hereiswherethereisnt I think the point is that you want to have a measure of how much is actually going in. If you choose a gas that is 100% diffused (nothing remains in the alveoli) then you don't actually know what is the maximal diffusion value. You could maybe double the concentration and it would still diffuse 100% by the end of the capillary network. By choosing CO you know the concentration of gas going into the system, and you can measure the amount of CO going out of the system. Subtract the first by the second and you have the amount actually diffused. From there you can get to a diffusion rate, which is compared to a predicted value of DLCO based on age, sex of patient etc (some factors like that influence on the value) and if there is a variation from the predicted value it might indicate some problem or another (she explains what kind of diseases influence the diffusion rate in the video, like emphysema or bronchitis)

Matan J thanks for your reply. thanks for clarifying that part. part i dont get is why we choose CO when it has 200x more affinity for hemoglobin than oxygen and yet oxygen is perfusion limited (with the exception of o2 being diffusion limited during heavy exercise). perhaps its an inherent property of the alveolar membrane that doesnt allow CO to cross it as easily as o2? and that CO only outcompetes o2 once actually crosses the barrier, but CO actually requires a higher pCO to cross the barrier vs the p02 required?

hereiswherethereisnt I'm not sure if I understand your question, but we need to differentiate the two things: There's a degree of diffusion through the membrane of the alveolus, and a degree of "diffusion" in the blood itself, and these are not the same thing.. The affinity of a gas to Hb is just about how much of the gas is transported in the blood with the Hb protein, it's not related to how much passes through the membrane. Let's imagine this situation: You have ten molecules of gas, lets say it's CO with the high affinity to Hb but low diffusion ability through the membrane. The molecules enter the alveolus, because the CO gas has low diffusion rate through the alveolus membrane, only 5 molecules pass the membrane into the blood. Because it has a huge affinity to Hb, it binds immediately and all 5 molecules that passed the membrane binded to Hb. But still you have five molecules in the alveolus, and they are exhaled out of the lung. Let's imagine a second situation, same thing, but instead of ten molecules of CO you have ten molecules of O2. O2 has a higher diffusion ability than CO through the membrane, so all 10 molecules of O2 will pass the membrane into the blood. Inside the blood the O2 has still good affinity to bind to Hb, but lower than CO, so let's say seven molecules bind to Hb and three don't. No O2 is exhaled from the alveolus because the diffusion is so efficient. Obviously these are just numbers I invented to make a point, but the point is that O2 is a perfusion limited gas, it goes through the membrane so efficiently, that it equilibrates (the pressure of O2 in the alveolus is equal to the pressure of O2 that's in the blood) before the volume of blood reaches the end of the respiratory capillary that surrounds the alveolus. So what is limiting the amount of O2 the system receives is not the passage of O2 into the blood, that is fast as hell, but the amount of blood coming in. If you somehow manage to bring more blood into the same capillary, it will be more rich in O2 at the end of the capillary (and that's what happens in respiratory vasodilation, if I'm not terribly wrong). In contrast, CO is a diffusion limited gas, so no matter how large the volume of blood that is passing through the capillary, the amount of CO at the end capillary will always be exactly the same, because the gas won't diffuse through the membrane fast enough by the time that certain volume of blood reaches the end of the capillary and into the pulmonary vein. Hopefully this helped clear things up because this is really really long and I hope I didn't write all of this for nothing :)

thanks a lot for taking the time. it certainly did help since you answered my question exactly -- that CO is limited by the membrane more than o2. you just helped a med student grasp a concept 4 days before his boards exam

hereiswherethereisnt No problem man, I'm a med student myself and I have an oral exam on this stuff (physiology) in four days as well. I consider this a self-test since I get to organise my thoughts and shoot everything I know in a way that hopefully makes an acceptable answer! Now I will definitely not choke up if they ask me about diffusion limited or perfusion limited gasses :)

no. lol.

Yes!John 14:6

kzfaq.info/get/bejne/i7mHrZiS1qypdZs.html

CO2 is carried in the blood as dissolved gas mostly, also as bicarb and a small amount bound to hemoglobin as carbaminohemoglobin. She didn't say anything inaccurate. Hemoglobin first preference is CO, then O2 then CO2.

yh that is what I mean too, she should have mentioned that.

@@edwardherrera846 the whole concept is inaccurate what is being tested for is the Transfer of CO across alveoli not Diffusion because there is NO Diffusion. Also CO2 is NOT a waste product it plays an important part in Respiratory Physiology. It is because of all the wrong concepts described here that Covid was not understood as a Respiratory Disease

www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=www.ucl.ac.uk/anaesthesia/sites/anaesthesia/files/co2_transport.pdf&ved=2ahUKEwjVi_e4jKuBAxUDWEEAHZ8fBLEQFnoECBMQBg&usg=AOvVaw0Xs61p5bOtwgRNjViUXgxF