 blood flow to the liver is approximately 25% of the cardiac output and it receives blood flow from two sources. One is the venous circulation via the portal vein which includes the blood coming from the stomach, small intestine, large intestine, pancreas and spleen. So this is the venous supply which is basically containing most of the deoxygenated blood but this has to enter into the liver because the blood here undergoes some kind of processing that means the content of the blood they will be filtered here, they will be detoxification because this blood is coming from intestine right. So it is important that there is some barrier for this blood to flow into the main circulation and there is also some metabolism because nutrients are entering into the liver. And the second blood supply is via hepatic artery which provides oxygenated blood to the liver and it is responsible for one fourth of the blood supply to the liver. But remember here that even though portal vein blood is deoxygenated still it contributes to some oxygen supply to the liver. So with this we will see certain important concepts in liver blood supply. One is the concept of hepatic lobule versus portal lobule and hepatic asinus a difficult topic to understand but we will try to clarify it. Then we will see one important aspect of portal hypertension and something related to regulation of blood flow in the liver. So let us see that how blood is entering into the liver. So here this diagram is showing a hepatic lobule and if you see that this hepatic lobule is a hexagonal structure. So you see this is a hexagonal structure and in this hepatic lobule we are seeing various hepatocytes you see these are the hepatocytes various hepatocytes are there which are arranged as plates. So two plates of cells are there this is the one plate of cells and this is the other plate of cells and in between we have bile duct right. Now let us focus on the blood supply. See here that we have hepatic artery right that is oxygenated blood is going to come and here we have the portal vein and both are entering into the liver and they are dividing into the branches okay and these branches are going at the corners of this hexagon right. So we have the branches of hepatic artery also which ultimately forms the hepatic arterial and there is portal vein which ultimately form the portal venules and then these further break up. So here you see this channel right so this channel so the blood from the hepatic arterial and portal venules enter enter these channels and these channels are known as sinusoids okay and you see that the blood from these sinusoids ultimately is draining into a central vein. So this central vein will be present in different different hepatic lobules so there will be one hepatic lobule here as well okay and central vein will be here and these blood from these central veins will combine and form the hepatic vein okay hepatic vein which ultimately drains into the inferior vena keva. So this is the basic hexagonal structure of a hepatic lobule. Let us see another diagram which will magnify this portion. So this is the diagram which we have discussed you notice here that this is the corner part where we have the portal vein then we have the hepatic artery and they are just producing further branches and you see what is happening blood is mixing from the hepatic artery with that of the portal vein and these are the sinusoids and why are they called sinusoids? Because they are highly porous so what happens that as the oxygenated and deoxygenated blood mixes in these sinusoids and these sinusoids are porous contents from these sinusoids can leave these capillaries okay so it's like this and there are spaces between the cells which are lining these sinusoids. Basically sinusoids is a capillary network which is highly porous and the contents can leave out of these spaces near to the hepatocytes so they can be taking up of the nutrients by the hepatocytes plus there are other cells lining here so there are macrophages actually and these are known as you see cup for cells so as the blood is coming from the intestine towards the liver so there is kind of a monitoring of presence of any infectious particle or for an antigen by these cup for cells which are nothing but local macrophages and you see how the blood is draining into the central vein so that was about hepatic lobule so this hepatic lobule is a structural unit of the liver structural unit of the liver but functionally we consider some other units of the liver as well but before we proceed here I just want to mention about bile ducts also you see in between these two plates there is bile duct also and the blood flow is from the hepatic portavane and hepatic artery to the sinusoid it's towards the central vein but the bile flow is in the opposite direction towards the bile duct fine coming to the next very important part sometimes difficult to understand is what is a hepatic lobule versus what is a portal lobule there is another term that is portal lobule and what is a hepatic acinus okay so this I told you this we have discussed that this hexagonal part is the hepatic lobule and you see that the branches of the hepatic arterial portal vein they are present at the corners and they will drain into the central vein so this is the central vein now these branches of the hepatic arterial and portal vein you see are