276 HANDBOOK OF PHYSIOLOGY. 



other hand, if the substance be not saturated, i.e., if it be not combined 

 with as much of the gas as it is capable of taking up, further combina- 

 tion leads to no increase of its tension. However, there is a point at 

 which the haemoglobin gives up its oxygen when it is exposed to a 

 low partial pressure of oxygen, and there is also a point at which it 

 neither takes up nor gives out oxygen; in the case of arterial blood of 

 the dog, this is found to be when the oxygen tension of the atmosphere 

 is equal to 3.9 per cent (or 29.0 mm. of mercury), which is equivalent 

 to saying that the oxygen tension of arterial blood is 3.9 percent; venous 

 blood, in a similar manner, has been found to have an oxygen tension of 

 2.8 per cent. At a higher temperature, the tension is raised, as there is 

 a greater tendency at a high temperature for the chemical compound to 

 undergo dissociation. It is therefore easy to see that the oxygen tension 

 of the air of the pulmonary alveoli is quite sufficient, even supposing it 

 much less than that of the expired air, to enable the venous blood to 

 take up oxygen, and what is more, it will take it up until the haemo- 

 globin is very nearly saturated with the gas. 



As regards the elimination of carbon dioxide from the blood, there 

 is evidence to show that it is given up by a process of simple diffusion, 

 the only condition necessary for the process being that the tension of 

 the carbonic acid of the air in the pulmonary alveoli should be less than 

 the tension of the carbonic acid in venous blood. The carbonic acid 

 tension of the alveolar air probably does not exceed (in the dog) 3 or 4 

 per cent, while that of the venous blood is 5.4 per cent, or equal to 41 

 mm. of mercury. 



Respiratory Changes in the Blood. 



Circulation of Blood in the Respiratory Organs. To be exposed to 

 the air thus alternately moved into and out of the air-cells and minute 

 bronchial tubes, the blood is propelled from the right ventricle through 

 the pulmonary capillaries in steady streams, and slowly enough to per- 

 mit every minute portion of it to be for a few seconds exposed to the 

 air, with only the thin walls of the capillary vessels and the air-cells 

 intervening. The pulmonary circulation is of the simplest kind: for 

 the pulmonary artery branches regularly; its successive branches run in 

 straight lines, and do not anastomose : the capillary plexus is uniformly 

 spread over the air-cells and intercellular passages; and the veins de- 

 rived from it proceed in a course as simple and uniform as that of the 

 arteries, their branches converging but not anastomosing. The veins 

 have no valves, or only small imperfect ones prolonged from their angles 

 of junction, and incapable of closing the orifice of either of the veins 

 between which they are placed. The pulmonary circulation also is un- 

 affected by changes of atmospheric pressure, and is not exposed to the 



