 Let us consider two different tubes of different diameters. So, one is larger diameter and one is smaller diameter and both of them are connected to tabs. Okay, now if we open the tabs fully for both of these tubes in which of these tubes do you think that the flow rate will be more and when we consider flow remember it is not only the amount it is actually the volume which flows per unit time. So, time is also considered when we are talking about flow. So, in which of these tubes will flow rate be more when in the larger diameter tube flow rate will be more. Now, let us consider another version in which maybe the diameter of the tubes is same. Okay, so same diameter tubes but the tabs to which they are connected one of them is opened fully. So, maybe this one is opened fully and this one is open only half. Now, in which of these tubes will flow be more? Well, obviously in the fully open tab. So, with this simple example what we have considered is that flow is dependent on two very important things and that is resistance. So, in first one we dealt with resistance and in the second we spoke about pressure. So, when we talk about diameter then we are referring to resistance and the pushing force which is there for the flow that is the pressure. So, let us see in detail about both of these aspects which determine the flow. First let us see the effect of the pressure on flow. Now, what are the factors which affect the pressure that is the push force for the flow? Pressure is determined by rho Hg where rho is the density of the fluid, g is the gravity and H is the height the column of the fluid which is there. Now, for a given fluid and obviously we are on the planet earth then g is constant for a given fluid, rho is constant the density of the fluid is constant only the height will change. So, again let us see certain simple examples. Suppose there are two cylinders which are filled with certain fluid suppose this one is fully filled with fluid and there is another one which is empty and both of them are connected by a tube. Now, from where to where will flow occur? Obviously from the region of higher pressure to that of the lower pressure. Why we are saying higher pressure here? Because the fluid is present here first of all and this we are showing empty. So, obviously the pressure here is 0, but here depending on the height of the column pressure is there. So, similarly if we take another example say suppose there are two cylinders again, but in this there is half filled cylinder and again no filled cylinder and again they are connected with tubes. Now, if you see both these examples in which of these tubes say this is A and B, flow rate will be more? Flow rate will be more in A. So, what we are saying here is that it is the pressure difference which determines the flow rate, pressure difference. So, when we consider absolute pressure that is determined by rho h e right. So, here height will determine the absolute pressure in this cylinder and here obviously height will be 0. So, pressure will be 0 here. On the other hand here the pressure will be half of the first cylinder like this one. So, in this the pressure difference between these two cylinders is less compared to in the first example. So, pressure difference determines the flow rate, more the pressure difference more is the flow rate fine. So, first thing to remember is that it is not the absolute pressure at a place which determines the flow rate, it is the pressure difference from one region to that of the another. Just to prove my point suppose if I give you another example say two cylinders one is full and the other is maybe half filled then will there be any difference in the flow rate of say this example right and this example no there will not be any difference because we are talking about the pressure difference right. So, if you see in both of these even though the absolute pressures in the cylinders is different, but pressure difference will be same. So, in both these examples flow rate will be same. So, how this concept of pressure determining the flow rate is important for physiology? Well, you see the blood flow in heart how it is determined it is going from left ventricle via the aorta, it is going via various vessels and it is going to reach to the right atrium. Now, the pressure in the right atrium is very less while that of the left ventricle maximum is 120 millimeter mercury right that is why the flow always occurs from left side of the heart to the right side of the heart that is from the region of high pressure to low pressure. Also by changing this pressure difference actually body can increase the flow rate you know about how blood pressure basically is the driving force for the blood flow right and whenever the blood pressure increases this blood flow is going to increase okay. So, our neural mechanisms and the hormonal mechanisms of rendering angiotensin system work to maintain this blood pressure. So, that the blood flow is maintained and in certain instances like that in exercise since the requirement of blood flow increases this blood pressure is increased in case of exercise so that the adequate force for the blood flow is available. So, that is one very important concept when we consider the relationship between the pressure and flow. Second thing is that when we talk about individual tissue beds then also for calculation of the amount of blood flow what we do is we take pressure difference between the various vessels. So, if we consider the driving force in suppose the renal vessels then what we do we have to take the pressure in the renal artery and the pressure in the renal vein so that gives us the driving pressure in the renal vessels fine. Now, let us move on to the second concept to which determines flow that is the resistance. So, how resistance effects the flow rate and we have seen that the diameter of the tube or when we talk about physiology diameter of the vessel is very important as far as flow is considered. So, let us see the same example what we saw in case of pressure that two cylinders okay and we will consider that two examples pressure difference is same right. So, maybe in both these conditions both are equally filled but the tubes by which they are connected they have different diameters right. So, this one is narrower. So, in which of these flow rate will be more obviously in the first example flow rate will be more why because the resistance to flow in this case is less right. So, two factors are determining the flow rate and actually this relationship between flow resistance and pressure is almost equivalent to that of the Ohm's law which is given in electrical theory where we say that voltage difference delta V is equal to IR that is the current flow rate and the resistance. So, here it is replaced by delta P is equal to blood flow into