shared by nearby hepatic lobule as well so say suppose this is the branches now the blood from these are going into three different hepatic lobule this side this side and also this side so if we see here example I've drawn a triangle here that these are the branches okay there will be one branch only so one hepatic arterial one portal venule and one bile duct and from the portal venule and hepatic arterial the blood will flow towards the central vein of these three different hepatic lobule so here this triangle is showing what it is showing a portal lobule that means a combination of three hepatic lobules to which one portal triad contributes the blood because it is sharing the same corner fine so that is portal lobule coming to the concept of acinus this is very important as far as blood flow of the liver is concerned now this acinus is a diamond shaped area so here we have discussed in hepatic lobule we told that there are six portal trial at the corners when we are talking about portal lobule we have talked about that the hepatic lobule shared by a single portal triad now when we are talking about acinus we are considering two portal triad okay so this is the diamond shaped area which is common to these two portal triad because blood will flow from this portal triad to this side and this side right and this side and this side now you see I was telling that oxygenated blood flows from this side to the other side to the central vein so this portion of the liver will have the maximum amount of the oxygenated blood then as the blood flows because oxygen is being utilized this portion will have less amount of oxygenated blood and least amount of oxygenated blood will be towards the central vein so based on this we define zones in liver okay zones in liver and this diamond shaped area is the acinus in this acinus we have three zones one area of maximum oxygenation then intermediate oxygenation and the least oxygenation so let us divide it into zones and see what are the various functions which are occurring in these zones because this acinus actually forms a functional unit of the liver okay so structural unit is this hexagonal area that is the hepatic lobule and functional unit is the acinus so let me clear all other hepatic lobules here so we will talk about like what are the functions which are occurring in this different zones so this first area that is the zone known as periportal zone because it is near to the portal right so this is periportal zone then we have the next one that is the intermediate zone okay and the last one which is near to the central vein that is known as the peri central zone okay so if you remember that central so it is peri central surrounding the central vein periportal surrounding the portal tribe now as I told you periportal zone maximum oxygenation then intermediate oxygenation and the least oxygenation in the peri central zone and because of this difference in oxygenation there is functional division in the zones as well so periportal zone the activities which occur are the main metabolic activities by the hepatocytes mostly involving gluconeogenesis okay then some synthetic activities as well like urea synthesis okay then intermediate zone we have the detoxification activities and there is a lipid metabolism which occurs in intermediate zone then bile acid synthesis also occurs in the intermediate zone coming to peri central zone peri central zone mainly it is responsible for glycogenolysis okay so there is functional division also in these different zones so hepatocytes which are located near the center will be performing different kind of metabolic activities plus one very important factor in this asinus is that because of different oxygenation in different zones the central area peri central area which is least oxygenated that is most prone to ischemic injury so in case of decrease in blood flow it is the peri central zone which is most likely to be affected by ischemia so that was about asinus now coming on to little bit regulation aspect not much we will talk but there is something known as hepatic arterial buffer system what it says that we said earlier that there are two types of blood supply portal vein and the hepatic arterial what this arterial buffer system says that when portal vein supply decreases hepatic arterial supply increases to the liver right so it's kind of a buffer mechanism which is going on and when hepatic arterial supply decreases portal vein circulation actually increases it is not like it is totally compensated that if there is 50% decrease there will be 50% increase but some buffering does happen however the mechanism that why it is occurring how it is occurring that is not much known so that is one aspect of regulation second aspect is that as I told you these are hepatic arterioles and just like any other arterioles of the body they are supplied by sympathetic nerves so in response to sympathetic nerve supply they can be vasoconstriction of this resistance vessels which ultimately leading to the sinusoids okay so here there will be vasoconstriction so that is another method of regulation of liver blood supply now coming on to the most important aspect that is the portal hypertension so first thing that the mean blood pressure of the portal vein is around 10 millimeter mercury okay portal hypertension means that there is increase in the portal venous