 Lungs are supplied by two types of circulation. One is bronchial circulation and the other is pulmonary circulation. Bronchial circulation is the one which supplies the nutrients to the lungs and takes away the metabolic based product. These bronchial vessels arise directly from aorta and they are like just another systemic vessels so they supply the tracheobronchial up to the terminal bronchioles for the metabolic requirements. This bronchial circulation constitutes about 1% of the cardiac output and one very important characteristic of bronchial circulation is that the bronchial veins empty directly into the pulmonary veins which enter into the left atrium. So the blood of the pulmonary veins doesn't get oxygenated. It directly enters into the left side of the heart and is responsible for some of the physiological shunting in the blood which will decrease the percentage oxygenation of hemoglobul. So since these bronchial vessels arise from the aorta and are responsible for the metabolic requirements of the lungs, they have the characteristics similar to that of the systemic arteries. But when we talk about pulmonary blood flow, pulmonary vessels are very much different from that of the systemic vessels and this pulmonary blood flow is basically for the oxygenation of the entire blood. So this pulmonary blood flow from where does it arise? It arises from the right ventricle that is the pulmonary artery is there and this enters into the lungs where it breaks down into the capillaries. Then after oxygenation of the blood from the capillaries, it continues as venules and the venules combine together to form the pulmonary vein which enters into the left atrium. So this pulmonary circulation characteristics are much different from that of the systemic circulation because it is suited for the oxygenation and remember that since entire right ventricular output is entering into the pulmonary circulation, it should have the capacity to accommodate the entire blood in case the right ventricular output increases. So suited to its function, the pulmonary vessels have certain anatomical characteristics and that is these vessels are thin-walled compared to the systemic vessels. They have less smooth muscle and because of decreased smooth muscle and there are no arterioles as well, there is very less resistance in the pulmonary vessels. Remember that arterioles are the resistance vessels. So if arterioles are not there and there is less smooth muscle, so there will be less vasoconstriction and that is important for resistance. So owing to this, there is very less resistance in the pulmonary vessels. In fact, the resistance is only one-tenth of the systemic circulation. So due to these anatomical characteristics, these vessels are low pressure vessels, low resistance vessels and high compliance vessels that is they have the ability to accommodate the blood without much increase in the pressure within the pulmonary arteries and what are the pressures? If we compare with the systemic circulation, what are the pressures in the pulmonary circulation? The pressures in pulmonary circulation is 25 millimeters systolic and 8 to 10 millimeter diastolic. So they are so less pressure if you compare with the systemic. Systemic pressures are 120 by 80, right? Here it is only 25 and 8 millimeter mercury and the mean pressure is just 15 millimeter mercury. Coming to capillary hydrostatic pressure, it is also very less. It ranges from 7 to 10 millimeter mercury. Understanding? So this is important for the functioning of the pulmonary circulation. We will see in detail how this pulmonary blood flow is changing due to various factors and how it is important for the function. So first very important thing when we talk about pulmonary circulation is that these pulmonary blood vessels are subjected to alveolar pressures and also to intra-plural pressures. So these pressures also determine the blood flow which is happening in the vessels. So based on this we have a concept known as alveolar and extraalveolar vessels. Let's have a look. So here in this schematic diagram what I have shown is that these circles represent the alveoli, okay? So you see that this is the part of the blood vessel which is running between these alveoli. So these vessels are known as alveolar vessels and the surrounding vessels, they are known as the extraalveolar vessels. So what happens? Actually these extraalveolar vessels are the large artery and veins and these alveolar vessels are the capillaries. So the capillaries are running in between the alveoli. So here this may be the artery, then capillaries and these are the veins. So what is the difference between this extraalveolar and alveolar vessels? See during inspiration what happens that the lung starts expanding. There is decrease in the intraplural pressure, it becomes more negative and the lungs have started expanding. So surrounding these vessels there is interstitial tissue, lung interstitial tissue is there. So when the lungs are expanding there will be stretch of these interstitial tissue as well and that is going to distend the vessels. So something is pulling the vessel wall. So they get distended and when the vessel distends what is happening? There is increase in the diameter of the vessel. So with increase in the diameter of the vessels resistance of these vessels decreases with inspiration. On the other hand alveolar vessels behave differently, they are subjected to alveolar pressure. So when there is expansion of these alveoli, actually these alveolar vessels become elongated, they elongate understanding. So one is like increase in the diameter, vessel is like that, distention will pull the vessel wall. So that is increase in the diameter and that is happening with the extra alveolar vessels. But with these alveolar vessels, the expansion what it does? It actually stretches the vessel wall like this, it is elongating. So this is going to decrease the diameter of the vessel. So you see that decrease diameter means that resistance within this vessels is going to increase with inspiration. Or in simple terms we can remember that expansion of the alveoli is going to compress this vessels. So outside vessels distend with inspiration and the alveolar vessels compress with inspiration. So there is difference in resistance change in these vessels. So we will keep this concept in mind and we will just see