Alright. For this example, it says, place the following areas in order from highest to lowest according to their expected partial pressure of oxygen, or that \( p_{{O}_2} \) there that we see. Then do the same for partial pressure of carbon dioxide. In some cases, the values are the same. If that is the case, circle the equal sign between the two values. If the values are not the same between locations, circle the greater than sign between each value. Alright. So here we have, where we need to fill in for partial pressure of oxygen and then partial pressure of carbon dioxide. And the places that we need to fill in are the body tissues, arterial blood, venous blood, the external air, and the air in the alveoli. Alright. So let's start with partial pressure of oxygen. As you look at those five areas, which one do you expect the partial pressure of oxygen to be the greatest? Well, I expect it to be the greatest in d, the external air. Right? The air around us has the most oxygen, the highest partial pressure of oxygen. Alright. Where do we go from there? Well, remember, the air comes into our alveoli, and there's a lower partial pressure of oxygen in the alveoli than in the external air. So air in the alveoli is going to come next, and that external air definitely has a greater partial pressure. So I'm going to circle that greater than sign there and I'll cross out the equal sign. Alright. Well, where do we go next? Alright. Well, the blood comes through those capillaries in the alveoli, and the partial pressure of oxygen is going to equalize to that air in the alveoli, and then that blood goes from those capillaries into the arterial blood. So that's going to come next. I'm going to put b here for the arterial blood. Now we got to think do we circle the greater than sign or the equal to sign? Well, I just said it equalizes too, but remember as it leaves, the partial pressure just goes down just a little bit as some of that oxygen gets picked up by the hemoglobin. So it's technically just a little bit different. The partial pressure of oxygen in the alveoli is just a little bit greater than in the arterial blood. Alright. From the arterial blood, where do we go next? Alright. Well, from there, the arterial blood goes to the capillaries through the tissues, and that partial pressure in the tissues is definitely lower than in the arterial blood. So I'm going to put that one next. Body tissues, that is a. And so, the arterial blood is greater than, so I'll circle the greater than sign, I'll cross out the equals to sign, and that leaves us with the venous blood is left. So remember that blood goes through the tissues, it equalizes to the tissues, and then it leaves, and now it's in the venous blood. So venous blood comes last, but that is going to be equal to the partial pressure in the tissues. Alright. That's our \( c \). I'm sorry. That's our oxygen. Now let's do our \( CO_2 \), our carbon dioxide. So carbon dioxide, where do you expect the partial pressure of carbon dioxide to be the greatest? Well, that's going to sort of be the opposite of the oxygen. It's going to be the greatest in the body tissues where cellular respiration is happening and that carbon dioxide is being produced. Alright. So high partial pressure in the body tissues. Well, what's next after that? Well, the blood goes through the capillaries, through those tissues, and the partial pressure of carbon dioxide is going to equalize to the partial pressure in those tissues, and then that blood leaves and then it's in the venous blood. So venous blood is going to come next. That's c. And we said it's going to equalize to that, so I'm going to circle the equal sign here and cross out the greater side. Alright. Where do we go next? Well next, that blood goes through the capillaries through the alveoli, And so that partial pressure in the alveoli is definitely lower than the partial pressure in the venous blood. So I'm going to put air in the alveoli next, and I said it's lower there. So I'm going to circle this greater than sign that's sort of pointing out that venous blood, and I'll cross out the equal sign. Alright. From there, that blood leaves the alveoli, and it enters into the arterial blood. So I'm going to put arterial blood next. And again, here as it goes through the alveoli, that blood equalizes to the partial pressure in the alveoli. So I'm going to circle equal to here and cross out the greater than, and that leaves us with one more thing to put in here. The external air needs to come last and, well, external air, do you remember, is that partial pressure going to be equal to or much less than the partial pressure in the arterial blood? Well, for carbon dioxide, remember, the partial pressure of carbon dioxide in the external air is really, really low. There's not much carbon dioxide in the air around us. So I'm definitely circling this greater than sign here that's sort of pointing toward that arterial blood. There's not much carbon dioxide, not much partial pressure of carbon dioxide in that external air. Alright. That felt a little complicated, but we straightened it out. We put them all in order. Practice problems after this. Give them a try.