As an illustration of the practical working of the respiration apparatus the first experiment of Pettenkofer and Voit,1 which gives the metabolism in a starving man, will be described.

1 Pettenkofer and Voit: "Zeitschrift fur Biologie," 1866, ii, 478.

The man was allowed a small quantity of Liebig's extract of beef, as the experimenters did not at that time realize the very slight discomfort usually entailed by total abstinence from food. As Liebig's extract has no nutritive value, its effect has been counted out in the following description.

The subject, on entering the living-room of the apparatus, weighed 71.090 kilograms, and he drank during the day 1.0548 liters of water, making a total body weight of 72.1448 kilograms. Twenty-four hours later he weighed 70.160 kilograms and his excreta had amounted to 0.7383 kilogram carbon dioxid, 0.8289 kilogram water from lungs and skin, and 1.1975 kilograms of urine. The final body weight plus all the excreta amounted to 72.9247 kilograms. A total body weight of 72.1448 kilograms was converted into a body weight plus excreta amounting to 72.9247 kilograms. The difference is due to oxygen absorbed. The difference of 0.7799 kilogram represents the amount of oxygen needed to convert the body substance lost into the excretory products obtained. The tabular statement reads:

Man - Starvation

The analysis of the urine showed 12.51 grams of nitrogen and 8.25 grams of carbon. A calculation gives the amount of carbon in the respiration as 201.3 grams. If we neglect the feces as being too small in starvation to influence the results, we find that the total carbon elimination for twenty-four hours was 209.55 grams, and the total nitrogen 12.51. In the Liebig extract ingested there were 2.44 grams of carbon and 1.18 grams of nitrogen, which must be deducted from the above in order to obtain the strict loss of carbon and nitrogen from the body during the period of starvation. These values are:

C........................................ 207.11 grams.

N........................................ 11.33 "

These two figures enabled Pettenkofer and Voit to calculate what substances had burned in the body. As every gram of nitrogen in the excreta is approximately represented by the destruction of 6.25 grams of meat protein, the amount of such protein destroyed by the man was 70.81 grams. It has been found that for every gram of nitrogen present in meat protein there are 3.28 grams of carbon. It is therefore easy to estimate that destruction of protein represented by 11.33 grams of nitrogen involved the elimination of 37.16 grams of carbon. Now, the man eliminated 207.11 grams of total carbon, from which this protein carbon may be deducted, leaving as residue 169.95 grams, which must have originated from a source other than protein. The possible sources are two in number - carbohydrates and fats. In starvation no carbohydrates are ingested and their supply in the form of reserve glycogen is usually counted as being negligible in such experiments as these. The only other source from which the 169.95 grams of extra carbon could have been derived is fat, and as fat contains 76.52 per cent, of carbon, a destruction of 222.1 grams of fat may be calculated. This fasting man therefore destroyed:

Protein................................... 70.81 grams.

Fat...................................... 222.1 "

That such metabolism actually did take place was further indicated by the comparison of the amount of oxygen needed for the destruction of the above constituents, and the amount of oxygen absorption as determined by the experiment.

From the constituents of the protein and fat destroyed, Pettenkofer and Voit deducted the constituents of the urine, which contains part of the C and H belonging to protein. The balance of the carbon and hydrogen was fit for oxidation to carbon dioxid and water. Their calculation may thus be presented:

We may reach the same result by using the most modern figures for the oxygen requirement in the metabolism of the food-stuffs. We now know that to burn 100 grams of meat protein requires 133.43 grams of oxygen, and to burn 100 grams of fat requires 288.5 grams, and to burn 100 grams of starch 118.5 grams. This being true, there are required:

Had carbohydrates burned, less oxygen would have been needed, since carbohydrates contain a larger proportion of oxygen than fats. Had the extra 169.95 grams of carbon been due to the combustion of starch (or glycogen), 382 grams would have burned, requiring 452.7 grams of oxygen instead of 639.5 grams for fat. Pettenkofer and Voit found in the amount of oxygen absorption a confirmation of their belief that the fasting organism supports itself by the combustion of its own protein and fat.

It is apparent from this discussion that the quantity of oxygen needed in metabolism depends upon the chemical composition of the material that burns in the organism, and also that the relation between the amount of oxygen absorbed and carbon dioxid excreted depends on the same factor. Regnault and Reiset frequently observed that this latter relationship was variable. The ratio of the volume of carbon dioxid expired to the volume of oxygen inspired during the same time is called the respiratory quotient (see p. 57). When carbohydrates burn, the R. Q. is unity; that is, for every hundred volumes of carbon dioxid excreted a hundred volumes of oxygen are absorbed. When protein burns the quotient is Vol.CO2 / Vol.O2 =78.1 / 100 or 0.781, and when fat burns the quotient is 0.71. Pettenkofer and Voit calculated that the respiratory quotient in their fasting man was 0.69. This indicated a combustion of fat in the organism.

The further researches of Pettenkofer and Voit were founded on the principles described in the above experiment on a fasting man. If meat and fat were ingested, the carbon and nitrogen excreta were collected, and from these data it was determined how much of each food-stuff was oxidized and whether there was a storage of either in the body or a loss of either from the body. If a mixed diet which included carbohydrates were given, the carbon dioxid elimination increased and the oxygen absorption was such as indicated the combustion of carbohydrates. It was assumed that after deducting the protein carbon from the total carbon eliminated, the balance of extra carbon was derived from the destruction of the carbohydrates in so far as these were ingested; any carbon in excess of this was attributed to fat combustion.