 like any other flow Venus return is also dependent on pressure gradient divided by the resistance to Venus return. We know that Ohm's law when considered in physiology we write it as flow is equal to delta P the pressure gradient for the flow then the resistance to flow. So pressure gradient is divided by resistance to flow this determines the flow. So same equation we will put to Venus return and the delta P that is the pressure gradient for Venus return is determined by mean systemic filling pressure minus the right atrial pressure we will see what is this mean systemic filling pressure. So mean systemic filling pressure minus the right atrial pressure is the pressure gradient for the Venus return and then there is resistance to Venus return. So first let us try to understand this equation mean systemic filling pressure and resistance to venous return. See, in the systemic circulation, we have these parallel vessels. There is a parallel flow to the circulation and these are the veins we are depicting. And from here, the blood is collected into veins and then it enters into the right atrium via the large veins. Now, this mean systemic filling pressure is basically the pressure in the systemic vessels when the large vessels of the heart are clamped. Meaning, meaning that the heart is pumping, isn't it? And that pump basically pushes the blood from behind. But if the effect of the pump is blocked, what will happen that everywhere in the circulation, there will be stasis of blood, isn't it? And because these are the vessels, they are like pipes which are filled with water. So, these are the vessels which are filled with blood. Obviously, just by the virtue of the filling, there will be some pressure in the vessels and that is known as mean systemic filling pressure. There is another term, mean circulatory filling pressure. I just want to tell you this because in graphs, this mean circulatory filling pressure is used. Actually, when suppose the heart stops pumping, in mean systemic filling pressure, we said that we have clamped the vessels, isn't it? Both sides we have clamped. So, the blood will be filling the systemic vessels and that pressure is mean systemic filling pressure. When we are talking about mean circulatory filling pressure, we say when the heart stops pumping. So, there we are considering the pressure in the pulmonary vessels as well, isn't it? Everywhere the blood is going to stop moving and we are considering the total pressure when we are including the pulmonary vessels in the scene. So, that is known as mean circulatory filling pressure. For all practical purposes, we take mean systemic filling pressure is equal to mean circulatory filling pressure. So, even if it is given in the graph, don't get confused between the terms and why we take it almost equal because pulmonary vessels actually do not offer much resistance to the flow. Their resistance is one eighth of that of the systemic circulation. So, we don't consider it for practical purposes. Fine. So, we are talking about the mean systemic filling pressure. What we said is that everywhere in the circulation, one single pressure will be there because the flow everywhere has stopped and that we are taking as mean systemic filling pressure. Now, what are the factors on which this mean systemic filling pressure will depend? It will depend on blood volume. Blood volume. So, you consider certain pipes if they are filled with more water, will the pressure be more? Yes, the pressure will be more. So, increased blood volume will increase the mean systemic filling pressure and it also depends on venous capacitance. What is that? See, veins actually are capacitance vessels and veins are supplied by the sympathetic system. So, when increase in sympathetic activity occur, this venous capacitance decreases. That is, there is a venous constriction. So, in our analogy of the pipe and the water, what is happening? Basically, the diameter of the pipes is decreasing. So, the system is filled more tightly with the same blood and when this happens, obviously, there will be increase in the mean systemic filling pressure. If you try to understand it by means of a graph, you see this red one. What is happening? When the blood volume is 4000 ml, that is 4 liters. So, we are considering this red graph there. One volume we are considering of 4000 ml and in case of normal circulatory system, you see, as the blood volume is increasing, the graph is going to the right side, the mean systemic filling pressure is also increasing. So, when the blood volume is around 5000 ml, the mean systemic filling pressure is 7 millimeter mercury. However, at 4000 ml, the filling pressure is 0. Why is it so? Isn't the blood vessels still filled with blood? Yes, they are, but the blood volume is so less that there is no tightness in the circulatory system. It is like the mostly relaxed state of the circulatory system. So, it does not exert any pressure at this time. Only when the blood volume is 5 liters, that is the mean filling pressure of 7 millimeter mercury, which is a normal circulatory system. And as the blood volume is increasing, the mean systemic filling pressure is increasing quite tremendously. You see, the slope is quite high in these cases. So, in this red graph, basically we saw the effect of blood volume, increase in blood volume on the mean systemic filling pressure, given the sympathetic activity is at a constant level. But if you see that sympathetic stimulation increases or decreases, then also this pressure changes. So, we have just seen this blue graph, there is elevation of the sympathetic activity. So, if we see what will be the mean