22.6 Understanding Gas Exchange and Respiration

Okay, moving on to section six, we're going to be talking about gas exchange between our blood in our lungs and our blood in our tissues. When we're exchanging gases between these two different places, we're talking about diffusion, which means it doesn't require any energy. We have external respiration. That's where we have oxygen entering and carbon dioxide leaving the blood and going into the lungs. If we're talking about internal respiration, we're talking about gas movement between our blood and our body tissues. The gases that we're talking about. our oxygen and carbon dioxide. And these are going to be moving in opposite directions, depending if we're talking about external or internal respiration. So for example, if we're talking about external respiration, we have carbon dioxide that's moving from our blood to our lungs to be dropped off. And if we're talking about internal respiration, we have carbon dioxide moving from our tissues into our blood. If we're talking about oxygen, external respiration. respiration, we have oxygen moving from our lungs into our blood. If we're talking about internal respiration, we're talking about the movement of oxygen from our blood to our tissues. So oxygen and carbon dioxide are moving in opposite directions. Regardless if we're talking about external respiration or internal respiration, we have to include two other things. The basic properties of gases or the laws of the gases, as well as what the the composition of gas is that's in our alveoli to begin with. So if we're talking about the basic properties of gases, we have two important laws that we need to kind of touch on. One is Dalton's law of partial pressures, which basically tells us how a gas is going to behave when it's a component of a mixture. Henry's law is going to basically explain how gases are going to be able to move into and out of a solution, such as water or blood. so Dalton's law of partial pressures basically states that the total pressure that's exerted by a mixture of gases is the sum of the pressures exerted independently by each of the gases in that mixture so each gas and its partial pressure is directly proportional to the percentage of that gas in the mixture so let me show you an example if we're talking about atmospheric gas or atmospheric air that is a mixture of gases. It doesn't just contain oxygen and it doesn't contain just carbon dioxide. The actual major component of atmospheric gas or air is nitrogen. So 78.6% of our atmospheric air is actually made up of nitrogen. Because it's a greater percentage, makes up a greater percentage of our air, that means it's going to contribute more to the total atmospheric pressure. So how do you calculate exactly how much is contributed pressure-wise by nitrogen? You basically convert its percentage, which is 78.6, into its decimal form. So 78.6% is the same thing as putting into decimal form 0.786. Once you have it in decimal form, you multiply it by the total atmospheric pressure, which is 760 millimeters of mercury, which gives us 590. 197 millimeters of mercury. That's the partial pressure that's contributed by nitrogen gas specifically. You can do the same thing with the other gases that make up atmospheric air. So if we're talking about oxygen, about 20.9% of our atmospheric air is oxygen. We put it into decimal form. We multiply it by our total atmospheric pressure. And then we see that about 159 millimeters of mercury out of the 100. out of the 760 millimeters of mercury is contributed by oxygen. You do not need to perform any calculations on this exam. You don't need a calculator. Just kind of understand what Dalton's law of partial pressures represents and that the percentage of a gas is directly proportional to the pressure that it exerts in that gas mixture. Henry's Law, Henry's Law again, it's telling us how gases are moving into and out of a solution. For gases, for gas mixtures that are in contact with liquids, which is what we're talking about if we're talking about blood or water that's present on our alveoli, there's a couple things that Henry's Law states. Basically, Henry's Law states that when a gas is in contact with a liquid, the gas will dissolve into the liquid in proportion to its own partial pressure. So the greater the concentration of a particular gas, the faster the gas will go into solution or into that liquid. At equilibrium though, the partial pressures and the gases and the liquid phases are going to be the same. So there's not going to be any net movement of these gas molecules. A couple things are going to depend on the gas. depend how a gas is able, how much of a gas is able to dissolve in a liquid. One of them is the partial pressure of the gas that's in contact with the liquid. The next was going to be the solubility of the gas into that specific liquid and the temperature at which the liquid or the solution is. So if we're talking about the partial pressure of gas in contact with the liquid, the greater the partial pressure, the greater the ability for that gas to dissolve into that solution. Solubility, the greater the solubility, the greater it's going to be able to dissolve into that liquid. So our two main gases that we're focused on are oxygen and carbon dioxide. Carbon dioxide has a greater solubility than oxygen into water and our blood plasma. So about 20 times more soluble than oxygen. Temperature. temperature influences um solubility based off of um well let's just say if the temperature of a solution increases that actually decreases the solubility of a gas so think about like a soda can if you have a soda can and you open it and leave it um outside and you also take a soda can open it and leave it in your refrigerator where it's much much cooler cooler, where after, you know, 15 minutes, which one of those cans is going to have the greatest amount of gas still in it, the greatest carbonation, and that's going to be the one in the fridge, because the gas is more soluble at the lower temperature. If we increase the temperature, for example, for the can that we put outside, it's going to be what we call flat, all the gas is going to be gone, because the solubility is going to be gone. has been reduced because of the higher temperature. If we're talking about the partial pressure