4 o8 TEXT-BOOK OF PHYSIOLOGY. 



Nitrogen. This gas exists in both arterial and venous blood in a state 

 of solution. There is no evidence that it enters into combination with any 

 other element. 



Tension of the Gases in the Blood. It will be recalled that a liquid 

 holding in solution one or more gases will on exposure to an atmosphere 

 composed of the same gases either give up or absorb volumes varying in 

 amount and in accordance with their partial pressures until equilibrium is 

 established. If the pressure of any one gas in the atmosphere is greater than 

 the pressure of the same gas in the liquid, it is absorbed; if the pressure is 

 less the gas is discharged. Knowing the pressure of the gases in percent- 

 ages of an atmosphere, at the beginning and the end of an experiment, the 

 original tension or pressure of the gases in the liquid can be easily calculated. 

 On this principle various forms of apparatus known as aerotonometer s have 

 been devised by which the tension of the gases in the blood can be determined. 



These appliances consist essentially of a glass tube containing oxygen, 

 carbon dioxid, and nitrogen in known amounts and tensions. The blood 

 from an animal is then allowed to flow directly from an artery or vein into the 

 tube. As it flows down its sides in a thin layer it presents a large surface 

 to the action of the contained gases. In the aerotonometer of Fredericq 

 the blood, made non-coagulable by the injection of peptone, is returned 

 from the opposite extremity of the tube to the animal. This enables the 

 experiment to be continued for an hour or more. A knowledge of the 

 tensions of the blood gases is of interest, as it affords a clue to the mechanism 

 by which the interchange takes place between the lungs and the blood, on the 

 one hand, and the blood and tissues, on the other. The results, however, of 

 different observers are not sufficiently in accord to permit of positive 

 deductions. 



In the well-known experiments of Strassburger, the tension of the 

 oxygen in the arterial blood of the dog was found to be 29.64 mm. Hg., or 

 3.9 per cent, of an atmosphere, and in the venous blood 22.04 mm - Hg., or 

 2.9 per cent. The tension of the carbon dioxid in the venous blood was 

 found to be 41.14 mm. Hg., or 5.4 per cent, of an atmosphere, and in the 

 arterial blood 21.8 mm. Hg., or 2.8 per cent. Very different results have 

 been obtained by Fredericq with the aerotonometer devised by him and by 

 the employment of a method different from that of Strassburger. Thus he 

 states that the oxygen tension in the pulmonary alveoli is 136 mm. Hg., or 

 18 per cent, of an atmosphere while in the arterial blood it is 106 mm. Hg., 

 or 14 per cent.; while the carbon-dioxid tension in the tissues varies from 

 45 to 68 mm. Hg., or from 6 to 9 per cent, of an atmosphere; while in the 

 venous blood it varies from 30 to 41 mm. Hg., or from 3.8 to 5.4 per cent, 

 and in the pulmonary alveoli it is about 21 mm. or 2.8 per cent. 



CHANGES IN THE COMPOSITION OF THE TISSUES AND LYMPH. 



From previous statements the inferences can be drawn that the oxygen 

 leaves the blood as the latter flows through the capillaries; that it passes 

 through the capillary wall into the surrounding lymph and so to the tissue- 

 cells; that it oxidizes food materials in the tissue-cells whereby the potential 

 energy of the former is liberated as kinetic energy; that the carbon dioxid 