systemic filling pressure at a volume of 5000 ml, in the red graph it is 7 millimeter mercury. But you see, in the blue graph, just extend this line and the mean systemic filling pressure has become so less when there is complete sympathetic inhibition. So, with sympathetic inhibition, there is shift in the graph to the right. On the other hand, with sympathetic stimulation, you see even at 4000 ml, there is some systemic filling pressure. So, why is it happening? Why the mean systemic filling pressure is changing with sympathetic stimulation or inhibition on the sympathetic system because of its effect on venous capacitance? When there is sympathetic stimulation, veins constrict, there is decrease in venous capacitance and there is increase in the filling pressure. And when there is sympathetic inhibition, there is actually increase in the venous capacitance, the storage capacity of the veins is increasing. So, blood will get damped in the veins and it is not going to exert that much pressure. So, there will be decrease in the mean systemic filling pressure. So, I hope you understand this concept of mean systemic filling pressure. So, this mean systemic filling pressure is basically a push force for the flow to occur to the heart, right? And then the right heated pressure is basically as a backward push. So, one pressure that is mean systemic filling pressure is promoting the venous return. On the other hand, right heated pressure is opposing the venous return. So, that is why this is the pressure gradient for the venous return. Then we have to take into account resistance to venous return. This resistance to venous return, again, is determined by the venous capacitance because if the veins are not constricted, they are relaxed. Then there is increased resistance to the flow. So, basically much of the resistance to venous return is given by the veins themselves. Let some amount of resistance is also provided by the arterioles. For understanding venous return curves, so, we will restrict to the concept of that most of the resistance to venous return is provided by the veins themselves. So, with this equation, now let us move on to the venous return curves. Venous return curve describes the relationship between the right heated pressure and the venous return. So, you see on x-axis, it is given right heated pressure in millimeter mercury from the range of minus 8 to plus 8 millimeter mercury and venous return is in millimeter permanent on the y-axis. So, what we see here in the beginning of the graph, there is a plateau, there is some venous return going on, but it is not decreasing when right heated pressure is changing from minus 8 to minus 4 millimeter mercury. Then from minus 4 to 0 millimeter mercury, there is a transitional zone that is venous return slightly starts decreasing from minus 4 to 0 millimeter mercury, but from 0 to a particular point in the graph where the right heated pressure becomes equal to mean systemic filling pressure, the down slope that is the decrease in the venous return occurs tremendously that is from 0 to 7 millimeter mercury and as the mean systemic filling pressure is the push force for the venous return, it is driving the venous return and right heated pressure is opposing the venous return. The point where right heated pressure becomes equal to that of the mean systemic filling pressure that is 7 millimeter mercury, there is no venous return. So, I think this down slope is quite clear that when the push force is being countered by the right heated pressure from 0 to 7 millimeter mercury, there is a decrease in the venous return. But to this part why it is like this that even when right heated pressure is negative, the venous return is not increasing. Well, here it is occurring because of the total collapse of the central veins which are present in the thoracic cage. So, whenever the right heated pressure becomes a negative because of the suction force it exerts on the veins, there is collapse of the veins and even though the push pressure is there, it cannot increase the venous return beyond a certain point. So, that is why there is a plateau and the transition is on because here now the opposition to the mean systemic filling pressure is occurring and venous return has started decreasing. So, that is the relationship between the right heated pressure and the venous return. But in this graph we are showing only the effect of right heated pressure on venous return. But we know that venous return is equal to how much mean systemic filling pressure minus the right heated pressure divided by resistance to venous return. So, in this graph we are not seeing the effect of mean systemic filling pressure and resistance to venous return on the venous return. So, let us see what will be the effect of these factors on the venous return. So, this is another graph where we are seeing how the change in mean systemic filling pressure that is the push pressure how the venous return is going to change at a particular right heated pressure. So, this is the normal graph which we saw that at mean systemic filling pressure of 7 millimeter mercury the venous return has become 0 is not it because the pressure gradient for the venous return has become 0. But when there is increase in the mean systemic filling pressure say for example, when there is increase in blood volume or there is a decrease in the venous capacitance or this decrease in the venous capacitance which may occur due to increase in the sympathetic activity. So, when that occurs what we are seeing that venous return is continuing up to 14 millimeter mercury right heated pressure. So, there is a rightward and