RESPIRATION. 



193 



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. 



B. 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 inter- 

 vening. 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 derived 

 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 junc- 

 tion, and incapable of closing the orifice of either of the veins between 

 which they are placed. The pulmonary circulation also is unaffected by 

 changes of atmospheric pressure, and is not exposed to the influence of 

 the pressure of muscles : the force by which it is accomplished, and the 

 course of the blood are alike, simple. 



Changes in the Blood. The most obvious change which the blood 

 of the pulmonary artery undergoes in its passage through the lungs is 

 1st, that of color, the dark crimson of venous blood being exchanged for 

 the bright scarlet of arterial blood ; %d, and in connection with the pre- 

 ceding change, it gains oxygen ; 3d, it loses carbonic acid ; th, it be- 

 comes slightly cooler ; 5th, it coagulates sooner and more firmly, appar- 

 ently containing more fibrin. The oxygen absorbed into the blood from 

 the atmospheric air in the lungs is combined chemically with the haemo- 

 globin of the red-corpuscles. In this condition it is carried in the arterial 

 blood to the various parts of the body, and brought into near relation or 

 contact with the tissues. In these tissues, and in the blood which cir- 

 culates in them, a certain portion of the oxygen, which the arterial 

 blood contains, disappears, and a proportionate quantity of carbonic acid 

 and water is formed. The venous blood, containing the new-formed 

 carbonic acid returns to the lungs, where a portion of the carbonic acid 

 is exhaled, and a fresh supply of oxygen is taken in. 



Mechanism of Various Respiratory Actions. 



It will be well here, perhaps, to explain some respiratory acts, which 

 appear at first sight somewhat complicated, but cease to be so when the 

 mechanism by which they are performed is clearly understood. The ac- 

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