of carbon dioxide in our pulmonary capillaries, that's going to be much higher than that of our lungs. Why? Because if we're talking about carbon dioxide, that's going to diffuse out of our blood and into the air of the alveoli themselves so that we can go through the process of expiration and breathe it out. That would be the goal. this table is on here um you don't need to memorize any specific percentages or any specific partial pressures i would just kind of be aware of the gases that do make up atmospheric air nitrogen is the highest component then comes oxygen, carbon dioxide actually makes up very, very, very, very little of our atmospheric air. And then understand the relationship when we're talking about Dalton's law, why we have to take into account partial pressures. And that's because our atmospheric air is a mixture of different gases. So that was the mixture of gases that we have in our atmosphere, but we also have to take into account the composition or the mixture of gases that we have in our alveoli. And this is going to be different than that of what we have in our atmosphere. because we have gas exchange occurring. We have gas exchanges occurring in our lungs where we have oxygen diffusing from our alveoli and into our blood and then again carbon dioxide diffusing in the opposite direction. We also have humidification of air that's accomplished by our conducting passageways. And we also have mixing of alveolar gas that occurs with each breath. Every time we bring in air, so we inspire gas, that's going to mix with the gases that were left over in our respiratory passageways between our breaths. Okay, moving on to external respiration. During external respiration, which is also called pulmonary gas exchange, we have blood flowing through the pulmonary circuit that is going to be returned to the heart. So eventually we can distribute it to all of our body tissues. During this time, there's going to be a color change, which we talked about in the last lecture material. And the color change that occurs of our blood is going to be due to the oxygenation levels that we have. So if we bring in oxygen and it's able to bind to the hemoglobin and our erythrocytes, and then we also have carbon. dioxide exchange where the unloading of that, those are occurring at the same time. But remember, the higher level of oxygen means the brighter red blood that we actually have. A couple things can influence the gas exchange of oxygen and carbon dioxide. One of them is going to be the partial pressure gradients and gas solubilities that we already talked about. So we have Dalton's law here and then Henry's law here. Another thing that we haven't mentioned yet that can influence external respiration is the thickness and the surface area of the respiratory membrane. And just as a reminder, the respiratory membrane is the alveolar wall connected to the capillary wall by their basement membranes. The last thing is going to be ventilation-perfusion coupling, where we're going to be trying to match the alveolar ventilation with the pulmonary ventilation. blood diffusion. So let's talk first about the partial pressure gradients and gas solubilities. The partial pressure gradients of both of the gases, oxygen and carbon dioxide, are going to be the driving force for diffusion of these gases across the respiratory membrane. So the partial pressures are the main driving force. If we're talking about the pressure gradients, there's going to be a steep gradient for the partial pressure of oxygen. oxygen across the respiratory membrane because the partial pressure of oxygen of the deoxygenated blood in those pulmonary arteries is only about 40 millimeters of mercury as opposed to about 104 millimeters of mercury in our alveoli. So because of that, oxygen is going to rapidly diffuse from the alveoli and into the pulmonary capillary blood. so again we're driving oxygen flow from the alveoli into the pulmonary capillaries until equilibrium is reached across that respiratory membrane it only takes about a quarter of a second for this to occur but it takes about 0.75 seconds to travel from start to end of the pulmonary capillary. The reason why it's very, very quick as far as equilibrium being... reached between the two different gases relative to the amount of time it takes for the actual erythrocyte to travel is to make sure that it's able to receive enough oxygen and we get adequate oxygenation even if we increase our blood flow three times. Say in the event we're exercising and our blood flow is much much faster it's okay because it only takes us a quarter of a second to actually pick up the oxygen that we need. So we have this buffer period of about a half a second I just in case we increase our blood flow, we still get enough oxygen picked up. The partial pressure gradient for carbon dioxide is much less steep, where our venous blood partial pressure is only about 45 millimeters of mercury, and our alveolar partial pressure of carbon dioxide is about 40 millimeters of mercury. So we only have about a 5 millimeter of mercury difference. When we're looking at... Pressure pressure of oxygen, we had about a 64 millimeter mercury pressure difference. So we do have a much smaller pressure gradient for carbon dioxide. So you wouldn't think as much carbon dioxide would be able to diffuse across the membrane. But we also have to take into account solubility. So remember I mentioned previously that carbon dioxide is 20 times more soluble than oxygen. So because... Carbon dioxide is more soluble than oxygen. It's able to still move even though we only have a 5 millimeter of mercury pressure difference. The next thing is going to be thickness and surface area. So respiratory membranes are very, very thin. If you have a thin membrane, that's going to mean that gas exchange is pretty efficient. We also have a large surface area if we're thinking about our alveoli in general. About 40 times the surface area of all of our alveoli relative to our entire surface area of our skin. Because we have such a large surface area, that means we can have more gas diffusing across this place at any given time. The last thing is going to be ventilation-perfusion coupling. Ventilation-perfusion coupling can kind of be tricky. Um, so I have extra supplemental videos under the