During the passage of the blood through the capillaries of the lungs an outward diffusion of carbon dioxide into the air in the lung sacs is going on simultaneously with the inward movement of oxygen from this air into the blood. The diffusion is never so complete as to deprive the blood of all its carbon dioxide; there is in fact only a little less of it in arterial blood than in venous, although the diffusion is sufficiently rapid so that as much carbon dioxide as is produced in the whole body in a minute is passed out into the air of the lung sacs in the same time. The effect of this outward diffusion is naturally to increase the amount of carbon dioxide in the air of the lung sacs, and if this increase is allowed to go on unhampered, there will presently be so much carbon dioxide there as to stop further outward diffusion, and so to put an end to the escape of carbon dioxide from the blood. This is avoided by lung ventilation. Every time a breath is drawn some air that is almost free from carbon dioxide enters the lung spaces to replace the carbon dioxide-laden air that was expelled at the previous exhalation. The description of gas transportation that we have just given opens the way for an account of the control of breathing. From what has just been said it should be clear that the amount of carbon dioxide in the blood corresponds closely with the amount that is in the air of the lung sacs. As the percentage of carbon dioxide in this air goes up, outward diffusion becomes less free, and so the amount of carbon dioxide in the blood will have to increase. The tissues are all the time producing and pouring out carbon dioxide, and so there will be a steady increase in the amount of carbon dioxide in the blood. This applies to the arterial blood as well as to the venous, since, as we saw a moment ago, there is nearly as much carbon dioxide in the former as in the latter. This is the fact which is utilized in the body for operating the breathing machinery. The respiratory center in the brain stem is susceptible to carbon dioxide; the more of this gas there is in the blood, the more tendency there will be for the center to discharge. There is a certain level of carbon dioxide below which it is entirely inactive, but when this level is passed nervous discharges begin and become more and more powerful as the amount of carbon dioxide in the blood goes up. Now we can see what makes us breathe. Let us imagine that there is not very much carbon dioxide in our blood, but that the tissues are constantly producing it and giving it off. Since we are supposing the amount is not enough to excite the respiratory center, there will be no movements of breathing. There will be a steady increase of the amount of carbon dioxide in the blood and at the same time a corresponding increase of the amount of carbon dioxide in the air sacs of the lungs; presently there will be enough in the blood to arouse the respiratory center to discharge.