136 NUTRITION OF FARM ANIMALS 



converted partly or wholly into oxy haemoglobin, the amount 

 of the oxygen taken up ranging from eight to twelve volume 

 per cent. The color of haemoglobin is a dark red or purplish, 

 while that of oxyhagmoglobin is bright scarlet. To this differ- 

 ence of color is due the marked difference in appearance be- 

 tween venous and arterial blood. 



192. Respiration of tissues. The term respiration is very 

 commonly applied to the mechanical processes of breathing 

 just described or to the exchange of gases in the lungs. In 

 reality all these are preliminary to the real respiration, which 

 takes place in the tissues. The vital processes in the body 

 cells consist, broadly speaking, as will appear in detail in the 

 next chapter, of a series of oxidations. The requisite oxygen 

 is necessarily drawn from the lymph in which the cells exist (185), 

 while the carbon dioxid produced by oxidation is discharged 

 into it. The lymph, therefore, tends continually to become 

 richer in carbon dioxid and poorer in oxygen. In the manner 

 just described the blood takes up oxygen in the lungs and acts 

 as a carrier through the body. Through the capillary blood 

 vessels of the body generally, therefore, there are continually 

 passing red blood corpuscles charged with loosely combined 

 oxygen, while on the other side of the capillary wall is a fluid 

 (the lymph) in which the partial pressure of oxygen is relatively 

 low. Accordingly, the combination of oxygen and haemoglo- 

 bin is dissociated to a greater or less extent and oxygen passes 

 into the lymph as required to supply the needs of the cells. At 

 the same time the excess of carbon dioxid in the lymph passes 

 in the opposite direction into the blood and is thus removed 

 from the neighborhood of the cell. 1 It is this continual con- 

 sumption of oxygen and elimination of carbon dioxid by the 

 cells which constitutes the real act of respiration, while the 

 complex structure of the lungs and the elaborate mechanism of 

 breathing and of the blood corpuscles are simply means for 

 providing oxygen to the cells and taking away carbon dioxid. 

 That the movements of breathing are not an essential part of 

 respiration is strikingly shown by the fact that it is perfectly 

 possible by suitable devices to maintain oxygenation of the blood 



1 In these exchanges, as in other similar ones, while diffusion doubtless plays a 

 large part, its effects are no doubt modified by the special properties of the living 

 cells. 