supplemental material, um, header in this lecture exam module. So if any of this is confusing, go back and watch the supplemental videos. Cause I only talk about the conceptual stuff and kind of leave out everything else. Okay. Ventilation, perfusion, coupling. The goal is for there to be optimal gas exchange. And in order for there to be optimal gas exchange. we have to have a close match or coupling between ventilation and perfusion. So if we're talking about ventilation, what is ventilation? That's the amount of gas that's actually reaching the alveoli and perfusion is going to be the blood flow in the pulmonary capillaries. So the goal of ventilation perfusion coupling or matching is that we are going to try to match or couple the amount of gas that we have coming to the alveoli with the blood flow. We don't want our blood flow to be higher than the amount of gas that we have coming to our alveoli because then our blood is moving more quickly and we're not able to pick up enough oxygen because we don't have enough gas reaching the alveoli. So we want to match or couple again the amount of gas in our alveoli to the blood flow that's actually occurring at the alveoli. so that we can have optimal gas exchange there. Okay, so if we're talking about ventilation and perfusion, they're both controlled by local autoregulatory mechanisms. The partial pressure of oxygen controls perfusion. So again, perfusion is going to be the blood flow in the pulmonary capillaries. How does it control the blood flow into the pulmonary capillaries? Well, by changing the arteriole diameter. The partial pressure of carbon dioxide is going to control the ventilation portion, which is the amount of gas that's actually reaching the alveoli. How does it change the amount of gas reaching the alveoli? By changing the diameter of the bronchioles themselves. So we have influence of the local... Partial pressure of oxygen on perfusion, a couple of things can influence this. So changes in the partial pressure of oxygen in alveoli cause the changes in the diameters of the arterioles where we just talked about. If our oxygen is high in our alveoli, then it's going to cause our arterioles to dilate. Where alveolar oxygen is low, it's going to cause the arterioles to constrict. So that's the influence of the partial pressure of oxygen on the arterioles themselves. It's going to direct blood to go to the alveoli where oxygen is high so it can pick up more oxygen. The opposite mechanism is going to be seen in systemic arterioles that are going to dilate when the oxygen level is low to increase the blood flow and constrict when it's high. If we're talking about the influence of the partial pressure of carbon dioxide on perfusion, basically when the concentration in our alveoli of carbon dioxide is high, that's going to cause our bronchioles to dilate. If we have too much carbon dioxide, we want to get rid of it. How do we get rid of it? We breathe it out. How do we breathe it out faster? By dilating our bronchioles. Wherever our carbon dioxide is low in our alveoli, our bronchioles are going to constrict so that we're not releasing as much carbon dioxide. So again, we have to... balance our ventilation and our perfusion. So the changing of the diameter of those local bronchioles and arterioles is going to help synchronize the alveolar ventilation to the pulmonary perfusion. If we have poor alveolar ventilation, then we're going to have low oxygen and high carbon dioxide levels in the alveoli. So what's going to happen if we have low oxygen and high carbon dioxide in the alveoli, our pulmonary arterioles are going to constrict and our airways are going to dilate. And this is going to allow blood flow and airflow to match. High partial pressures of oxygen and low partial pressures of carbon dioxide in the alveoli cause the bronchioles to constrict. And that's going to promote the release of blood into the pulmonary capillaries. So here's a good little table kind of walking you through ventilation and perfusion. What's going to happen if one is mismatched? So for example, on the left side, if ventilation is less than perfusion and it's mismatched, what's going to happen? Same thing if our ventilation is greater than our perfusion and it's mismatched. what's going to happen. So these diagrams are really good to kind of simplify what I just talked about. So go in and review these. The next thing we have to talk about is internal respiration. Internal respiration, we're talking about gas exchange between our capillaries and our body tissues. The partial pressure and the diffusion gradients are going to be reversed from what we just talked about in external respiration, which I told you previously. Carbon dioxide and oxygen are going to be moving in opposite directions from that of external respiration. So just something to think about. Our tissues are constantly using oxygen for their metabolic activities, for cellular respiration, so that they can produce ATP. One of the byproducts of producing ATP is carbon dioxide, which is considered a waste product. So we have to drop off oxygen to our tissues and pick up carbon dioxide as a waste product to transport it back to our lungs. So because the partial pressure of oxygen is always lower in the tissues than in the arterial blood, that's going to cause oxygen to move from our blood into the tissues until eventually equilibrium is reached. At the same time oxygen is moving into our tissues, we have carbon dioxide moving along its pressure gradient from the tissues to the blood. and into the blood. This is kind of showing you the partial pressures of oxygen and carbon dioxide in different areas, depending on whether or not we're in external respiration and internal respiration. Again, you don't need to know the exact pressures of oxygen or carbon dioxide during these respiration processes. Just kind of have a general understanding of which one is greater. Is the partial pressure of oxygen greater in the tissues or in the blood? Or is the carbon dioxide pressure greater in the tissues or the blood if we're talking about internal respiration? Because it's these pressure differences that cause the gas to move or to diffuse across these different membranes. So again, which one's greater in which area, which is going to influence the gases movement.

