38(3 THE HUMAN BODY. 



venous blood, returning to the heart, is sent on to the pul- 

 monary capillaries. Here, the partial pressure of oxygen 

 in the air-cells being 130 mm. (5.2 inches) and that in the 

 blood plasma much less, oxygen will be taken up by the 

 latter, and the tension of that gas in the plasma tend to be 

 raised above the limit at which haemoglobin combines with 

 it. Hence, as fast as the plasma gets oxygen those red cor- 

 puscles which 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 concerned, as when we commenced to fol- 

 low 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 com- 

 bination 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 pulmonary air-cells is about 40mm. (1.6 

 inches) of mercury. Therefore 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 60 of this gas, and 100 vols. of arterial only 50. 



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 pro- 

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

 however, tolerably certain: 



1. The amount of carbon dioxide produced in an organ 



