RESPIRATION. 367 



Nitrogen. This gas exists in both arterial and venous blood 

 in a state of solution. There is no evidence that it enters into com- 

 bination 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, vlf the pressure of any one 

 gas in the atmosphere is greater than in the IJquidriFjsabsorbed; 



Knowing the pressure of the 



gases in percentages 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 aerotonometers 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 and carbon dioxid 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 and generally accepted experiments of Strass- 

 burger, 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 41.04 mm. 

 Hg, or 5.4 per cent, of an atmosphere, and in the arterial blood 21.8 

 mm. Hg, or 2.8 per cent. In the experiments of Fredericq the oxygen 

 tension in the arterial blood was found to be 106 mm. Hg, or 14 per 

 cent, of an atmosphere. 



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- 