that what are the factors which affect the blood flow that is the pulmonary blood flow. So factors affecting pulmonary blood flow means these changes in alveolar and intraplural pressure. So there will be change in blood flow in inspiration and expiration because there is change in the resistance. Then if there is change in the right ventricular output, suppose right ventricular output increases then pulmonary blood flow is going to increase. Then there is the effect of gravity also. So what are the intravascular pressures? See pressures are the driving force. The pressure within the vessel is the driving force for the blood flow. So these intravascular pressures also determine the amount of the blood flow. So outside pressure that is the alveolar pressure, intraplural pressure also determine the blood flow and the intravascular pressures also determine the blood flow. Then there is effect of the lung volume and that is because of these alveolar pressure and intraplural pressure only because when the lung volume is increasing that means inspiration is taking place. So there will be change in this pressures right and finally there is tone of vessel smooth muscle that whether the smooth muscle is constricted or it is dilated that is also going to determine the pulmonary blood flow. We will see each of these one by one. So first one is what is the effect of this alveolar pressure or what we can say what is the effect of this lung volume on the pulmonary blood flow. This is a graph representing effect of lung volume on resistance. On x-axis we see increasing lung volume that is RV's residual volume then FRC total lung capacity and y-axis shows the pulmonary vascular resistance. So what we see here that at residual volume that means when the lung volume is very less pulmonary vascular resistance is high and when the lung volume is very high then also pulmonary vascular resistance is high and you know that we have an equation when we are talking about blood flow. Flow is equal to pressure upon resistance. So this change in resistance is going to determine the flow isn't it when resistance is more flow will be less and when resistance is less flow will be more. So why is this that at different lung volume so we are getting this kind of graph. See at residual volume what is happening that there is least expansion of the lungs right and we have talked about the extra alveolar and the intra alveolar vessels. So suppose this is intra alveolar and this is extra alveolar. Now at lung volume equal to residual volume the extra alveolar vessels will be least distended isn't it because we said that during inspiration there is a stretch to the walls. So during expiration there is minimal stretch to the walls right so they are least distended. So here actually extra alveolar vessels are having very high resistance extra alveolar vessels are very high resistance. On the other hand at total lung capacity when the lung is fully expanded then alveolar pressure is very high and there is compression of these alveolar vessels. So at total lung capacity there is increased resistance very high resistance on the alveolar vessels right. It is somewhere in between that is at functional residual capacity that pulmonary vascular resistance is least because as the lung volume starts increasing this resistance is going to decrease right and alveolar vessels resistance is going to increase. So it's a balance between these two resistance that what happens that at FRC pulmonary vascular resistance is least. So that is the effect of lung volume on resistance and that in turn is going to affect the pulmonary blood flow. Coming to the effect of right ventricular output on pulmonary blood flow. See whenever right ventricular output increases say suppose when a person is exercising in that case cardiac output is going to increase right and also the requirement of the oxygen by the tissues is increasing. So that means the entire blood which is going out of the right ventricular should be accommodated in the pulmonary circulation and it should get oxygenated and enter into the systemic circulation. So for that there should be the characteristic of pulmonary flow such that it accommodates the entire blood. So what we know that pressure is equal to flow into resistance right and we are just rearranging the equation again and again depending on what we require. So resistance is equal to pressure upon flow. Now you see if the right ventricular output increases what will be the effect on the pressure it is going to increase at least that happens in systemic circulation. Whenever the cardiac output increases then the systolic blood pressure increases actually the cardiac output is the main determinant also of the systolic blood pressure. So pressure is increasing but that is not good in pulmonary circulation. Why? Because say suppose this is the alveoli and here this is the capillaries. When the pulmonary arterial pressure increases then the hydrostatic pressure in the capillaries is also going to increase and since hydrostatic pressure is the push force what will happen there will be increased fluid movement and that is going to increase the fluid accumulation first is in the interstitial tissue and later on in the alveoli. So we don't want that these pressures should increase too much instead what happens to accommodate the extra blood it is the resistance which falls understanding. So that means that flow when increases there is fall in the pulmonary vessel resistance such that these pressures don't rise. So basically what is happening since resistance is equal to pressure gradient that is input pressure minus output pressure or in case of pulmonary vessels it will be pulmonary artery pressure minus the left atrial pressure divided by the blood flow. So in case of pulmonary circulation it is equal to the right ventricular output. So with the fall in resistance actually this pulmonary artery pressure is not increasing. Let us try to understand it with means of a graph. So here this graph is taken from Gaetan where x axis is showing the cardiac output in liters per minute and y axis is showing the pulmonary arterial pressures. So you see that normal cardiac output is how much it is approximately five liters per minute and this is the pulmonary artery pressure. So pulmonary artery pressure is not increasing too much even with the rise in the cardiac