Carbon dioxide, unlike oxygen, is carried largely in the plasma; a small quantity is in physical solution, but the major portion is in chemical combina tion as sodium bicarbonate. The relative quantities in solution and in chemical combination regulate the reaction (degree of acidity or alkalinity) of the blood. The equation which connects the concentration of hydrogen ions CH to the concentration of carbon dioxide
in solution,
and in chemical combina tion
is To say that the carbon dioxide is present chiefly in chemical combination as sodium bicarbonate
gives but a par tial picture of its relation to the blood. Such a combination by itself would be very stable, and while it might provide a medium of suitable hydrogen ion concentration would not present the all important property of absorbing and parting with large quan tities of carbon-dioxide with very little reaction and with very little alteration in the partial pressure of carbon dioxide to which the blood is exposed. This double purpose is achieved by the presence in the blood of other acids, notably haemoglobin, which do not unite with carbon dioxide, but which compete with it for the sodium. The full beauty of the mechanism is only seen, how ever, when it is realized that haemoglobin is a stronger acid in combination with oxygen than when reduced. Now in passing through the tissues the moment when the blood requires to unite with carbon dioxide is also the moment at which it looses oxygen; at that moment, therefore, the haemoglobin becomes less strongly acid, and a base is therefore liberated with which the carbon di oxide can unite. The reverse series of changes takes place in the lung. As the oxygen unites with the haemoglobin, that material becomes more strongly acid, claims more of the base, and so displaces carbon dioxide, raising the partial pressure of that gas and therefore assisting its diffusion from the blood.
Diffusion of Gases Through the Pulmonary Epithelium. —With this understanding of the chemical processes which enable large quantities both of oxygen and carbon dioxide to pass into and out of the blood, as the result of only very small alterations in the pressure of those gases in the medium to which the blood is exposed, let us return to the proof of the general thesis that the passage of gases through the pulmonary epithelium is due simply to diffusion.
The basal principle of diffusion is that the quantity of gas which passes through a given membrane depends upon the dif ference of pressure of the gas on the two sides of the membrane. Regarding the lung as a membrane through which gas diffuses, Q being the quantity of oxygen which will pass through it per minute, P the pressure of oxygen in the pulmonary alveoli, T the mean pressure of oxygen in the capillaries of the lung, and k a coefficient depending upon the area and nature of the lung, then will pass through the pulmonary epithelium per minute, with a difference of pressure of i mm. of mercury between the oxygen on the two sides of the membrane.
If the diffusion theory is correct, then not only must P be always greater than T, but their relations must be such as to allow of quantities of oxygen ranging from 200 cu.cm. at rest to perhaps 3,00o during extreme activity, being driven through the pulmonary epithelium per minute. As all the quantities Q, k, P and T are susceptible of independent measurement, it should be possible to form a judgment of the applicability of the equation. The sim
plest case is the condition of rest. The following measurements of k are given for I I men, all of good physique at rest.
Assuming that each of these men was absorbing 25o cu.cm. per minute, the mean difference of pressure (P—T) between the oxygen in the alveolar air and capillary blood would have to be These figures lead to the conclusion that at rest the pressure of oxygen in the alveolar air exceeds the average pressure in the capillary blood by about 6 mm. of mercury in the majority of well-developed persons. The average pressure in the capillaries is, of course, less than the pressure in the arteries and greater than that in the veins. Other data known concerning the individ uals on whom these experiments were made lead to the conclu sion that 6 mm. for the value of (P—T) when Q is 25o cu.cm. may likely enough be correct. Greater difficulties arise when Q becomes, say, 2,500 cu.cm. during degrees of activity of which probably all the above persons would have been capable. If the diffusion coefficient still remains on the average 4o cu.cm. the value of (P—T) would become 62.5 mm. It is hardly possible that the pressure of oxygen in the lungs should exceed the aver age pressure in the capillaries by so great an amount. We are therefore thrown back upon the position that in violent exercise the diffusion coefficient must alter, and there seems little doubt that an alteration on a sufficient scale takes place.
When active exercise is taken both the depth and the rate of respiration are as a rule increased. The increase is effected by one or both of two mechan isms; of these the first to be considered is nervous, the second chemical. The nervous factor in the regulation of respiration has been well illustrated by the following experiment, devised by Krogh and Lindhart. The subject is placed on a bicycle ergometer of a special type, i.e., a bicycle which, instead of progressing, is made to work against a brake, the actual work done being meas ured by the brake. In this case the brake was an electromotor, the resistance to the worker and hence the work which he per formed in overcoming it could be regulated by adjustment of the current passed through the motor. When work was commenced there was an immediate increase in the rate of depth of respira tion and also in the pulse rate. Had these alterations been due to the stimulating action of chemical products formed in the muscle on the respiratory centre, enough time must have elapsed to allow of the products being taken up by the blood, carried to the heart, passed through the lungs and driven to the brain. These processes would have occupied upwards of half a minute. In point of fact, the augmentation of respiration came about much more quickly—in about five seconds from the commencement of the exercise. An even more striking experiment of the same sort was the following: The apparatus being as before the subject was led to suppose that the load on the machine (and consequently the exercise he was to take) was to be suddenly and largely in creased by throwing in a powerful current. Actually the current was not thrown in, though the pantomime of closing the switches, etc., was gone through. The pulse and respirations were aug mented as before, though no extra work was done by the subject. Clearly, therefore, the increased respiratory efforts were not due to chemical products produced by the work.