 dead space is the space in the respiratory system where no gas exchange takes place. So, when we inhale the air what happens that the air goes into the respiratory passages and via the respiratory passages it enters into the respiratory unit which consists of respiratory bronchioles, alveolar ducts, alveolar sacs and it is in this respiratory unit that gas exchange takes place. The air which remains in the conducting zone of the respiratory system there no gas exchange takes place. And if you know that this tracheobronchial tree actually consists of 23 divisions 23 divisions and in this 23 divisions the first 16 divisions are what we are considering the conducting zone or the dead space and because it is anatomically present means it cannot be avoided the air will go through that passage itself that is known as anatomical dead space and it is only in the last seven generations that is from the 17th to 23 divisions that gas exchange takes place and this is known as the respiratory zone or what I said was respiratory unit consisting of respiratory bronchioles then there is alveolar ducts and alveolar sacs. So that is anatomical dead space we spoke about but there is another kind of dead space known as physiological dead space and actually even though the name suggests it is physiological dead space there is nothing physiological about it rather it happens in pathological conditions in which there might be certain areas in the respiratory unit in which the gas is going the air is going but there is no perfusion in the area so what will happen if perfusion is not there will gas exchange take place no gas exchange will not take place. So these are areas where ventilation is there in the respiratory unit but perfusion is not there so that is kind of a wasted ventilation okay. So these areas become part of physiological dead space which is basically anatomical dead space because it is already existing plus the alveoli which are only ventilated and not perfused so that constitutes the physiological dead space. So let us see that how we have to measure these anatomical and physiological dead space. So first let us talk about the anatomical dead space measurement. Anatomical dead space is measured by a method known as Fowler's method. This is basically a technique in which the person takes a single breath of 100% oxygen that is not he is not inhaling the air but he is inhaling oxygen and after that he exhales completely as much as he can and we measure the amount of nitrogen washout understanding he is inhaling oxygen and we are measuring the nitrogen concentration in the exhaled air. Let us see the concept here what is happening see normally we are inhaling the air atmospheric air and this atmospheric air what is the composition around 79% nitrogen is there and 21% oxygen is there. So when we are inhaling the atmospheric air what is happening the alveoli what is its composition it is having both nitrogen as well as oxygen understanding nitrogen as well as oxygen and remember what is the amount of air which is present in the lungs if we are taking tidal expiration then the amount of air which is present in the lung is functional residual capacity which is approximately 2.3 liters so in this 2.3 liters there is a mixture of nitrogen oxygen and some carbon dioxide also is there. Now normally this air is filled everywhere right this is filled everywhere and this is filled here also in the respiratory passages. Now at this point we ask a person to inhale 100% oxygen so when the person inhales 100% oxygen what will happen that it will first go into the respiratory passages whenever we are inhaling the air is first going into the respiratory passes so this part will be filled with oxygen and whatever air was there which is the mixture of nitrogen oxygen carbon dioxide it was moving inside understanding so this portion first becomes 100% oxygen and there is pushing of air which was already present in the respiratory passages into the lungs. Now say suppose this air is the amount 500 ml so some of it will remain in the respiratory passages and the rest will mix with the air which is already present in the lungs. Now the person starts exhaling and we measure the concentration of nitrogen in the exhale layer so let us see what will be the concentration of nitrogen as the person exhales. So we will draw it graphically what will happen when the person is exhaling the nitrogen concentration how it will change so suppose on y axis there is nitrogen concentration and on x axis with time we will record that what is the changes in the nitrogen concentration. So initially when the person starts exhaling which air will come out you see the air in the respiratory passages is going to come out and what is this air made up of it is made up of oxygen only because initially it gets filled then it is pushing right and the last portion of the inhaled oxygen remains in the dead space itself so when he exhales this last portion comes out first so what will be the nitrogen concentration in the exhaled air it will be zero initially because it is only 100% oxygen which is coming out right so there will be no nitrogen detected initially but as he is exhaling the air from down here will start coming and this air is basically a mix of the air which is coming out from the respiratory passages and which was present in the dead space so now the nitrogen concentration will start rising why because it consists of mixed alveolar air as well as the air in the dead space so let us draw that nitrogen concentration starts rising up till a point where its concentration almost becomes constant that means now the air which is present in the alveoli only is coming all the mixed air has come out and now only the alveolar air is coming so here what is this it is the pure 100% oxygen then this is the mixed alveolar air right and now here it is only the alveolar air so basically we can say this is only the dead space right this is dead space plus alveolar air okay and this is only the alveolar air so to determine the dead space what we do is that this rising portion in this rising portion at the midpoint we draw a vertical line such that it divides this line into two equal areas understanding so this and this will become equal and from the beginning to this point the