If we're talking about internal respiration, we're talking about gas movement between our blood and our body tissues. The gases that we're talking about. our oxygen and carbon dioxide. And these are going to be moving in opposite directions, depending if we're talking about external or internal respiration. So for example, if we're talking about external respiration, we have carbon dioxide that's moving from our blood to our lungs to be dropped off.

And if we're talking about internal respiration, we have carbon dioxide moving from our tissues into our blood. If we're talking about oxygen, external respiration. respiration, we have oxygen moving from our lungs into our blood. If we're talking about internal respiration, we're talking about the movement of oxygen from our blood to our tissues. So oxygen and carbon dioxide are moving in opposite directions.

Regardless if we're talking about external respiration or internal respiration, we have to include two other things. The basic properties of gases or the laws of the gases, as well as what the the composition of gas is that's in our alveoli to begin with. So if we're talking about the basic properties of gases, we have two important laws that we need to kind of touch on. One is Dalton's law of partial pressures, which basically tells us how a gas is going to behave when it's a component of a mixture.

Henry's law is going to basically explain how gases are going to be able to move into and out of a solution, such as water or blood. so Dalton's law of partial pressures basically states that the total pressure that's exerted by a mixture of gases is the sum of the pressures exerted independently by each of the gases in that mixture so each gas and its partial pressure is directly proportional to the percentage of that gas in the mixture so let me show you an example if we're talking about atmospheric gas or atmospheric air that is a mixture of gases. It doesn't just contain oxygen and it doesn't contain just carbon dioxide. The actual major component of atmospheric gas or air is nitrogen. So 78.6% of our atmospheric air is actually made up of nitrogen.

Because it's a greater percentage, makes up a greater percentage of our air, that means it's going to contribute more to the total atmospheric pressure. So how do you calculate exactly how much is contributed pressure-wise by nitrogen? You basically convert its percentage, which is 78.6, into its decimal form.

So 78.6% is the same thing as putting into decimal form 0.786. Once you have it in decimal form, you multiply it by the total atmospheric pressure, which is 760 millimeters of mercury, which gives us 590. 197 millimeters of mercury. That's the partial pressure that's contributed by nitrogen gas specifically. You can do the same thing with the other gases that make up atmospheric air.

