THE CHEMISTRY OF RESPIRATION. 411 



contain any reduced haemoglobin rob it, and so its oxygen 

 tension is kept down below that in the air-cells until all the 

 haemoglobin is satisfied. Then the oxygen tension of the 

 plasma rises to that of the gas in the air-cells; no more 

 oxygen is absorbed, and the blood returns to the left auricle 

 of the heart in the same condition, so far as oxygen is con- 

 cerned, as when we commenced to follow it. 



The Carbon Dioxide of the Blood. The same general 

 laws apply to this as to the blood oxygen. The gas is partly 

 merely dissolved and partly in a loose chemical combination 

 much like that of oxygen with haemoglobin, but the body 

 with which it combines probably exists in the plasma more 

 than in the red corpuscles; what it may be is not certainly 

 known. Besides this, some more carbon dioxide is stably 

 combined and is only given off on the addition of a stronger 

 acid. The partial pressure of carbon dioxide in the pulmo- 

 nary air-cells is about 40 mm. (1.6 inches) of mercury. There- 

 fore the tension of that gas in the pulmonary capillaries 

 must be more than this. On the other hand its tension in 

 arterial blood must be less than that in the lymph around 

 the tissues; otherwise it could not enter the blood in the 

 systemic circulation, which it does, as proved by the fact 

 that 100 vols. of venous blood give off 46 of this gas, and 100 

 vols. of arterial only 40. 



The nitrogen dissolved in the blood is, so far as we know, 

 quite unimportant. 



Internal Respiration. As to the amount of oxygen used 

 by each tissue and the quantity of carbon dioxide produced 

 by it we know but little; the following points seem, however, 

 tolerably certain: 



1. The amount of carbon dioxide produced in an organ 

 in a given time bears no constant ratio to the amount of 

 oxygen taken up by it simultaneously. This is certainly 

 true of muscle, for experiment shows that muscular work 

 if really severe leads to an elimination of carbon dioxide 

 containing more oxygen than the total oxygen taken up from 

 the lungs at the same time. The balance is of course made 

 up in subsequent periods of rest, when more free oxygen is 

 taken up than is eliminated in combination during the same 

 time. Moreover, a frog's muscle excised from the body and 

 put in an atmosphere containing no oxygen and made there 

 to contract, will evolve with each contraction considerable 