output. Only when the cardiac output increases more than say suppose five times right then there is sudden steep rise in the pulmonary arterial pressure and how this is happening this is happening due to the fall in the resistance. Now the point comes at how the resistance is falling that is because of two phenomena capillary distention and capillary recruitment. So here the schematic diagram is showing the concepts. So suppose blood is flowing from this side and these are the various capillaries. At normal resting condition there are certain capillaries which are in closed state. Now when the cardiac output increases the right ventricular output increases so what is happening there is distention of certain capillaries right so that is known as capillary distention. So obviously they will be able to accommodate more blood and the capillaries which were closed before because of the push they are opened. So blood flows through them also so additional pathway is being created for the blood flow. This phenomenon is known as capillary recruitment. So two terms you remember that resistance falls due to capillary distention and capillary recruitment. Now when we are talking about this one very good concept we have touched that the pulmonary circulation accommodates the entire blood flow which is coming from the right ventricle and this concept is opposite to the concept of auto regulation. Actually in pulmonary circulation there is no auto regulation that means the blood flow to the pulmonary circulation is not according to the requirements of the lungs and obviously we have seen that the requirements of the lungs are fulfilled by the bronchial circulation. If we see other tissues what is happening that suppose the blood pressure is increasing or suppose the cardiac output is increasing then the local metabolic requirements of the tissue are important that how much blood flow is going to happen to the tissues that is known as metabolic theory of auto regulation. Say suppose when we are exercising then blood flow increases more to the muscles and in fact in other issue beds there is vasoconstriction so that they do not receive any extra blood flow. Here that is not the case. Here when the blood flow is coming from the right ventricle then everything should be accommodated in the pulmonary circulation. Anyways so that was about capillary distention and capillary recruitment. Now coming to another very important concept of blood flow in the lungs that is zones in the lungs and that is due to the effect of the gravity which affect the intrabuscular pressures. See in the video on ventilation perfusion ratio we saw that pulmonary circulation decreases from base to apex and why that is happening that is happening due to the effect of the gravity because there is fall in the hydrostatic pressure as we go from the level of the heart to above right. So at apex if we see the hydrostatic pressure in the pulmonary arteries it will be much less compared to that what is the pulmonary arterial pressure at the base at the base it will be more at apex it will be less and that is causing the decreased blood flow at the apex compared to that of the base. Now what is the driving flow for the blood flow? It is the pressure gradient okay pressure gradient. If from one vessel to other pressure gradient is not there everywhere the pressure is same then the blood flow will not occur even if the pressure is high in all the vessels okay blood flow is not going to occur what we need is pressure gradient. So normally the pressure gradient is gradient in the arteries minus the gradient in the vessels where the flow is happening so gradient in the veins or what we have written before the left atrial pressure left atrial pressure is also very less it is almost equivalent to pulmonary venous pressure right. So here we will talk as pulmonary arterial pressure and pulmonary venous pressure that is the gradient. However we have seen the concept of how the alveoli effect to the capillary diameter isn't it? So that also comes into effect here don't get confused we are handling it very slowly we'll go one by one. So first thing let us look at the base at base what we saw pulmonary arterial pressure is high isn't it? So at base what is the driving force pulmonary arterial pressure minus pulmonary venous pressure that is one thing. Second as we go up what happens pulmonary arterial pressure is decreasing okay what happens as the flow continues on the path there will be always falling pressure right. Now here the vessels are surrounded by the alveoli a point comes where the alveolar pressure will start compressing these vessels here also the compression is there but the pressure is more so the vessels are not collapsing you see when will the vessel completely compress when the outside pressure becomes more than the inside pressure that is known as the transmural pressure. So here inside pressure the pressure inside the capillaries is much more so they are not getting compressed so the driving force is beginning and the end that is arterial pressure minus the venous pressure. Coming to little above what we are seeing that because of the fall in the pressures inside the blood vessels that is the intravascular pressure these alveoli will be able to compress the blood vessels and what happens there occurs a point only a single point where this compression is little bit more here you see right because fall here continues okay so at this point they are able to compress little bit more so it is like from a point of constriction afterwards suddenly there is increase in the flow and this phenomena is known as waterfall effect okay we are not talking about zones right now we will just touch upon them this is waterfall effect we are talking let's go little bit above more now what happens that this pulmonary arterial pressure will fall further because of the gravity intravascular pressure is going to fall that is the hydrostatic pressure so now these alveoli will be able to compress the vessels much more right so the pressure inside the vessels intravascular pressure is so less that the alveolar pressure is able to compress these vessels so in this case there is no flow okay by the way when we were talking here waterfall effect actually the driving force here is pulmonary arterial pressure minus pulmonary alveolar pressure capital A denotes alveoli so that is the driving