So if we're talking about oxygen, about 20.9% of our atmospheric air is oxygen. We put it into decimal form. We multiply it by our total atmospheric pressure.

And then we see that about 159 millimeters of mercury out of the 100. out of the 760 millimeters of mercury is contributed by oxygen. You do not need to perform any calculations on this exam. You don't need a calculator.

Just kind of understand what Dalton's law of partial pressures represents and that the percentage of a gas is directly proportional to the pressure that it exerts in that gas mixture. Henry's Law, Henry's Law again, it's telling us how gases are moving into and out of a solution. For gases, for gas mixtures that are in contact with liquids, which is what we're talking about if we're talking about blood or water that's present on our alveoli, there's a couple things that Henry's Law states.

Basically, Henry's Law states that when a gas is in contact with a liquid, the gas will dissolve into the liquid in proportion to its own partial pressure. So the greater the concentration of a particular gas, the faster the gas will go into solution or into that liquid. At equilibrium though, the partial pressures and the gases and the liquid phases are going to be the same.

So there's not going to be any net movement of these gas molecules. A couple things are going to depend on the gas. depend how a gas is able, how much of a gas is able to dissolve in a liquid.

One of them is the partial pressure of the gas that's in contact with the liquid. The next was going to be the solubility of the gas into that specific liquid and the temperature at which the liquid or the solution is. So if we're talking about the partial pressure of gas in contact with the liquid, the greater the partial pressure, the greater the ability for that gas to dissolve into that solution.

Solubility, the greater the solubility, the greater it's going to be able to dissolve into that liquid. So our two main gases that we're focused on are oxygen and carbon dioxide. Carbon dioxide has a greater solubility than oxygen into water and our blood plasma.

So about 20 times more soluble than oxygen. Temperature. temperature influences um solubility based off of um well let's just say if the temperature of a solution increases that actually decreases the solubility of a gas so think about like a soda can if you have a soda can and you open it and leave it um outside and you also take a soda can open it and leave it in your refrigerator where it's much much cooler cooler, where after, you know, 15 minutes, which one of those cans is going to have the greatest amount of gas still in it, the greatest carbonation, and that's going to be the one in the fridge, because the gas is more soluble at the lower temperature. If we increase the temperature, for example, for the can that we put outside, it's going to be what we call flat, all the gas is going to be gone, because the solubility is going to be gone.

has been reduced because of the higher temperature. If we're talking about the partial pressure of carbon dioxide in our pulmonary capillaries, that's going to be much higher than that of our lungs. Why?

Because if we're talking about carbon dioxide, that's going to diffuse out of our blood and into the air of the alveoli themselves so that we can go through the process of expiration and breathe it out. That would be the goal. this table is on here um you don't need to memorize any specific percentages or any specific partial pressures i would just kind of be aware of the gases that do make up atmospheric air nitrogen is the highest component then comes oxygen, carbon dioxide actually makes up very, very, very, very little of our atmospheric air. And then understand the relationship when we're talking about Dalton's law, why we have to take into account partial pressures.

And that's because our atmospheric air is a mixture of different gases. So that was the mixture of gases that we have in our atmosphere, but we also have to take into account the composition or the mixture of gases that we have in our alveoli. And this is going to be different than that of what we have in our atmosphere.

because we have gas exchange occurring. We have gas exchanges occurring in our lungs where we have oxygen diffusing from our alveoli and into our blood and then again carbon dioxide diffusing in the opposite direction. We also have humidification of air that's accomplished by our conducting passageways.

And we also have mixing of alveolar gas that occurs with each breath. Every time we bring in air, so we inspire gas, that's going to mix with the gases that were left over in our respiratory passageways between our breaths. Okay, moving on to external respiration.

During external respiration, which is also called pulmonary gas exchange, we have blood flowing through the pulmonary circuit that is going to be returned to the heart. So eventually we can distribute it to all of our body tissues. During this time, there's going to be a color change, which we talked about in the last lecture material.

And the color change that occurs of our blood is going to be due to the oxygenation levels that we have. So if we bring in oxygen and it's able to bind to the hemoglobin and our erythrocytes, and then we also have carbon. dioxide exchange where the unloading of that, those are occurring at the same time.

