IV THE ESSENCE OF RESPIRATION 127 



solutions of hfemoglobin absorb oxygen as readily and 

 largely as blood does. Finally, the oxygen is known to 

 be loosely combined with the lipenioglobin, because when 

 blood is subjected to a gradually increasing vacuum, the 

 oxygen does not come off uniformly and progressively, 

 as the vacuum is made greater, in the way it would if 

 it were in mere solution ; on the contrary it escapes 

 icith a sxuiden rush after the 2Jressiire has been considerably 

 rediiced. 



The conditions under which carbonic acid exists in the 

 blood may also be shown to be those of a loose chemical 

 combination ; but beyond this fact our knowledge is 

 somewhat incomplete. It is known, however, that the 

 carbonic acid is combined chiefly in some constituents of 

 the plasma rather than with the corpuscles, and most 

 authorities consider that the larger part is present in 

 plasma united with sodium in the form of sodium bicar- 

 bonate, NaHCO,. 



2. The Nature and Essence of Respiration. — All the 

 tissues, as we have seen, are continually using up oxygen. 

 Their life in fact is dependent on a continual succession 

 of oxidations. Hence they are greedy of oxygen, while at 

 the same time they are continually producing carbonic 

 acid (and other waste products). The demand for oxygen 

 is met by a supply fi'om the red corpuscles, and the 

 oxygen they give up passes through the walls of the 

 capillaries, across the lymph and so to the cells of which 

 the tissue is composed. At the same time the carbonic 

 acid passes across the lymph in the opposite direction, 

 through the capillary walls and into the blood, by which it 

 is at once whii-led away into the veins. The blood there- 

 fore leaves the tissue poorer in oxygen and richer in 

 carbonic acid than when it came to it : and this change is 

 the change from the arterial to the venous condition. 

 This gaseous interchange between the blood and the 



