RESPIRATION 115 



render the plasma alkaline namely, sodium carbonate and sodium phos- 

 phate. Walter found in the blood of rabbits poisoned with hydrochloric 

 acid (thus rendering the plasma acid) only 2.5 volumes per cent, of carbon 

 dioxide. According to Fernet and to Heidenhain, the dioxide is also 

 combined in part with sodium acid phosphate, Na 2 HPO 4 , but only to a 

 small extent. Serum globulin is another substance of the plasma which 

 undoubtedly holds some of the carbon dioxide during its transit to the 

 lungs. The corpuscles contain about a third of the carbon dioxide, 

 it being "in loose chemical combination probably with the alkali of the 

 phosphates, globulin, and hemoglobin of the corpuscles, and directly 

 with the hemoglobin." (Starling.) Setschenow found in the erythro- 

 cytes 10 per cent, by volume of carbon dioxide and in the leukocytes 

 2.5 per cent. 



There is a small amount (about 1.8 per cent.) of nitrogen simply 

 dissolved in the blood, but as it appears not to have any respiratory 

 function, its further consideration need not detain us. It is absorbed 

 by the blood in the lungs on purely physical principles, and does not 

 enter into chemical combination when in its free gaseous state, being 

 only a diluent of the oxygen of the atmosphere. 



Having summarized now the chemical information as to the relation 

 of the respiratory gases in the blood to and from the lungs and the 

 tissues, let us see in general terms the mechanism of this transit. 



The blood circulating in its closed system of tubes and transuding 

 (as lymph) through the capillary walls is the means of the distribution 

 of oxygen and of the excretion of the carbon dioxide into air in the alveoli. 

 As the arterial blood -current moves a meter or less in a second, and the 

 venous current somewhat more slowly (the capillaries are only \ mm. 

 long), a complete circulation from any point through the heart and the 

 lungs and back again to the point of starting might occur in thirty seconds. 

 This fact shows how prompt a carrier the blood (plasma and corpuscles) 

 is, and as it is a continual flow, unceasing for an instant during life, the 

 service is very efficient. Owing to the minute caliber of the capillaries 

 of the lungs and tissues generally, the speed of the blood through them is 

 small, \ mm. per second. The capillaries average J mm. in length, and 

 there is just a second, therefore, on the average, for the plasma, its dis- 

 solved sodium carbonate and phosphate, and the two main sorts of 

 corpuscles floating in its stream to dissolve and absorb from the lungs 

 their load of oxygen and from the tissues their load of carbon dioxide. 

 During this second also the respective burdens must be dropped when 

 they have made their transits. 



The structures through which the two respiratory gases pass in internal 

 respiration that is, between the tissue-capillaries and the interior of 

 tissue-cells are not unlike the homologous structures of external respira- 

 tion in the lungs. A molecule of oxygen bound from the blood for a 

 tissue-cell must pass through the plasma intervening between the red 

 corpuscle from which it starts and the capillary wall; through the 

 endothelium constituting the latter; through a layer of lymph, prac- 



