430 BLOOD AND LYMPH. 



as carbon monoxid hemoglobin. In this compound also the union 

 takes place in the proportion of one molecule of hemoglobin to one 

 molecule of the gas. The compound formed differs, however, 

 from oxyhemoglobin in being much more stable, and it is for this 

 reason that the breathing of carbon monoxid gas is liable to prove 

 fatal. The CO unites with the hemoglobin, forming a firm com- 

 pound; the tissues of the body are thereby prevented from obtain- 

 ing their necessary oxygen, and death results from suffocation or 

 asphyxia. Carbon monoxid forms one of the constituents of 

 coal-gas. The well-known fatal effect of breathing coal-gas for 

 some time, as in the case of individuals sleeping in a room in which 

 gas is escaping, is traceable directly to the carbon monoxid. Nitric 

 oxid (NO) forms also with hemoglobin a definite compound that 

 is even more stable than the CO hemoglobin; if, therefore, this 

 gas were brought into contact with the blood, it would cause death 

 in the same way as the CO. 



Oxyhemoglobin, carbon monoxid hemoglobin, and nitric oxid 

 hemoglobin are similar compounds. Each is formed, apparently, 

 by a definite combination of the gas with the hematin portion of the 

 hemoglobin molecule, and a given weight of hemoglobin unites 

 presumably with an equal volume of each gas. In marked contrast 

 to these facts, Bohr* has shown that hemoglobin forms a compound 

 with carbon dioxid gas, carbohemoglobin, in which the quantitative 

 relationship of the gas to the hemoglobin differs from that shown 

 by oxygen. In a mixture of O and CO 2 the latter gas is absorbed by 

 hemoglobin solutions independently of the oxygen, so that a solu- 

 tion of hemoglobin nearly saturated with oxygen will take up CO 2 

 as though it held no oxygen in combination. Bohr suggests, there- 

 fore, that the O and the CO 2 must unite with different portions of the 

 hemoglobin the oxygen with the pigment portion and the CO 2 possi- 

 bly with the protein portion. Although the amount of CO 2 taken up 

 by the hemoglobin is not influenced by the amount of O already in 

 combination, the reverse relationship does not hold in all cases. It is 

 found that the presence of the CO 2 loosens, as it were, the combina- 

 tion between the hemoglobin and the oxygen so that the oxyhemo- 

 globin dissociates more readily than would otherwise be the case. 

 This is observed at least when the oxygen is under a low pressure, 

 ?uch as occurs, for instance, in the capillaries of the tissues. The 

 importance of this fact in regard to the oxygen supply to the tissues 

 is referred to more explicitly in the section on Respiration. 



Presence of Iron in the Molecule. It is probable that iron 

 is quite generally present in the animal tissues in connection with 

 nuclein compounds, but its existence in hemoglobin is noteworthy 

 because it has long been known, and because the important property 

 of combining with oxygen seems to be connected with the presence 



* "Skandinavisches Archiv f. Physiologic/' 3, 47, 1892, and 16, 402, 1904. 