force because see this becomes a limiting factor and when we talk about limiting factor it has a major importance so pulmonary arterial pressure minus pulmonary alveolar pressure and above what is happening blood flow is not occurring because pulmonary alveolar pressure becomes greater than the pulmonary arterial pressure so what we have seen three different ways of blood flow below blood flow is not getting obstructed so that is known as zone three above that there is zone two where blood flow is there at a single point there is a constriction and then there is sudden flow of blood after that that is zone two and further above that there is zone one where is where there is no blood flow so these are the various zones in the lungs now this zone one actually doesn't exist physiologically so physiologically always there is a state where pulmonary arterial pressure is greater than that of the alveolar pressure so this state where pulmonary alveoli is greater than pulmonary arterial this state doesn't exist in physiological conditions but it may occur in certain conditions one let me rub this clear the screen little bit so I was talking that this may occur in certain conditions one when actually there is decrease in the pulmonary arterial pressure so it may occur in case when cardiac output decreases second it may occur in case when the alveolar pressure is increasing and that happens in case of positive pressure ventilation when the person is on ventilator so that is positive pressure ventilation so in these two pathological conditions zone one may occur physiologically we have zone two and zone three now one more concept you should remember here in zone two we talked about waterfall effect you might be little bit confused here that how this constriction is happening actually there is difference between the systole and diastole so during the diastole constriction is happening and during the systole that construction is suddenly being released and the water is moving out so it's kind of an intermittent flow what happens during systole you see pulmonary arterial pressure will be more right what is the pulmonary arterial pressure it is 25 millimeter mercury so during systole it is like that but during diastole it becomes 8 millimeter mercury so if you see the pressure gradient will be more during the systole obviously the fall with height will be there so here it is 25 millimeter mercury as we go above the level of the heart this value is are going to decrease so the decrease even for systolic blood pressure is not like that it is going to stop the blood flow but the diastolic pressure decreases so much that the blood flow stops so that was a concept of different zones in La so physiologically remember it is only zone two and zone three and one more thing that during exercise when cardiac output increases what happens this pressure the push pressure is going to increase we have seen the diagram that when the cardiac output is increasing to four to five times the pressure is increasing isn't it let us go back to the graph see this graph as it is increasing to four times or five times the pulmonary arterial pressure is increasing so in that case throughout the lungs it becomes on three only because the systolic as well as the diastolic pressure is going to increase so remember certain exceptions right zone one doesn't exist except in case if cardiac output falls too much or there is positive pressure ventilation then during exercise throughout the lungs the blood flow is like as in zone three and also in supine position because we are talking here of the effect of the gravity so when the person is supine the effect of the gravity is nullified and everywhere the pulmonary arterial pressure will be same so during supine position also the blood flow throughout the lungs will be like as in zone three let's come to the last concept of hypoxic pulmonary vasoconstriction here our pulmonary vessels are different from that of the systemic vessels see in systemic vessels whenever there is hypoxia there is vasodilation that is one of the factor of metabolic theory of otoregulation but in pulmonary vessels when hypoxia occurs it leads to pulmonary vasoconstriction and this is functionally very important because it leads to matching of ventilation and perfusion so here in this diagram if you see there are two alveoli being shown and they are like normally ventilated then the vessels are dilated right now if there is an alveoli which is not properly ventilated then in that alveoli there will be decrease in the partial pressure of oxygen and this decrease in partial pressure of oxygen will cause decrease in partial pressure of oxygen in the blood vessels as well and this decrease in partial pressure of oxygen is going to lead to vasoconstriction understanding so is it good or bad it is good because when vasoconstriction occurs in these vessels the blood flow is directed to alveoli which are normally ventilated so there is no wastage of the blood all the blood flow is going to the normally ventilated alveoli so the ventilation and perfusion become matched however in certain conditions like in high altitude pulmonary edema we have discussed in the video on high altitude physiology that how this becomes harmful before closing let us just see what is the mechanism of this hypoxic pulmonary vasoconstriction when there is hypoxia there is presence of actually otosensitive potassium channels on these vessels so these otosensitive potassium channels close okay these otosensitive potassium channels are going to close when potassium channels close what happens potassiums are not able to move out of the cells normally in the cells potassium is more so closure of the channels potassium movement out of the cells decreases so this is a positive ion so that leads to depolarization of the cells depolarization leads to opening of the voltage gated calcium channels and when these voltage gated calcium channels open what happens entry of calcium within the cells happens and this leads to vasoconstriction so simple mechanism just remember that there are otosensitive potassium channels which close in case of hypoxia so that was all about the characteristics of pulmonary circulation the applied aspect of this that is how pulmonary edema develops that we are going to deal with in another video 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