RESPIRATION 421 



with the formation of sodium bicarbonate and monobasic sodium phosphate, 

 as shown in the following equation: 



3. In combination with the protein constituents. The percentage com- 

 bined with protein constituents of the red corpuscles and plasma has been 

 estimated at 20 per cent. As this is the case the corpuscles must be 

 regarded as carriers of CO 2 from the tissues to the lungs as well as carriers 

 of oxygen. The same holds true for the protein of the plasma. It has 

 also been suggested that the presence of the C0 2 in the corpuscle, assists in 

 the dissociation of the oxygen as the blood passes through the capillaries. 

 With the diffusion of the carbon dioxid from the blood into the alveoli its 

 tension in the venous blood falls, its mass influence diminishes, while that 

 of the phosphoric acid relatively increases. As a result, the sodium is with- 

 drawn from the sodium bicarbonate, an additional liberation of carbon 

 dioxid takes place and dibasic sodium phosphate is re-formed. The asso- 

 ciation or combination of the carbon dioxid with the basic salts depends on 

 its partial pressure; dissociation in the lungs, on a diminution of pressure. 



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 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, 

 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 