But remember, the higher level of oxygen means the brighter red blood that we actually have. A couple things can influence the gas exchange of oxygen and carbon dioxide. One of them is going to be the partial pressure gradients and gas solubilities that we already talked about.

So we have Dalton's law here and then Henry's law here. Another thing that we haven't mentioned yet that can influence external respiration is the thickness and the surface area of the respiratory membrane. And just as a reminder, the respiratory membrane is the alveolar wall connected to the capillary wall by their basement membranes. The last thing is going to be ventilation-perfusion coupling, where we're going to be trying to match the alveolar ventilation with the pulmonary ventilation. blood diffusion.

So let's talk first about the partial pressure gradients and gas solubilities. The partial pressure gradients of both of the gases, oxygen and carbon dioxide, are going to be the driving force for diffusion of these gases across the respiratory membrane. So the partial pressures are the main driving force.

If we're talking about the pressure gradients, there's going to be a steep gradient for the partial pressure of oxygen. oxygen across the respiratory membrane because the partial pressure of oxygen of the deoxygenated blood in those pulmonary arteries is only about 40 millimeters of mercury as opposed to about 104 millimeters of mercury in our alveoli. So because of that, oxygen is going to rapidly diffuse from the alveoli and into the pulmonary capillary blood. so again we're driving oxygen flow from the alveoli into the pulmonary capillaries until equilibrium is reached across that respiratory membrane it only takes about a quarter of a second for this to occur but it takes about 0.75 seconds to travel from start to end of the pulmonary capillary.

The reason why it's very, very quick as far as equilibrium being... reached between the two different gases relative to the amount of time it takes for the actual erythrocyte to travel is to make sure that it's able to receive enough oxygen and we get adequate oxygenation even if we increase our blood flow three times. Say in the event we're exercising and our blood flow is much much faster it's okay because it only takes us a quarter of a second to actually pick up the oxygen that we need.

So we have this buffer period of about a half a second I just in case we increase our blood flow, we still get enough oxygen picked up. The partial pressure gradient for carbon dioxide is much less steep, where our venous blood partial pressure is only about 45 millimeters of mercury, and our alveolar partial pressure of carbon dioxide is about 40 millimeters of mercury. So we only have about a 5 millimeter of mercury difference.

When we're looking at... Pressure pressure of oxygen, we had about a 64 millimeter mercury pressure difference. So we do have a much smaller pressure gradient for carbon dioxide.

So you wouldn't think as much carbon dioxide would be able to diffuse across the membrane. But we also have to take into account solubility. So remember I mentioned previously that carbon dioxide is 20 times more soluble than oxygen. So because... Carbon dioxide is more soluble than oxygen.

It's able to still move even though we only have a 5 millimeter of mercury pressure difference. The next thing is going to be thickness and surface area. So respiratory membranes are very, very thin.

If you have a thin membrane, that's going to mean that gas exchange is pretty efficient. We also have a large surface area if we're thinking about our alveoli in general. About 40 times the surface area of all of our alveoli relative to our entire surface area of our skin. Because we have such a large surface area, that means we can have more gas diffusing across this place at any given time.

The last thing is going to be ventilation-perfusion coupling. Ventilation-perfusion coupling can kind of be tricky. Um, so I have extra supplemental videos under the supplemental material, um, header in this lecture exam module.

So if any of this is confusing, go back and watch the supplemental videos. Cause I only talk about the conceptual stuff and kind of leave out everything else. Okay. Ventilation, perfusion, coupling.

The goal is for there to be optimal gas exchange. And in order for there to be optimal gas exchange. we have to have a close match or coupling between ventilation and perfusion.

So if we're talking about ventilation, what is ventilation? That's the amount of gas that's actually reaching the alveoli and perfusion is going to be the blood flow in the pulmonary capillaries. So the goal of ventilation perfusion coupling or matching is that we are going to try to match or couple the amount of gas that we have coming to the alveoli with the blood flow. We don't want our blood flow to be higher than the amount of gas that we have coming to our alveoli because then our blood is moving more quickly and we're not able to pick up enough oxygen because we don't have enough gas reaching the alveoli.