pressure and when can that occur see portal vein ultimately is going to form the sinusoids and it is going into the central vein forming the hepatic vein which will drain into the inferior vena cava so if there is increase in pressure upstream one or there is any blockage which will prevent the blood flow that will lead to portal hypertension so if we see into the causes they can be post hepatic causes okay post hepatic causes that is beyond the liver which are going to occur now don't confuse it with the jaundice because in jaundice we say post hepatic causes here in the bile duct it is in the direction of the flow which we have to consider pre-hepatic will be before the portal vein enters into the liver because the flow is going like that bile duct flow is in the opposite direction so there after the liver that is considered as post hepatic fine so if there is inferior vena cava obstruction then there will be resistance to the blood flow right so portal vein pressure will increase one then if there is suppose right heart failure in right heart failure the right heart will not be able to pump blood further that is going to increase the back pressure and that back pressure will be reflected to increase pressure in pira vena cava increase pressure in the portal vein right so that is also leading to portal hypertension second there can be hepatic causes so anything which is obstructing the blood flow within the liver that is the hepatic causes and common causes liver cirrhosis right and then there can be pre-hepatic causes also so here if it occurs that will be pre-hepatic causes okay so for example portal vein thrombosis so that is pre-hepatic cause which will lead to increased portal vein is pressure so when that happens increased portal vein is pressure what is going to be affected so for that we should know that how the portal vein is formed and what are the different veins which are draining into the portal vein well portal vein is formed basically when a splenic vein combines with the superior mesentric vein so you see here it is the portal vein and in this circulation we have other veins as well gastric vein also drains into the portal vein then umbilical vein also drains into the portal vein however after birth you know that this umbilical vein is obliterated okay so blood flow doesn't happen here but it has connection with the portal vein then inferior mesentric vein formed the large intestine also drained into the portal vein so superior mesentric vein is from small intestine's plenic vein is from its plane and gastric vein is we know it is from stomach okay so all this blood is being drained into the portal vein now when back pressure develops that means flow will not go in the above direction this back pressure will increase the pressure everywhere where these veins are connected so what will happen that these veins are going to increase in size right there will be abnormal dilation of these veins and where these veins connect with the systemic circulation that means see what happens like in a stomach stomach right it is draining into the gastric veins but there are other venous channels as well okay and they are making connections further into the inferior vena gava so that when back pressure develops what happens ultimately via these venous channels it will enter into the systemic vein main vein so that blood can drain into the right atrium and this happens everywhere actually here a small intestine and vein here also back pressure is developing so this is known as development of potosystemic shunts potosystemic shunts right and because of this and abnormal dilation of the veins everywhere we get some manifestations of portal hypertension and what are these one is dilation of the esophageal veins that is located at the end of the esophagus and that is known as esophageal varices okay very common very important because the rupture of these varices can lead to massive amount of blood loss okay that is esophageal varices then because we are draining from the large intestine as well so dilation of veins there leads to hemorrhoids okay then splinic vein is forming the portal vein ultimately by combining with superior mesentric vein so back pressure there will lead to splinomegaly because the congestion of blood will happen in the spleen right splinomegaly and then this vessel which has been obliterated may also open up and it is leading to the umbilicus so that may lead to formation of caput medusa that is the visible veins which appear at the umbilicus like spokes of wheel radiating from the umbilicus so that is known as caput medusa then as this systemic veins they are actually coming from the capillaries right all are coming from the capillaries capillaries from the veins so they are also back pressure is going to increase so as the capillary pressure increases what is happening the capillary hydrostatic pressure is increasing and whenever the capillary hydrostatic pressure increases there is fluid movement from the capillaries into the interstitium and in case of portal hypertension it is manifested in the form of acytus right so accumulation of fluid in the peritoneal cavity is known as acytus so I hope you understood portal hypertension with this and what are the various manifestations of portal hypertension and what is its physiological basis 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 thank you