596 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



gases by liquids, and increases with the gaseous tension; while, on the 

 other hand, a larger amount must be held in some form of chemical com- 

 bination, since blood is capable of absorbing under equal pressures much 

 more carbon dioxide than water. 



On the other hand, it is also clear that this chemical combination 

 must be a very loose one, since it is broken up by exposure to a vacuum. 

 It is probable that part of this carbon dioxide is present in the serum as 

 a bicarbonate of sodium, a combination which, by reduced pressure, is 

 readily broken up into sodium carbonate and free carbon dioxide. It is 

 further possible that another part of this carbon dioxide may be loosely 

 combined with a sodium biphosphate, a salt which is likewise present 

 in the blood-serum, and which has been shown to readily combine with 

 carbon dioxide. It is, however, to be noted that this salt, sodium biphos- 

 phate, is solely present in considerable amounts in the blood of carnivora 

 and omnivora, while only traces of it are present in the blood of her- 

 bivora, so that in the latter this combination cannot exist. It, in any 

 case, seems evident that the red blood-corpuscles in some way govern 

 the presence of this gas in the serum. 



The nitrogen, which may be removed from the blood by exposure to 

 a vacuum in from 1 to 3 volumes per cent., is without doubt simply 

 held in solution by the serum, since water likewise is capable of absorb- 

 ing this gas to the same amount. 



The exchange which takes place between these gases in the blood 

 and the atmospheric air in the lungs is governed almost purely by the 

 simple physical laws of gaseous diffusion. As already mentioned, the 

 air within the pulmonary vesicles is separated from the blood in the 

 capillaries only by the delicate epithelial walls of the vesicles and the 

 structureless walls of the capillaries. The oxygen of the blood ? as 

 already indicated, is loosely combined with the haemoglobin. When re- 

 duced haemoglobin is exposed to an atmosphere of oxygen it readily 

 absorbs that gas, provided the tension of the oxygen in the surrounding 

 medium be greater than the tension of the oxygen in combination with 

 the haemoglobin. When, therefore, the blood from the pulmonary arter- 

 ies reaches the capillaries of the lungs a large part of the haemoglobin 

 there present exists in the form of reduced haemoglobin, while the atmos- 

 phere within the pulmonary alveoli contains oxygen, perhaps in a lower 

 tension than in the atmosphere, but at any rate in a higher tension than 

 is present in the haemoglobin. As a consequence, diffusion phenomena 

 set in, the oxygen passing from the interior of the alveoli through the 

 moist walls of the alveoli and blood-capillaries into the interior of 

 the latter vessels, and there combining with the haemoglobin. It may 

 be assumed that the oxygen tension in the alveoli of the lungs amounts 

 to about 10 per cent. The oxygen tension in the venous blood will, 