So we want to match or couple again the amount of gas in our alveoli to the blood flow that's actually occurring at the alveoli. so that we can have optimal gas exchange there. Okay, so if we're talking about ventilation and perfusion, they're both controlled by local autoregulatory mechanisms. The partial pressure of oxygen controls perfusion.

So again, perfusion is going to be the blood flow in the pulmonary capillaries. How does it control the blood flow into the pulmonary capillaries? Well, by changing the arteriole diameter. The partial pressure of carbon dioxide is going to control the ventilation portion, which is the amount of gas that's actually reaching the alveoli.

How does it change the amount of gas reaching the alveoli? By changing the diameter of the bronchioles themselves. So we have influence of the local...

Partial pressure of oxygen on perfusion, a couple of things can influence this. So changes in the partial pressure of oxygen in alveoli cause the changes in the diameters of the arterioles where we just talked about. If our oxygen is high in our alveoli, then it's going to cause our arterioles to dilate. Where alveolar oxygen is low, it's going to cause the arterioles to constrict. So that's the influence of the partial pressure of oxygen on the arterioles themselves.

It's going to direct blood to go to the alveoli where oxygen is high so it can pick up more oxygen. The opposite mechanism is going to be seen in systemic arterioles that are going to dilate when the oxygen level is low to increase the blood flow and constrict when it's high. If we're talking about the influence of the partial pressure of carbon dioxide on perfusion, basically when the concentration in our alveoli of carbon dioxide is high, that's going to cause our bronchioles to dilate.

If we have too much carbon dioxide, we want to get rid of it. How do we get rid of it? We breathe it out.

How do we breathe it out faster? By dilating our bronchioles. Wherever our carbon dioxide is low in our alveoli, our bronchioles are going to constrict so that we're not releasing as much carbon dioxide.

So again, we have to... balance our ventilation and our perfusion. So the changing of the diameter of those local bronchioles and arterioles is going to help synchronize the alveolar ventilation to the pulmonary perfusion. If we have poor alveolar ventilation, then we're going to have low oxygen and high carbon dioxide levels in the alveoli.

So what's going to happen if we have low oxygen and high carbon dioxide in the alveoli, our pulmonary arterioles are going to constrict and our airways are going to dilate. And this is going to allow blood flow and airflow to match. High partial pressures of oxygen and low partial pressures of carbon dioxide in the alveoli cause the bronchioles to constrict.

And that's going to promote the release of blood into the pulmonary capillaries. So here's a good little table kind of walking you through ventilation and perfusion. What's going to happen if one is mismatched? So for example, on the left side, if ventilation is less than perfusion and it's mismatched, what's going to happen? Same thing if our ventilation is greater than our perfusion and it's mismatched.

what's going to happen. So these diagrams are really good to kind of simplify what I just talked about. So go in and review these. The next thing we have to talk about is internal respiration. Internal respiration, we're talking about gas exchange between our capillaries and our body tissues.

The partial pressure and the diffusion gradients are going to be reversed from what we just talked about in external respiration, which I told you previously. Carbon dioxide and oxygen are going to be moving in opposite directions from that of external respiration. So just something to think about.

Our tissues are constantly using oxygen for their metabolic activities, for cellular respiration, so that they can produce ATP. One of the byproducts of producing ATP is carbon dioxide, which is considered a waste product. So we have to drop off oxygen to our tissues and pick up carbon dioxide as a waste product to transport it back to our lungs.

So because the partial pressure of oxygen is always lower in the tissues than in the arterial blood, that's going to cause oxygen to move from our blood into the tissues until eventually equilibrium is reached. At the same time oxygen is moving into our tissues, we have carbon dioxide moving along its pressure gradient from the tissues to the blood. and into the blood.

This is kind of showing you the partial pressures of oxygen and carbon dioxide in different areas, depending on whether or not we're in external respiration and internal respiration. Again, you don't need to know the exact pressures of oxygen or carbon dioxide during these respiration processes. Just kind of have a general understanding of which one is greater. Is the partial pressure of oxygen greater in the tissues or in the blood? Or is the carbon dioxide pressure greater in the tissues or the blood if we're talking about internal respiration?

Because it's these pressure differences that cause the gas to move or to diffuse across these different membranes. So again, which one's greater in which area, which is going to influence the gases movement.

Transcript for:22.6 Understanding Gas Exchange and Respiration

Transcript for:
22.6 Understanding Gas Exchange and Respiration