow that is known as coop injury if injury is occurring on the opposite side of the blow that is known as the counter coop injury so our CSF which was protective it is because of that only that coop injury is not occurring but actually counter coop injury is occurring so that was all about the concepts in cerebrospinal fluid physiology thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel physiology open thank you