366 TEXT-BOOK OF -PHYSIOLOGY. 



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 experiment 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 phos- 

 phoric, 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 oxyhemoglobin. 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 physically 

 absorb carbon dioxid and begins to chemically combine it 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 re- 

 mainder 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 combination, 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. 



fi At its ^ origin in the tissues the carbon dioxid acquires a consider- 

 able tension, and its mass influence is correspondingly largey^)n' 

 entering the blood it combines with sodium carbonate, with the 

 formation of sodium bicarbonate, as shown in the following equation: 



Na 2 CO 3 + CO 2 + H 2 O = 2 NaHCO 3 . 



At the same time, having a greater mass influence than the phos- 

 phoric acid, it will withdraw from the dibasic sodium phosphate 

 one-half of its sodium, with the formation of sodium bicarbonate and 

 3basic sodium phosphate, as shown in the following equation: 



Na 2 H}>0 4 + C0 2 + H 2 = NaHCO 3 + NaH 2 PO 4 . 



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

 alveoli its tension in the venous blood falls, its mass influence dimin- 

 ishes while that of the phosphoric acid relatively increases. As a 

 SJ i e i? f ls 7 lth u raWn from the sodium bicarbonate, an 



DhJsZt r ^ n dl Xid takes P lace and dibasic s dium 



phosphate is re-formed. The association or combination of the 

 carbon dioxid with the basic, salts depends on its partial pressure' I 

 dissociation in the lungs, on a diminution of pressure 



