268 KESPIRATION 



they would be in water ; but with oxygen and carbon dioxide the quan- 

 tities present are much greater than can be explained by simple solu- 

 tion. 



In the case of oxygen, for example, the amount present does not 

 vary appreciably with the pressure of the gas in the lungs, whereas 

 the amount of a gas dissolved by a liquid is directly proportional to the 

 pressure of the gas. The greater portion of the oxygen (probably all 

 but about one volume per 100 volumes of blood) is in the state of com- 

 bination with haemoglobin. The oxyhaemoglobin is, however, easily 

 dissociated, and if the quantity of dissolved oxygen be diminished from 

 any cause, the combined oxygen is diminished proportionately. The 

 great bulk of the oxygen of the blood is thus to be found in the red 

 corpuscles, while only a small quantity is in the plasma. 



With carbon dioxide the case is different. Of the 40 to 46 volumes 

 of the gas present in 100 volumes of blood about two are in the state 

 of simple solution, the remainder in a state of weak combination, 

 partly with the haemoglobin of the red corpuscles, but mainly as bi- 

 carbonates in solution in the plasma. 



Respiration consists in bringing air into close proximity to the blood 

 stream in the lungs, separation being only maintained by the thin 

 walls of the capillaries and air cells or alveoli. The air in the alveoli 

 is not renewed by mechanical expulsion due to inspiration and ex- 

 piration, but by diffusion from and into the bronchial tubes. 



The partial pressure of the dissolved carbon dioxide in the venous 

 blood is greater than that in the air of the alveoli ; consequently the 

 blood plasma loses carbon dioxide, thus producing dissociation of the 

 bicarbonates in solution. The partial pressure of the dissolved oxygen 

 in the venous blood is, however, less than that of the alveolar air, 

 and consequently the plasma takes in oxygen. This disturbs the 

 equilibrium between the dissolved oxygen of the blood plasma and the 

 combined oxygen of the oxyhaemoglobin, causing the formation of more 

 of the latter with a simultaneous diminution of the former. The blood 

 plasma is thus able to take in a further quantity of oxygen from the 

 air. In this way it is probable that all the oxygen which goes into 

 the blood in the lungs does so by passing through the stage of dissolved 

 oxygen in the plasma. 



At the same time the air in the lungs becomes saturated with 

 aqueous vapour at the temperature of the body. 



The number of respirations per minute varies with the age, etc., 

 of the animal. For adult animals the following is the average : 



Horse . 9 to 12 



Ox 15 to 18 



Sheep 13 to 16 



The change produced in the composition of air by respiration is 

 indicated in the table on the following page, which gives the average 

 composition of air before and after inspiration. 



The most important changes undergone by the air are the loss of 

 oxygen and the gain in carbon dioxide and aqueous vapour. Oxygen 

 by its union with carbon gives rise to its own volume of carbon dioxide. 



