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From my understanding, the process that displaces the carbon dioxide and oxygen in our erythrocytes and lungs is diffusion. I've been taught that diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, until an equillibrium is reached.

For there to be an equillibrium in carbon dioxide and oxygen, doesn't that mean that there should be equal portions of those elements present at all times? So therefore, erythrocytes should contain both of those chemicals simultaneously. Am I correct in assuming this? If I am, then I have a follow-up question: wouldn't carbon dioxide floating about in our blood indefinitely cause some unhealthy side effects, and is there something that prevents it from doing so?

I'm currently only in GCSEs, so apologies if this question might actually have a very simple answer.

Thanks

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For diffusion to occur there must be a "concentration gradient", that is one area where the substance that is diffusing from, to where it is diffusing to. Where the substance is diffusing from, the concentration is high and where it is going to, the concentration is low. Our bodies continually use up oxygen and produces carbon dioxide. Carbon dioxide is generally carried in the blood plasma, and oxygen is generally carried by the red blood cell (erythrocytes). Whereas, it is true that in diffusion an equilibrium is usually achieved, in this case you mention there is no equilibrium. Our physiological processes (respiration) continually maintains that concentration gradient and so carbon dioxide always flows out and oxygen always comes in. Too much carbon dioxide in the blood will raise the pH and cause serious metabolic problems with enzymes and their efficient function.

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  • $\begingroup$ Diffusion can (and in this case, typically does) lead to local equilibrium in lung gas exchange. This was determined by extensive work during the first 3/4 of the 20th century. Also, increasing carbon dioxide lowers rather than raises blood pH; acidification is a lowering of pH. $\endgroup$ – EdM Oct 7 '15 at 2:59
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There in fact typically is an equilibrium of diffusion for both oxygen and for carbon dioxide by the time the blood finishes gas exchange in the alveoli of the lung. One confusion in the OP is that there is no real meaning for a chemical equilibrium between carbon dioxide and oxygen, as they are separate chemical entities. Carbon dioxide in the lung capillary blood equilibrates with carbon dioxide in the lung alveolar gas; oxygen similarly equilibrates between blood and gas. Each effectively on its own.

Yes, oxygen is continually being consumed and carbon dioxide being produced by the body, so from that perspective we have a whole body steady state. At overall equilibrium we are dead. But at the site of gas exchange in the lung, there is generally no difference in the chemical activity of either gas between the alveolar air and the capillary blood leaving the alveolus. So from a chemical perspective there is generally local equilibrium of both gases at that point.

In some circumstances this local equilibrium is not reached. The transport of carbon dioxide is very complicated, as it exists in several inter-convertible forms: dissolved carbon dioxide gas, a form called carbonic acid where it has chemically reacted with water, the bicarbonate and carbonate ions that come from dissociation of carbonic acid, and a carbamate form that is a reversible chemical bond with amino groups on proteins. The reaction with water is slow on its own; it is catalyzed by the enzyme carbonic anhydrase in red cells. If that enzyme is inhibited (for example, by the drug acetazolamide) then there might not be chemical equilibrium of total carbon dioxide between alveolar air and capillary blood as there might not be enough time, as blood flows through the alveolus, for all the other forms to convert to the dissolved gas that can diffuse from blood into alveolar air.

Another complication is that transport of oxygen and carbon dioxide in the blood are somewhat interdependent. Acidification by carbon dioxide (lower pH is more acid) affects the ability of hemoglobin to bind oxygen, tending to release more oxygen. Nevertheless, in most circumstances in humans, both gases are individually at equilibrium between alveolar air and the blood that leaves the alveolus.

Finally, the continued presence of carbon dioxide in the body is a normal, healthy part of physiologic functioning. It does have to be kept in balance, but it does have to be there.

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