40 RESPIRATION. 



Mention some other changes in the respired air. 



Small quantities of organic matter and of ammonia are found. 



Has the nitrogen of the air a physiological bearing? 



A small amount of this gas is found in the blood, but it is 

 mostly in simple solution, and is not supposed to be of physio- 

 logical value. In respired air it is of use as a diluent of the 

 oxygen. 



Describe the respiratory changes of the air in the lungs. 



In the blood oxygen is in loose chemical combination with the 

 haemoglobin (oxy-haemoglobin). The amount of the oxygen in the 

 residual air of the pulmonary alveoli is estimated at about 10 per 

 cent., that of expired air being 16 per cent. It is found that unless 

 there is present about 4 per cent, of oxygen there is no tendency for 

 the blood of the pulmonary arteries (venous) to take up fresh oxygen, 

 or the tension of the oxygen in the reduced haemoglobin of venous 

 blood is about 4 per cent., and unless the oxygen tension in the 

 lungs is greater, there is no absorption of oxygen. But, as we 

 have seen, the amount of oxygen amounts to at least 10 per cent., 

 and therefore the excess is sufficient to exceed the demands, and 

 the exchange is readily made by diffusion through the thin capil- 

 lary walls in the alveoli. 



On the other hand, the tension of the carbonic acid in the 

 pulmonary arteries is much higher than in the alveoli, and hence 

 the extrusion of this gas by diffusion is accomplished. 



How is the blood changed by respiration? 



(1) Color, deep purple to bright scarlet by oxidation of the 

 reduced haemoglobin i. e. from venous becomes arterial blood ; 

 (2) gains oxygen ; (3) loses carbonic acid ; (4) becomes cooler ; 

 (5) coagulates more readily. 



Name some of the special actions of modified respiration. 



There are a number of involuntary and voluntary special respi- 

 ratory acts, largely reflex, which are dependent upon modifications 

 of inspiration and expiration e. g. sighing, hiccough, cough, sneez- 

 ing, speaking, singing, sniffing, sobbing, laughing, yawning. 



What is the nervous mechanism of respiration ? 



In the medulla oblongata are centres for each half of the body, 

 from which arise automatically the rhythmical impulses for the 

 respiratory acts. While these are automatic, they may be stimu- 



