THE RESPIRATORY SYSTEM. 263 



tension of carbonic acid in venous blood and in alveolar air, slight 

 though it is, is sufficient to allow carbonic acid to pass from the blood 

 into the air in the lungs by simple diffusion. 



Owing to the difference in the tension of oxygen and of carbonic 

 acid in the blood and in alveolar air, the venous blood as it flows 

 through the lungs takes up oxygen and leaves the lungs almost fully 

 saturated ; at the same time it loses 6 to 8 volumes of carbonic acid per 

 1 00 volumes of blood. 



SECTION III. 

 THE REGULATION OF THE RESPIRATORY MOVEMENTS. 



Oxygen is continually passing from the alveolar air into the blood, 

 and is replaced by the entrance of air from the atmosphere during 

 respiration. Similarly, carbonic acid is constantly diffusing from the 

 blood into the alveolar air, and with each expiration a certain amount 

 is expelled from the lungs. Assuming that the amount of air entering 

 and leaving the lungs with each breath is 400 c.c., and that the expired 

 air contains 4 per cent, of carbonic acid, then with each respiration 

 16 c.c. of carbonic acid are removed from the body. Since the composi- 

 tion of alveolar air remains constant, it is evident that 16 c.c. of carbonic 

 acid must, in the same time, have passed from the blood into the air 

 in the lungs. When an additional amount of carbonic acid is formed 

 in the body and taken up by the blood, it passes into the alveolar air 

 and tends to raise its percentage. Hence, if the percentage of carbonic 

 acid in alveolar air is to remain constant, as it actually does under 

 normal conditions, the amount of air passing into and out of the lungs 

 at each breath must be correspondingly increased. If 32 c.c. of carbonic 

 acid pass from the blood into the alveolar air during a breath, they can 

 be removed if the amount of expired air is 800 c.c. containing 4 per 

 cent, carbonic acid. In the same way, the needs of the body for more 

 oxygen are met by a larger amount of air entering the lungs at each 

 inspiration. This process, whereby any accumulation of carbonic acid 

 or deficiency of oxygen in the alveolar air, and consequently in the 

 blood, is prevented, is under the control of the central nervous system, 

 and is regulated in such a way that an excess of carbonic acid, or a lack 

 of oxygen, leads to deeper and more rapid respiratory movement^ 



The Respiratory Centre. The rhythmic alternation of inspiration 

 and expiration is brought about by the contraction of the respiratory 

 muscles, which are controlled and co-ordinated by a centre the 

 respiratory centre lying in the grey matter of the floor of the fourth 

 ventricle near the apex of the calamus scriptorrus. From this centre, 



