1118 PHYSIOLOGY 



the mixture is exposed to a vacuum, any sodium bicarbonate present will 

 undergo dissociation, carbon dioxide being given off and the carbonate 

 Na 2 C0 3 formed. This then reacts with the sodium phosphate in the 

 following way : 



2NaH 2 P0 4 -f Na 2 C0 3 = 2Na 2 HP0 4 + C0 2 + H 2 0. 



In this way the whole of the sodium enters into combination with the P0 4 

 and the carbon dioxide previously combined is given off. On exposing 

 the mixture to an atmosphere containing carbon dioxide, the reverse change 

 takes place, and we get once again sodium hydrogen phosphate and sodium 

 carbonate, and finally sodium bicarbonate. It was formerly thought that 

 in the blood plasma phosphates were an important factor in the evolution 

 of the carbon dioxide. Blood plasma however contains the merest trace 

 of phosphates, and the role of a weak acid competing with the carbon dioxide 

 for the sodium seems to be played chiefly by the proteins (including 

 haemoglobin) of the corpuscles and plasma. It may be assumed that in a 

 complete vacuum the whole of the soda is in combination with the protein 

 and haemoglobin. On exposure of the blood to increasing pressure of C0 2 , 

 the sodium leaves the protein to combine with this gas with the production 

 of sodium bicarbonate. Under ordinary circumstances, with a C0 2 tension 

 in the blood of 40 mm. Hg., rather more than half the sodium would thus 

 be in combination with protein, and only when the C0 2 pressure exceeds 

 100 mm. Hg. would the whole of the sodium be taken up by the carbonic 

 acid. 



The exact mode in which carbonic acid is carried into the blood cannot be regarded 

 as finally settled. If the above account is correct, it should be possible by the 

 addition of blood protein to sodium bicarbonate to induce this latter to give up the 

 whole of its carbonic acid to a vacuum. The protein would however have to be pre- 

 pared free from sodium, and this ought to be possible by dialysing a serum or haemo- 

 globin solution against normal saline in the presence of excess of C0 2 , i. e. maintaining 

 a tension of this gas in the dialysing fluid of something over 100 mm. Hg., since only 

 under these conditions would it be possible to free sodium from its attachment to the 

 protein. It was pointed out by Bohr that haemoglobin formed a combination with 

 carbonic acid and that the dissociation curve of this compound resembled closely that 

 of the carbonic acid in the whole blood. According to Buckmaster, carbonic acid 

 exists in the blood in two modes of combination, viz. as sodium bicarbonate and as 

 carbon dioxide haemoglobin. He considers that the function of the sodium bicar- 

 bonate is to maintain the neutrality of the blood by acting as a ' buffer ' substance, 

 while the haemoglobin compound is responsible for all the transference of carbonic 

 acid which takes place in respiration. If however the crystallised haemoglobin 

 employed contains sodium in combination, it might act as a sodium protein system in 

 the same manner as we have assumed for the whole blood. Further evidence is required 

 on this question. 



The red corpuscles may act in another way in favouring the giving up 

 of the carbon dioxide of the plasma to a vacuum. There is evidence that 

 an interchange of acid radicals takes place between the corpuscles and the 

 plasma on exposure of blood to varying tensions of C0 2 . According to 

 Hamburger, when carbon dioxide is passed into defibrinated blood, the 