air exhaled during this time that is the anatomical dead space so this technique known as Fowler's method or single breath nitrogen washout technique is used for measurement of the dead space now before we proceed on how to measure the physiological dead space just here I want to mention one interesting phenomena that happens that if a person continues exhaling what happens that this nitrogen concentration again starts rising okay so basically this graph has four phases initially one first phase only 100% oxygen second phase rising nitrogen concentration third almost constant nitrogen concentration and again there is a rising nitrogen concentration phase and by this phase we can determine what is known as closing volume so if we draw a line here okay actually at this volume what happens that some alveolar passages start to close see if you have seen my video on ventilation perfusion ratio there I explained that how the top alveoli are more distended and because of them being in distended position they are less ventilated while the base alveoli the alveoli at the base of the lung are more ventilated okay ventilated here means the air which is coming in and out with each breath now what happens that when the person actually inhaled this 100% oxygen because there is less ventilation in the apical alveoli less oxygen reached this apical alveoli and more of it mixed with the alveoli in the base so you see the nitrogen concentration at the apex is higher or lower because the mixing is less nitrogen concentration at the apical alveoli is more now here what I am talking that the nitrogen concentration starts rising actually what happens this basal alveoli their respiratory passages begin to close at the last part of the expiration and the air from the apical alveoli which is less mixed with oxygen starts coming out only so that's why we see here again a rise in nitrogen concentration so this volume at which the basal respiratory passages starts to close off that is known as closing volume and finally no air will come out right because almost all the air has been exhaled only the residual volume will be left in the lungs fine so that was about measurement of anatomical dead space let's go on to measurement of physiological dead space physiological dead space as I told you before what is that actually it is pathological combination of anatomical dead space with the space where alveoli are only ventilated and not perfused and this physiological dead space is measured by an equation known as Bohr's equation and what is this Bohr's equation for this we need to measure the partial pressure of carbon dioxide in the expired air and in the alveolar air so what we see here is a simple concept that amount of carbon dioxide which will be exhaled will be equal to the total amount of carbon dioxide which is present in the respiratory passages as well as in the alveoli total amount of carbon dioxide so we can write it as in respiratory system so in this simple equation what we do is we ask the person to exhale and measure the partial pressure of carbon dioxide in the exhaled air so what will happen partial pressure of exhaled carbon dioxide in the volume which is exhaled so amount is what amount is equal to concentration into volume so in place of concentration we are taking here partial pressure of carbon dioxide and into the volume exhaled so in a tidal breath that will be equal to the tidal volume and total amount of carbon dioxide in respiratory system partial pressure of carbon dioxide in alveoli where the diffusion has taken place okay so where will the diffusion take place that is a normally perfused alveoli so that we can write it as tidal volume minus that volume of the dead space that is not in the respiratory passages not in the alveoli which are not perfused only in normally perfused alveoli this will be the partial pressure of carbon dioxide plus the partial pressure of carbon dioxide which is inhaled because that is going to be present in the dead space so we have taken the alveolar component separate and the dead space component separate and both will mix to form the amount of carbon dioxide exhaled now we will simplify this equation you see partial pressure of inhaled carbon dioxide is almost zero so we literally neglect this part we ignore this part and we concentrate only in this part secondly this alveolar carbon dioxide partial pressure is equivalent to that of the arterial carbon dioxide partial pressure why because there the gas exchange is taking place and the equilibrium has occurred so we can take that value as well so let us rewrite this equation little bit it will be pecu2 into tidal volume is equal to pecu2 multiplied by tidal volume minus the dead space so let us solve this equation to determine this particular component that is the dead space so we'll open this part so it becomes pecu2 into tidal volume minus pecu2 into the dead space and when we rearrange this equation what we will get is dead space is equal to tidal volume multiplied by difference between the partial pressure of carbon dioxide in our trees subtracted with difference in the partial pressure of carbon dioxide in exhaled air upon partial pressure of carbon dioxide again in our trees so this is the simple Bohr's equation and let us try to take certain values and see how much is the dead space coming so normal say suppose tidal volume is 500 ml and alveolar or arterial carbon dioxide is how much it is 40 millimeter mercury then exhaled carbon dioxide let us say the partial pressure is 28 millimeter mercury normally it happens so with this if we solve how much will be the dead space 500 into 40 minus 28 is 12 divided by again 40 so how much this comes this comes to 150 ml and that is the normal value of the anatomical dead space because here we are taking a condition physiological condition in which there is no abnormal alveolar dead space however if alveolar dead space is there in that case this value will be much lesser right and we will get more dead space value so I hope you understood how we measure the anatomical dead space and physiological dead space thanks for watching the video if you liked it do press the like button do share the video with others and don't forget to subscribe 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