upward shift of the curve on the other hand when mean systemic filling pressure decreases for example, in case of blood loss or decrease in sympathetic activity in that case we are seeing that venous return is decreasing. So, that is the effect of mean systemic filling pressure on venous return curve coming to next factor that is what will be the effect of resistance to venous return on a venous return curve. So, here this particular graph is showing the effect of resistance to venous return on venous return curve there you see that the driving force is same mean systemic filling pressure is 7 and the pressure gradient is also same. So, as right heated pressure becomes 7 millimeter mercury the venous return is stopping. So, coming back to our equation on venous return is equal to the pressure gradient for the venous return divided by the resistance to venous return. You see what we are telling here is that if this becomes 0 venous return is going to stop that is why it is stopping at the same point what we are seeing in the normal graph this is a normal resistance graph normally means basically we are keeping everything constant. But as you see the resistance to venous return is increasing initially when the right heated pressure is less. So, what happens basically even the lesser pressure gradient is causing more increase in the venous return. So, as the resistance is halved venous return is doubling venous return has doubled. So, it is also clear in the equation you see resistance to venous return is in the denominator and these are inversely proportional. So, halving of the resistance to venous return will double the venous return. But obviously this will be applicable only to the point that there is some pressure gradient when this becomes 0 when mean systemically pressure becomes equal to right heated pressure there is no pressure gradient. So, you keep on decreasing the resistance it does not matter the venous return is going to become 0 when the pressure gradient becomes 0. Similarly, on the other hand you see when the resistance to venous return is increasing venous return is getting halved. But till now we are considering only one one effect. Things will become very complicated basically the effect on the venous return the final effect on the cardiac output depends on so many factors and if all the factors are changing the graphs will become very complicated. So, one graph they will try to see where we are seeing the combined effect of mean systemically pressure and the resistance to venous return on venous return curve. So, here again x axis is showing the right heated pressure and y axis is showing the venous return in the just per minute and this red graph is the one where the mean systemic filling pressure is 7 millimetre mercury and there is normal resistance. So, first let us see the graph where mean systemic filling pressure increases and resistance to venous return decreases that is both the factors are promoting the venous return. So, this blue graph if you see what has happened that there is shift of the curve to right complete shift is there right in the normal graph it is the stopping here venous return. But in the graph where mean systemic filling pressure is 10.5 the venous return is stopping at a much later right heated pressure value and the graph is also shifted upwards like what we saw that when resistance is decreasing. So, for the same right heated pressure say suppose at 0 millimetre mercury this was the venous return and now with decrease in the resistance this much is the venous return. So, resistance is half so venous return should double correct. But the driving force has also increased because the mean systemic filling pressure has increased. So, here we are seeing much greater rise in the venous return it is not just doubled. So, here at 0 millimetre it was 5 if there is change only in the earlier it should increase only to 10 but the driving force also increased. So, we are seeing the increase up to 40 millimetre. On the other hand in this black graph you see mean systemic filling pressure is decreased but resistance to venous return that has also decreased. So, for example this happens in a foundation when there is blood loss. So, mean systemic filling pressure is going to decrease but as a compensatory mechanism there is increase in sympathetic activation and this is going to constrict the veins decreasing its capacity and thus decreasing the resistance to the venous return. Also mean systemic filling pressure will little bit increase is not it because of sympathetic activation. So, if we consider only blood loss maybe the mean systemic filling pressure will fall to 2 millimetre mercury but if we consider blood loss plus the compensatory mechanism that is the sympathetic activation because of the decrease in the venous capacity there will be increase in the mean systemic filling pressure to maybe 3 millimetre mercury and obviously there will be decrease in the resistance as well. So, in that case we will get this particular graph and this green graph is showing where mean systemic filling pressure is more. So, you see that the flow will continue till a higher right atrial pressure but because of the increase in resistance for the same for a particular right atrial pressure the venous return is much less. So, for understanding these graphs basically you take one particular point and try to drop a perpendicular from that point to all the graphs and then extend that to the y-axis. So, you will get to know that how much is the change in the venous return depending on what are the various parameters. 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.