 In this video, we are going to discuss about the gaseous exchange in our body, how the oxygen from our lungs gets into our blood and how the carbon dioxide from the blood gets into our lungs and then it is exhaled out into the atmosphere. And the exchange happens again in the tissue level, how the carbon dioxide produced by the tissues are dumped into the blood stream that is in our blood capillaries. These are the blood capillaries and how the oxygen from the blood is again taken by the tissues. So in this video, let's discuss what drives these gases to move across membranes in our body. So there are a number of factors that plays crucial role in this gaseous exchange. But in this video, we are going to talk about the most crucial one and the most important one, which is the pressure of the gas. Now before we move ahead with what pressure does, let me show you something. Here I have two containers, closed containers, one filled with a mixture of different gases and another is empty. So here the pressure is high because there are a number of gases exerting pressure on the walls of the container. But here, since it's completely empty, the pressure here will be zero. Now if I somehow connect the two boxes or let me say that if I somehow made a passage between these two boxes, what do you think will happen here? Well, I'm sure you can predict already that the gases will start to move into this empty box, right? And why do this happen? Well, because of a very important property of gas, which says that the gases will always move from a region of higher pressure to a region of lower pressure. And the empty box had lower pressure than the other one, right? And this is exactly what drives the gases from one part of our body into some other part because there is a difference in pressure between the two parts. So if we talk about the cavity of the alveolus and the liquid blood, there must be a difference in pressure between the two and only because of that gases can diffuse in or out of the blood. Same in case of tissues and blood. So let me write down what we learned from here that for any gas to move from one part to the other, there should be a pressure difference or we also call it the pressure gradient. Okay, now here we discussed about the total pressure of air into different parts of the body, okay? But when we talk about the gases in human body, we are mostly concerned with oxygen and carbon dioxide and not other gases like nitrogen, okay? So if we need to calculate how much oxygen moves in and how much carbon dioxide moves out of the body, we need to know their individual pressure, right? So out of the total pressure of the gas that we inhale in, if we calculate only the percentage of pressure exerted by oxygen there, that will be called the partial pressure of oxygen out of the total pressure of the gas, okay? Same for carbon dioxide as well. So if we calculate the pressure of an individual gas, we call it the partial pressure. So if we consider oxygen, we write it as Po2, partial pressure of oxygen and PCO2, that is the partial pressure of carbon dioxide. Alright, now let's get into the numbers. What is the actual difference in pressure between alveolas, blood, blood and tissues, okay? And we will begin with the atmospheric pressure. The atmospheric pressure is 760 mmHg, mmHg is the unit for air pressure, okay? And if we consider this box here, this rectangle here is the total atmospheric pressure, that means 760 mmHg, okay? Out of this, 21% is oxygen, around 78% is nitrogen, less than 1% here is argon and very trace amount of other gases. So you can imagine how less carbon dioxide is out there in the atmosphere, okay? So we will not calculate the amount of carbon dioxide for now, but 21% of 760 is a good number to calculate, okay? So 21% of 760 will come around 159.6, let's just write 160 mmHg. So this is the partial pressure of oxygen out of the total atmospheric pressure, right? So this is Po2, 160 mmHg. That is the pressure of oxygen we inhale. Po2 that we inhale is 160, okay? This is what we inhale. So when we inhale, the air moves down our trachea and reach the LVLS, right? So what will be the partial pressure of oxygen in the LVLS? Well, the very first thought to this question would be, hey, it should be the same as it was outside, right? But, you know, the pressure inside, if you calculate, it changes. Pressure of oxygen here is just 104 mmHg and guess what would be the partial pressure of carbon dioxide? It increases to 40 mmHg. It was just in trace amount outside, somewhere around 0.3 mmHg. Interesting, right? And the reason behind this change partial pressure of gases is also very interesting. The change occurs because the air that we inhale in mixes with water vapor. Water vapor. Now, where is this water vapor