156 CHANGES OF AIR AND BLOOD IN RESPIRATION. 



tion of arterial and venous blood, aside from the proportion of gases. The 

 arterial blood contains less water and is richer in organic and in most inor- 

 ganic constituents than the venous blood. It also contains a larger propor- 

 tion of corpuscles. It is more coagulable and offers a larger and firmer clot 

 than the clot of venous blood. The only constituents which are constantly 

 more abundant in venous blood are water and the alkaline carbonates. Ac- 

 cording to Longet, 10,000 parts of venous blood contained 12'3 parts of car- 

 bon dioxide combined, and the same quantity of arterial blood contained but 

 8-3 parts. The deficiency of water in the blood which comes from the lungs 

 is readily explained by the escape of watery vapor in the expired air. 



An important distinction between arterial and venous blood is that the 

 former has a uniform composition in all parts of the arterial system, while 

 the composition of the latter varies very much in the blood coming from dif- 

 ferent organs. Arterial blood is capable of carrying on the processes of 

 nutrition, while venous blood is not, and it can not even circulate freely in 

 the systemic capillaries. 



Relations of Respiration to Nutrition, etc. It has been demonstrated 

 that all tissues, so long as they retain their absolute integrity of composition, 

 have the property of appropriating oxygen and exhaling carbon dioxide, in- 

 dependently of the presence of blood; and that the arterial blood carries 

 oxygen from the lungs to the tissues, there gives it up, and receives carbon 

 dioxide, which is carried by the venous blood to the lungs, to be exhaled. 

 This fact alone shows that respiration is inseparably connected with the 

 general act of nutrition. Its processes must be studied, therefore, as they 

 take place in the tissues and organs of the body. 



Oxygen taken from the air is immediately absorbed by the blood and en- 

 ters into the composition of the red corpuscles. Part of the oxygen disap- 

 pears in the red corpuscles themselves, and carbon dioxide is given oft. To 

 how great an extent this takes place, it is impossible to say ; but it is evident, 

 even from a study of the methods of analysis of the blood for gases, that the 

 property of absorbing oxygen and giving off carbon dioxide, which belongs 

 . to the tissues, is possessed as well by the red corpuscles. During life it is 

 not possible to determine how far this takes place in the blood and how far 

 it occurs in the tissues. The theory has been proposed that the respiratory 

 change takes place in the blood as it circulates ; but the avidity of the tissues 

 for oxygen and the readiness with which they exhale carbon dioxide leave no 

 room for doubt that much of this change is eifected in their substance. 



Oxygen, carried by the blood to the tissues, is appropriated and consumed 

 in their substance, together with the nutritive materials contained in the cir- 

 culating fluid. Physiologists are acquainted with some of the laws which 

 regulate its consumption, but have not been able to ascertain the exact nature 

 of the changes which take place. All that can be said definitely on this point 

 is that oxygen unites with the organic constituents of the body, satisfying 

 the " respiratory sense " and supplying an imperative want which is felt by 

 all animals and which extends to all parts of the organism. After its absorp- 

 tion, oxygen is lost in the processes of nutrition. There is no evidence in 