resistance or we can write as flow is equal to delta P by R. Now, this concept that what are the factors which affect the flow was studied thoroughly by Boise Lee and he gave an equation where he said that flow is equal to that is the Q is represented as flow rate Q is equal to P1 minus P2 that is the pressure difference between two regions then by R to the power 4 divided by 8 viscosity of the fluid into length of the tube. So, these were the various factors which affect the flow rate. So, this one is obviously the pressure difference but if you see this part it actually represents the resistance. So, here it is given inversely right. So, we can put this as resistance and resistance will become as R is equal to 8 viscosity into length of the tube divided by R to the power 4. So, what we see here is that resistance is inversely proportional to radius of the tube actually to the fourth power of the radius of the tube. So, more the radius less is the resistance and why this concept is important? You see in body apart from changing the pressure to the flow, the flow to the individual tissue beds is regulated by changing the resistance of the vessels of various tissue beds. So, let us see that how the vessels are arranged in body we have seen that basically from left ventricle the blood is going to the right atrium and then what happens that there is large artery which is coming and then there is division of this large artery supplying the blood to various tissue beds right. So, what is the serial flow from left ventricle which is glowing like this and reaching to the right atrium? So, this is a series but in between for supply to the individual tissue beds these vessels are arranged in parallel manner. So, yes they will combine together right and then the entire venous flow will reach to the problem do not bother it will reach to the right atrium. So, if we ask you that what is the resistance of this entire arrangement? So, what you will do is that you will add up the resistances whenever the tubes are in series we add up the resistance of the various vessels. So, suppose this is one tube right and then this is another tube but in between this is all parallel right. So, we will consider entire this arrangement as in series with full of this right this is continuous. So, maybe this is R1 R2 right. So, we have to add all of these resistances. So, that will contribute to the total peripheral resistance. Actually, what we see in the body is that this resistance in vessels is maximum in arterioles. So, that is why they are known as resistance vessels, veins, arteries, capillaries hardly offer any resistance. Resistance is maximum in arterioles fine. So, we have to add to the resistance of any arterioles which are in series but let us talk about individual tissue bits. So, maybe this is one tissue bit, this is the second tissue bit, this is the third tissue bit, this is having resistance 1, resistance 2 and this is having resistance 3. So, when we consider that how much is the resistance of this part and we said that these vessels are in parallel in that case the formula is 1 by R is equal to 1 by R1 plus 1 by R2 plus 1 by R3. So, in series we just add R1 plus R2 plus R3 but when in parallel this is the formula for calculation of the resistance. So, what is the importance of this kind of arrangement? That is where lines of physiology. See, I said that the flow rate is determined by the pressure head right. So, that is changed by left ventricular contraction by changing the pumping. So, that is going to increase the systolic blood pressure fine but once the pressure head is increased blood flow will go into individual tissue bits depending on what is the resistance of these tissue bits. So, suppose all these vessels have the same diameter then the blood which is coming will equally divide into these three tissue bits. However, suppose that if this particular tissue bit has a arterial with a smaller diameter then the resistance here will be much more. If resistance here is much more then the flow in this tissue bit will be less compared to the other tissue bits and that is one mechanism by which blood flow to various tissue bloods is regulated right. So, that occurs by sympathetic activation which leads to vasoconstriction. So, wherever blood flow required is less sympathetic supply will cause the vasoconstriction on the other hand the metabolites which accumulate what do they do? Suppose the third tissue bit is having high metabolism then the metabolites are going to accumulate and these metabolites cause the vasodilation in this tissue bit increasing its diameter and hence increasing the blood flow through this while other tissue bits because of the sympathetic supply they might be vasoconstriction that is the arteriolose will be constricted and the resistance will be more. So, that is how the blood flow to various tissue bits is regulated depending on the requirement of that particular region. Great, but one more thing happens actually see as there will be vasoconstriction ultimately this value is going to increase. So, it will add to the total peripheral vascular resistance. So, when that happens actually there is increased resistance to flow and there is development of the back pressure. So, suppose the blood is flowing right and here it encounters a block what will happen? Because flow is not occurring a back pressure is going to develop which is going to increase the pressure head and hence it will further push the blood flow to other vessels understanding or not. So, one mechanism is that obviously to this tissue bit the flow will be less, but because of the increase in the back pressure the flow is going to increase to other vessels and though I am using the term back pressure what we term it as diastolic blood pressure that is the minimum blood pressure during the diastolic and that is determined by the peripheral vascular resistance. I think hope you understand that how peripheral vascular resistance increases the diastolic blood pressure. So, strawberry blood pressure on the other hand is determined by the pumping ability of the heart. So, do you understand that how peripheral vascular resistance? One is the local one which is causing the regional distribution of the blood flow and the other one is the total peripheral resistance which affects the diastolic blood pressure. More the total peripheral vascular resistance more is the diastolic blood pressure. So, that was all about the relationship between pressure volume and resistance and how physiological systems are used it to increase the blood flow rate also for the regional distribution of blood to various tissues. Thanks for watching the video if you liked it to press the like button share the video with others and don't forget to subscribe to the channel if you are new to the channel. Thank you.