coming in from? The trachea has moisture in it, right? And our body is somewhere around 37 degrees Celsius. It is hot. It converts the moisture into water vapor. And when the inhaled air mixes with water vapor, the partial pressure of individual gases will change. And that is how we see the change pressure of oxygen there, okay? Okay, so we can accept the change partial pressure of oxygen, right? Water vapor is a cause. But what about carbon dioxide? Water vapor definitely is not contributing to the increased carbon dioxide pressure in the alveolus, right? Well, carbon dioxide here is produced by you and me, the human body itself. The body tissues continuously produce a lot of carbon dioxide which is released by the blood into the alveolar cavity. And the alveolar cavity, let me remind you, is never empty. Even after a forceful exhalation, there is a good amount of air left. And whatever amount of air left in the alveolus has carbon dioxide in it. And that is how the partial pressure of carbon dioxide in the alveolar cavity is 40. Okay, now for any of these two gases to move into or out from the blood, we need to know the partial pressure of gases in the blood. That is coming in from the heart. And it is found that the partial pressure of oxygen in the deoxygenated blood coming from the heart is 40. And for carbon dioxide is 45. So wherever the pressure is more, it will move towards the lower pressure region. So oxygen in the alveolus is 104. In the blood capillaries, it is 40. So the oxygen will move from alveolus to blood capillaries, right? Po2. And for carbon dioxide, in the alveolus, it is just 40. But in the blood capillaries, it is 45. So the carbon dioxide will move in the opposite direction. It will move from the blood capillaries to the alveolus, right? Okay, now as we discussed, the oxygen will move into the blood. Carbon dioxide will move out of the blood. And so the deoxygenated blood that came from the heart will now be oxygenated because it has got a fresh dose of oxygen from the alveoli, right? Now what will be the partial pressure of carbon dioxide and oxygen in the oxygenated blood that's moving towards the heart? Well, it is found that the new partial pressure in the blood capillaries now for oxygen and carbon dioxide is 95 and 40, respectively. Well, now you may think, why didn't we get more oxygen into the blood? And why didn't we get rid of the entire carbon dioxide from the blood into the alveoli? Well, this is because apart from the pressure gradient, a number of other factors plays crucial role when it comes to the gas exchange. So one of those factor is the solubility. Carbon dioxide is far, far, far more soluble in blood than oxygen. That is the reason we cannot get rid of the entire carbon dioxide, okay? It was so soluble that it needed more pressure to be pushed out from the blood. On the other hand, oxygen, since it's not as readily soluble as carbon dioxide, more pressure was required to push and dissolve oxygen into our bloodstream. So solubility plays a role here, okay? And that is how we got the new partial pressure of oxygen in the oxygenated blood as 95 and for carbon dioxide, we got 40. Okay? Now, this freshly oxygenated blood from the heart will move towards all different body parts, towards our body tissues. Now, let's see the partial pressure of oxygen and carbon dioxide in the body tissues. Since our body cells are always working, they are producing and utilizing a lot of ATP. In the process, they produce a lot of carbon dioxide and consume a lot of oxygen. So oxygen becomes scarce in the body tissues. It becomes 40. And carbon dioxide, since it's produced every time, it raises to 45. Now again, the same thing will happen. Gases will start to move towards the low pressure region. So carbon dioxide is higher in the tissues. So carbon dioxide will move out. It will be dumped into the blood. And oxygen from the blood will start to move into the body tissues. Okay? So the blood will lose oxygen and gain carbon dioxide. And therefore, the partial pressure of these two gases will finally be 40 and 45 for oxygen and carbon dioxide. Okay? This is the partial pressure of these two gases in the deoxygenated blood that is coming from the tissues towards the heart. This deoxygenated blood again reaches the alveoli through the heart. It gets oxygenated and the cycle keeps continuing. Okay? So this was about the gaseous exchange inside the body. Now I have made a better and neat diagram for you. I mean, I have written the numbers in a better way for you to grasp. So you can pause the video here and look at each step in this diagram and see how the partial pressure gradient helps gaseous to diffuse in and out of the blood. And if you're stuck anywhere, you can always go back and watch the video.