RESPIRATION. 407 



Atmospheric Pressure Partial Pressure of Oxygen Hemoglobin Oxy hemoglobin 



in mm. Hg. in mm. Hg. Percentage. Percentage. 



760 159.3 J -49 98-5 1 



524.8 no 2.14 97.86 



357.8 75 3-n 96-89 



238.5 50 4.60 95-40 



119.3 25 8.79 91.21 



47.7 10 I9-3 6 80.64 



23.8 5 3 2 -Si 67.49 



O.O O.O IOO.OO O.OO 



Carbon Dioxid. The blood yields up its contained carbon dioxid to the 

 vacuum of the gas-pump as completely as it does its oxygen. The same is 

 not the case, however, if the red corpuscles are first removed and the ex- 

 periment made with either plasma or serum. Even at zero pressure the fluid 

 contains carbon dioxid, as shown by its liberation on the addition of some 

 weak acid, as tartaric or phosphoric, an indication that it exists in a state of 

 firm combination. The same result follows the addition of the red blood- 

 corpuscles, which act in a manner similar to the acids just mentioned. 

 This property of the corpuscles has been attributed to hemoglobin, and 

 especially when in the state of oxy hemoglobin. It is for this reason that 

 blood yields all its carbon dioxid to the vacuum of the gas-pump. 



The limit of pressure at which the plasma ceases to absorb carbon 

 dioxid physically and begins to combine it chemically is not very clearly 

 defined. It has been estimated that of the entire amount, 38 to 45 volumes, 

 only about 2.5 volumes are so absorbed, the remainder being in a condition 

 of both loose and stable combination. 



An analysis of the serum, and presumably of the plasma, shows the 

 presence of sodium salts, with which the carbon dioxid could enter into com- 

 bination, viz.: sodium carbonate and dibasic sodium phosphate. The 

 sodium is thus partly divided between carbonic acid and phosphoric acid. 

 The amount of the sodium which falls to carbon dioxid will depend on the 

 mass influence of the latter; that is, its partial pressure. 



At its origin in the tissues the carbon dioxid acquires a considerable 

 tension, and its mass influence is correspondingly large. On entering the 

 blood it combines with sodium carbonate, with the formation of sodium 

 bicarbonate, as shown in the following equation: 



Na 2 C0 3 + C0 2 + H 2 0-2NaHC0 3 . 



At the same time, having a greater mass influence than the phosphoric acid, 

 it will withdraw from the dibasic sodium phosphate one-half of its sodium, 

 with the formation of sodium bicarbonate and monobasic sodium phosphate, 

 as shown in the following equation: 



Na 2 HP0 4 + CO 2 + H 2 0=NaHCO 8 +NaH 2 P0 4 . 



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 withdrawn 

 from the sodium bicarbonate, an additional liberation of carbon dioxid takes 

 place and dibasic sodium phosphate is re-formed. The association or com- 

 bination of the carbon dioxid with the basic salts depends on its partial 

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



