In man one-thirteenth part of the body weight is carried as blood to the lungs at least every minute and there exposed for a period of two seconds to the action of the alveolar air. The blood in the capillaries of the lungs may be estimated as a film 0.01 millimeter in thickness and 150 square meters in area, or nearly a hundred times the area of the surface of the body. Zuntz estimates the combined thickness of the alveolar wall and capillary wall at 0.004 mm. This is the total distance separating the alveolar air from the blood. The gaseous exchange between air and blood is thus readily made possible.

In an experiment by Henriques1 four different determinations were made upon an anesthetized dog: (1) The rate of flow of blood; (2) the carbon dioxid and oxygen content of the venous blood in the right heart; (3) the quantity of the same gases in the blood of the femoral artery, that is, after the lungs had been traversed, and (4) the extent of the gaseous exchange in the lungs was measured. The rapidity of the blood flow was 1806 c.c. in three minutes. The following calculations show that no oxidation took place in the lungs or in the blood, and in publishing these results Henriques recants a contrary opinion previously held by him:

1 Henriques: "Biochemische Zeitschrift," 1915, lxxi, 481.

The differences are within the limits of experimental error. It is evident that the place of oxidation is in the tissues (see P. 32).

Complete deprivation of oxygen results in asphyxiation and death. The question arises, Will there be any effect upon metabolism if the oxygen supply for the body be reduced?

Such a reduction of oxygen available for the tissues might be brought about by bloodletting, anemia, carbon-monoxid poisoning, by life on high mountains, or in balloons at high altitudes, or in pneumatic cabinets at reduced pressure, or by the artificial restriction of the free influx of atmospheric air into the lungs. Any of these methods if carried beyond a certain point is known to produce death.

It was noted by Lavoisier and confirmed by Regnault and Reiset that the respiration of pure oxygen did not increase the metabolism. Liebig was convinced that atmospheric pressure was without influence, for it was evident to him that fife at the sea-level was of the same character as on high mountains. In confirmation of these principles Zuntz1 has definitely shown that if air rich in oxygen be respired, there is an increased oxygen absorption lasting for about one minute, and then the normal quantity is absorbed. The primary increase in the quantity of oxygen absorbed is due to the filling of the lungs with oxygen and a further saturation of the blood with it, processes which are without effect on tissue metabolism. There is apparently no retention of such oxygen within the cells of the organism.

However, Hill and Flack2 show that in the fatigue of athletes oxygen inhalation increases the lasting power and decreases the fatigue, probably by maintaining or restoring the vigor of the heart. They believe that the fatigue which follows an athletic feat is mainly cardiac in origin and due to want of oxygen.

Pfluger3 first showed that frogs could live for a long period in an atmosphere which was free from oxygen when they were maintained at a temperature of 0°. After five hours they were capable of movement, and after seventeen hours, although apparently dead, they could be revived when placed in the air. Fletcher and Hopkins1 have found traces of lactic acid in normal resting frog's muscle, and also traces after a series of muscular contractions which were induced in an atmosphere of oxygen; but they found lactic acid in large quantity in the muscle if the contractions were brought about under anaerobic conditions.

1 Zuntz: "Archiv fur Physiologie," 1903, Suppl., p. 492.

2 Hill and Flack: "Journal of Physiology," 1909, xxxviii, p. xxviii.

3 Pfluger: "Pfluger's Archiv," 1875, x, 313.

Lesser2 has placed frogs in an ice calorimeter and filled the chamber in which they lived first with air and then with hydrogen. When living in air the animals produced more heat and only half as much carbon dioxid as they did when they lived in hydrogen gas. In the air each milligram of carbon dioxid exhaled corresponded to a production of 4.5 small calories; in hydrogen, to only 1.6 calories. Hence the processes taking place in the two cases could not have been the same. The anaerobic carbon dioxid production could not have been at the expense of oxygen stored in the tissues of the frog or the heat production per unit of carbon dioxid exhaled would have been the same as in air, instead of being only 35 per cent, as much. The processes involved in this case can only be conjectured. It has already been stated that ascaris, an anaerobic inhabitant of the intestine, may convert glycogen into fatty acid with the elimination of carbon dioxid and the evolution of heat. (See P. 305.) Similar processes might take place in the anaerobic frog.

Lesser3 has further shown that the quantity of oxygen absorbed by a frog at 150 is independent of the pressure of oxygen in the atmosphere until a percentage of 3.3 of oxygen is reached. At this point the respiratory quotient was 1.02. When 1.8 per cent, of oxygen was present the quantity of oxygen absorbed decreased to one-third the normal and the respiratory quotient rose to 2.40, indicating anaerobic cleavage of the food materials with the production of carbon dioxid. After eight hours of this treatment the frog became paralyzed.

1 Fletcher and Hopkins: "Journal of Physiology," 1907, xxxv, 247. 2 Lesser: "Zeitschrift fur Biologie," 1908, li, 287. 3 Lesser: "Biochemische Zeitschrift," 1914, lxv, 400.

According to Zunta,1 any anemic condition which results in the production of lactic acid makes demands on the glycogen reserves of the body, so that sugar may rise abnormally in the blood, and both sugar and lactic acid appear in the urine.